waterproof and water-repellent fabrics, materials treated with various substances so as to make them impervious to water. Permanent waterproofing is achieved by first coating fabrics with rubber or plasticized synthetic resins, then vulcanizing or baking them. Fabrics so treated lose porosity and lightness and when rubberized are subject to cracking. Water-repellent fabrics, sprayed with or immersed in synthetic resins, metallic compounds, oils, or waxes, tend to remain porous and to retain their natural characteristics. Earlier treatments, such as tarring the surface (as for tarpaulin) or oiling (as for oilskin), have been supplemented by highly technical and varied processes and by the method of coating the fibers prior to cloth construction. Some woolen fabrics, especially Navajo blankets and tweeds and other napped textiles, are naturally water repellent.

water, desalination of, process of removing soluble salts from water to render it suitable for drinking, irrigation, or industrial uses. The principal methods used for desalination include distillation (or evaporation), electrodialysis, freezing, ion exchange, and reverse osmosis.

In distillation saltwater is heated in one container to make the water evaporate, leaving the salt behind. The desalinated vapor is then condensed to form water in a separate container. Although long known, distillation has found limited application in water supply because of the fuel costs involved in converting saltwater to vapor. Representative of the early attempts in this direction were the solar distillation methods employed (c.49 B.C.) by the legions of Julius Caesar for using water from the Mediterranean. Modern technological advances led to the development of more efficient distillation units using solar energy; however, since these units have small capacities, their utility is restricted.

Distillation plants having high capacities and using combustible fuels employ various devices to conserve heat. In the most common system a vacuum is applied to reduce the boiling point of the water, or a spray or thin film of water is exposed to high heat, causing flash evaporation; the water is flashed repeatedly, yielding fresh distilled water. This multistage flash distillation method is used in more than 2,000 desalination plants, including one in Saudi Arabia that produces 250 million gallons of freshwater per day.

Another method of desalination is by electrodialysis. When salt dissolves in water, it splits up into charged particles called ions. Placed in a container with a negative electrode at one end and a positive electrode at the other, the ions are filtered by the membranes as they are attracted toward the electrodes; they become trapped between semipermeable membranes, leaving outside the membranes a supply of desalinated water that can be tapped. The first large installation using this process began operating in South Africa in 1958, but its electrical demands make it impractical except where such energy is abundant.

By far the most promising approach is the reverse osmosis process, in which pressure is applied to saltwater to force it through a special membrane. Only pure water passes, leaving concentrated seawater behind. Where multistage flash distillation costs about $4 per 1,000 gallons, reverse osmosis costs about half that amount. This process is used by a plant in the Tampa Bay area, Florida, that produces 25 million gallons of drinking water a day. Another type uses an empty hollow sphere of semipermeable material that is lowered into the sea. The water flowing into the sphere is fresh, since the salt is excluded by the membrane that covers the entire sphere and is its guard.

One final approach is under development in Hawaii, where different layers of seawater display a large temperature differential. Here an Ocean Thermal Energy Conversion plant is being built which will use steam produced by the flash method to produce energy, then condense the steam into freshwater. Three such plants could produce a hundred megawatts of power, as well as supply 30% of Hawaii's water needs.

For emergency use, i.e., in lifeboats, various systems are available in addition to solar or fuel-heated distillation devices. One device made of flexible plastic is worn around the waist of the user to employ body heat for evaporation.

water wheel, device for utilizing the power of flowing or falling water. The Norse wheel is the oldest type known. Despite its name it probably originated in the Middle East, where the swift stream required by this type of wheel is common. The Norse wheel has a vertical shaft directly connected at the top to a millstone; the lower end of the shaft, with vanes or paddles attached, dips into the flowing stream. In the 1st cent. B.C. a horizontal shaft came into use; the wheel attached to it had radial vanes around its edge. Among the early forms of this wheel are the overshot wheel, used where water falls from a height, striking the vanes from above; the breast wheel, employed where the height of the water is less than the height of the wheel so that the water strikes the wheel about midway; and the undershot wheel, usable where the water flows more or less on a level but with a swift current and strikes the vanes on the under part of the wheel. One of the first uses of the steam engine was to drive a pump that raised water into a millpond whose spillway drove a water wheel. Today the water wheel has been largely replaced by the turbine. See hydraulic machine.

water turkey: see darter.

water table, the top zone of soil and rock in which all voids are saturated with water. The level of the water table varies with topography and climate.

water supply, process or activity by which water is provided for some use, e.g., to a home, factory, or business. The term may also refer to the supply of water provided in this way.

In the United States, the average residential daily water supply demand is 100 gal (380 liters) per person, although it can go as high as 500 gal (1900 liters) per person. The stringency of the requirements that a supply of water must meet depends on the use to be made of it. For example, water used to wash semiconductor material from which transistors are made must be extraordinarily pure. The more usual requirements, however, are that water be free enough of harmful bacteria, chemicals, and other contamination to be drinkable; free of substances that make its taste or appearance unpleasant; and if the water is to be used for washing, free of salts of calcium and magnesium that will interfere with the action of soap.

The basic source of water is rainfall, which collects in rivers and lakes, under the ground, and in artificial reservoirs. Water from under the ground is called groundwater and is tapped by means of wells. Most often water must be raised from a well by pumping. In some cases a well will draw water from a permeable rock layer called an aquifer in which the water is under pressure; such a well needs little or no pumping (see artesian well). Water that collects in rivers, lakes, or reservoirs is called surface water. Most large water supply systems draw surface water through special intake pipes or tunnels and transport it to the area of use through canals, tunnels, or pipelines, which are known as mains or aqueducts. These feed a system of smaller conduits or pipes that take the water to its place of use. The California Water Plan, initiated in 1957, eventually entailed twenty dams, seven power plants, and more than 700 mi (1100 km) of canals, tunnels, and pipelines to meet the needs of southern California residents—a total of more than five billion cubic meters of water per year.

A complete water supply system is often known as a waterworks. Sometimes the term is specifically applied to pumping stations, treatment stations, or storage facilities. Storage facilities are provided to reserve extra water for use when demand is high and, when necessary, to help maintain water pressure. Treatment stations are places in which water may be filtered to remove suspended impurities, aerated to remove dissolved gases, or disinfected with chlorine, ozone, ultraviolet light, or some other agent that kills harmful bacteria and microorganisms. Sometimes hard water is softened through ion exchange, by which dissolved calcium and magnesium salts are replaced by sodium salts, which do not interfere with soap. Salts of iodine and fluorine, which are considered helpful in preventing goiter and tooth decay, are sometimes added to water in which they are lacking.

Not all water supply systems are used to deliver drinking water. Systems used for purposes such as irrigation and fire fighting operate in much the same way as systems for drinking water, but the water need not meet such high standards of purity. In most municipal systems hydrants are connected to the drinking water system except during periods of extreme water shortage. Because many cities draw water from the same body into which they discharge sewage, proper sewage treatment has become increasingly essential to the preservation of supplies of useful water (see sewerage; water pollution).

water strider: see water bug.

water skiing, sport of riding on skis along the water's surface while being towed by a motorboat. It probably originated on the French Riviera in the early 1920s, and was known in the United States by 1927. The American Water Ski Association, founded in 1939, held the first national championships that year. In recreational water skiing, the type of ski, length of tow rope, and speed of the boat vary with individual taste. In championship competitions, skis of hickory, ash, fiberglass, or aluminum are used. They must be at least 393/8 in. (1 m) long and not more than about 93/4 in. (25 cm) wide. There are usually three events in water ski tournaments: slalom, jumping, and trick riding. In the slalom the skiers must ski outside each of a staggered series of buoys. In jumping, contestants are towed over an inclined wooden ramp about 21 ft (6 m) long and 6 ft (1.8 m) high; jumps of 150 ft (46 m) from the point of takeoff have been recorded. In trick riding, contestants choose their own routines, many of them remarkably intricate. The World Water Ski Union (founded 1949) sponsors biennial international tournaments. Wakeboarding, which has similarities to snowboarding (see skiing), is gaining in popularity.

water skater: see water bug.

water scavenger: see water beetle.

water rights, in law, the qualified privilege of a landowner to use the water adjacent to or flowing through his property. The privilege, also known as riparian rights, may be modified or even denied because of the competing needs of other private-property holders or of the community at large. There is no private ownership of such water in most cases, and hence it cannot ordinarily be impounded and sold. The owner, however, may use the water for his ordinary private purposes, such as stock watering or irrigation, and then return the unused residue. Most uses of water affect its purity to some degree, and recent environmental legislation has greatly restricted the amount of permissible water-use pollution. Water projects such as dams that threaten the survival of rare species can be blocked under the Endangered Species Act. In certain parts of the United States—especially in the arid and semiarid regions of the Southwest—the prior appropriation rule applies, and the first user of water, whether or not he owns land abutting the water, has the unrestrained right to it without regard to his neighbor's needs. Throughout the United States, the rights of private owners in water can be set aside to construct public works, such as dams and irrigation projects. The ownership of a stream bed may depend upon whether the stream is or is not a navigable water. If it is navigable, some states claim title to the bed, whereas in other states the rule is the same as in the case of nonnavigable streams, namely that an abutting owner's property extends to the middle of the bed and that those with property along both banks of a stretch own the enclosed portion of the bed. If the stream is navigable, the owner must permit public use for passage and transportation; if it is nonnavigable, the owner may exclude all but other riparian owners from using the stream. If the stream shifts course, ownership of the former bed is not affected. Underground and percolating waters have no easily determined course, and the usual American practice is not to restrict a landowner who taps and exploits these waters; however, in some states the rights of those who may be adversely affected must be considered.

water power, mechanical energy derived from falling or flowing water, e.g., rivers, streams, and the overflow of dams. The wooden water wheel, long utilized for driving machinery in flour mills and factories, was largely supplanted by the steam engine in the early 19th cent. In modern practice, water flowing from a higher level to a lower level (as from a dam or waterfall) is used to activate a turbine that drives an electric generator, a process called hydroelectric power generation. The amount of power furnished is proportional to the rate of flow of the water and the vertical distance through which it falls. In a pumped-storage plant, water is pumped upward to a high-level reservoir during periods of low electricity demand by using the excess electricity available. During periods of high demand the facility produces electricity by using the water that flows down from the reservoir. The availability of water power along a fall line, which is a boundary between an upland region and a coastal plain, influenced the location of many cities in the E United States. Similarly, Canada, Norway, Sweden, and Switzerland, which have mountainous regions subject to heavy rainfall near industrialized areas, have highly developed hydroelectric programs. Asia, South America, and Africa have the greatest potential for further water power development, the nations of Europe and North America having developed their resources to the greatest extent. In the late 1990s the countries that produced the most hydroelectric power—about 51 percent of the world total—were the United States, Canada, Brazil, China, and Russia. For information on some important hydroelectric power projects, see Churchill Falls; Niagara Falls; Columbia basin project; Colorado River storage project; Saint Lawrence Seaway; Tennessee Valley Authority; Dniprohes.

water polo, swimming game encompassing features of soccer, football, basketball, and hockey. The object of the game is to maneuver, by head, feet, or hand, a leather-covered ball 27 to 28 in. (about 70 cm) in circumference into net-enclosed goals at opposite ends of a pool 19 to 30 yd (17.37 to 27.43 m) long and at most 20 yd (18.29 m) wide. The two competing teams consist of seven players each, one of whom is the goalie. Only one hand may be used to advance the ball, which must be carried on the surface. Rough defensive techniques permitted include ducking, i.e., holding a player underwater.

Water polo was devised in England in the 1870s and became popular in the United States in the early 20th cent. It is played mainly by club teams, although it is also popular in collegiate competition. Water polo has been an men's Olympic event since 1900; women's water polo was first played at the games in 2000. A far rougher version of the game, played with a soft rubber ball in a larger pool and known as American or softball water polo, was formerly popular in the United States. However, its extreme violence brought it into disfavor, and today only the international or hardball game is played throughout the world.

water pollution, contamination of water resources by harmful wastes; see also sewerage, water supply, pollution, and environmentalism.

Industrial Pollution

In the United States industry is the greatest source of pollution, accounting for more than half the volume of all water pollution and for the most deadly pollutants. Some 370,000 manufacturing facilities use huge quantities of freshwater to carry away wastes of many kinds. The waste-bearing water, or effluent, is discharged into streams, lakes, or oceans, which in turn disperse the polluting substances. In its National Water Quality Inventory, reported to Congress in 1996, the U.S. Environmental Protection Agency concluded that approximately 40% of the nation's surveyed lakes, rivers, and estuaries were too polluted for such basic uses as drinking supply, fishing, and swimming. The pollutants include grit, asbestos, phosphates and nitrates, mercury, lead, caustic soda and other sodium compounds, sulfur and sulfuric acid, oils, and petrochemicals.

In addition, numerous manufacturing plants pour off undiluted corrosives, poisons, and other noxious byproducts. The construction industry discharges slurries of gypsum, cement, abrasives, metals, and poisonous solvents. Another pervasive group of contaminants entering food chains is the polychlorinated biphenyl (PCB) compounds, components of lubricants, plastic wrappers, and adhesives. In yet another instance of pollution, hot water discharged by factories and power plants causes so-called thermal pollution by increasing water temperatures. Such increases change the level of oxygen dissolved in a body of water, thereby disrupting the water's ecological balance, killing off some plant and animal species while encouraging the overgrowth of others.

Other Sources of Water Pollution

Towns and municipalities are also major sources of water pollution. In many public water systems, pollution exceeds safe levels. One reason for this is that much groundwater has been contaminated by wastes pumped underground for disposal or by seepage from surface water. When contamination reaches underground water tables, it is difficult to correct and spreads over wide areas. In addition, many U.S. communities discharge untreated or only partially treated sewage into the waterways, threatening the health of their own and neighboring populations.

Along with domestic wastes, sewage carries industrial contaminants and a growing tonnage of paper and plastic refuse (see solid waste). Although thorough sewage treatment would destroy most disease-causing bacteria, the problem of the spread of viruses and viral illness remains. Additionally, most sewage treatment does not remove phosphorus compounds, contributed principally by detergents, which cause eutrophication of lakes and ponds. Excreted drugs and household chemicals also are not removed by present municipal treatment facilites, and can be recycled into the drinking water supply.

Rain drainage is another major polluting agent because it carries such substances as highway debris (including oil and chemicals from automobile exhausts), sediments from highway and building construction, and acids and radioactive wastes from mining operations into freshwater systems as well as into the ocean. Also transported by rain runoff and by irrigation return-flow are animal wastes from farms and feedlots, a widespread source of pollutants impairing rivers and streams, groundwater, and even some coastal waters. Antibiotics, hormones, and other chemicals used to raise livestock are components of such animal wastes. Pesticide and fertilizer residues from farms also contribute to water pollution via rain drainage.

Ocean Pollution

Large and small craft significantly pollute both inland and coastal waters by dumping their untreated sewage. Oil spilled accidentally or flushed from tankers and offshore rigs (900,000 metric tons annually) sullies beaches and smothers bird, fish, and plant life. In 1989 in one of the world's worst single instances of water pollution, the Exxon Valdez spilled 11 million gallons of oil in Prince William Sound, Alaska, causing great environmental destruction. In 1997, the 22 oil spills reported worldwide involved a total of 15 million gallons (57 million liters) of oil. In addition to its direct damage to wildlife, oil takes up fat-soluble poisons like DDT, allowing them to be concentrated in organisms that ingest the oil-contaminated water; thus such poisons enter the food chains leading to sea mammals and people (see ecology).

Both DDT, which has been banned in the United States since 1972, and PCBs are manufactured in many parts of the world and are now widespread in the Atlantic and Pacific oceans. In addition, tarry oil residues are encountered throughout the Atlantic, as are styrofoam and other plastic rubbish. Plastic bits litter sections of the Pacific as far north as Amchitka Island near Alaska. Garbage, solid industrial wastes, and sludge formed in sewage treatment, all commonly dumped into oceans, are other marine pollutants found worldwide, especially along coastal areas.

Dangers of Water Pollution

Virtually all water pollutants are hazardous to humans as well as lesser species; sodium is implicated in cardiovascular disease, nitrates in blood disorders. Mercury and lead can cause nervous disorders. Some contaminants are carcinogens. DDT is toxic to humans and can alter chromosomes. PCBs cause liver and nerve damage, skin eruptions, vomiting, fever, diarrhea, and fetal abnormalities. Along many shores, shellfish can no longer be taken because of contamination by DDT, sewage, or industrial wastes.

Dysentery, salmonellosis, cryptosporidium, and hepatitis are among the maladies transmitted by sewage in drinking and bathing water. In the United States, beaches along both coasts, riverbanks, and lake shores have been ruined for bathers by industrial wastes, municipal sewage, and medical waste. Water pollution is an even greater problem in the Third World, where millions of people obtain water for drinking and sanitation from unprotected streams and ponds that are contaminated with human waste. This type of contamination has been estimated to cause more than 3 million deaths annually from diarrhea in Third World countries, most of them children.

Legislation and Control

The United States has enacted extensive federal legislation to fight water pollution. Laws include the Federal Water Pollution Control Act (1972), the Marine Protection, Research, and Sanctuaries Act (1972), the Safe Drinking Water Act (1974), and the Federal Insecticide, Fungicide, and Rodenticide Act, as amended in 1988. In the United States in 1996, nearly $10 billion was spent on water and wastewater treatment alone. International cooperation is being promoted by the Inter-Governmental Maritime Consultive Organization (IMCO), a UN agency. Limitation of ocean dumping was proposed at the 80-nation London Conference of 1972, and in the same year 12 European nations meeting in Oslo adopted rules to regulate dumping in the North Atlantic. An international ban on ocean dumping in 1988 set further restrictions.

water on the brain: see hydrocephalus.

water oats: see wild rice.

water mold, common name for a group of multinucleated organisms that superficially resemble fungi but are now recognized as having an independent evolutionary lineage and are placed in the kingdom Protista. There are two important phyla (divisions) of water molds, the chytrids (Chytridiomycota) and the oomycetes (Oomycota).

The chytrids live in salt- and freshwater and in moist soil. They live as saprobes, obtaining their metabolic energy from decaying plant and animal material, or as parasites, attacking plants, fungi, and algae. They typically take the form of small coenocytic (multinucleated) masses, called sporangia, from which many hairlike rhizoids protrude. Like roots, the rhizoids absorb nutrients. Reproduction can be by simple division of a sporangium into individual motile, flagellated spores or by more complicated sexual processes that yield flagellated gametes. There are approximately 900 species of chytrids.

The oomycetes resemble fungi, taking the form of coenocytic filaments (hyphae). They differ from fungi, however, in that cellulose is present in their cell walls. The hyphae of oomycetes have specialized regions that can produce distinct male and female gametes. Oomycetes can also reproduce asexually. Many oomycetes are aquatic. Many of the others live in water in certain stages of the life cycle. Most of the 800 species of oomycetes are saprobes, but those that are parasitic are of great significance: they cause downy mildew, a disease often affecting grapes; late blight of potatoes, an outbreak of which led to the Great Potato Famine in Ireland (1845-49); sudden oak death syndrome (also known as ramorum leaf blight or ramorum dieback; redwoods, Douglas firs, and other plants also are harmed by the same water mold); and blue mold of tobacco. Other parasitic water molds cause diseases of fish and fish eggs.

water moccasin or cottonmouth, highly venomous snake, Ancistrodon piscivorus, of the swamps and bayous of the S United States. Like the closely related copperhead, it is a pit viper and has a heat-sensitive organ for detecting warm-blooded prey. The young are born live. The young snake is a pale reddish brown with transverse dark brown bands edged with white; as it ages the colors dull to a blotched olive or brown and then to an unmarked olive or blackish in old specimens. The maximum length is 6 ft (2 m), the average from 3 to 4 ft (90-120 cm). A good climber, the water moccasin often relaxes on branches overhanging the water. If startled it erects its head and shows the white interior of its mouth—hence the name cottonmouth. It eats both warm-blooded and cold-blooded animals. It is aggressive in the wild state but may become quite tame in captivity. It is classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Crotalidae.

water lily, common name for some members of the Nymphaeaceae, a family of freshwater perennial herbs found in most parts of the world and often characterized by large shield-shaped leaves and showy, fragrant blossoms of various colors. Among the plants of the family are the water lilies, lotuses, and pond lilies (called also cow lilies and spatterdocks) of the genera Nymphaea, Nelumbo, and Nuphar, respectively; however, the common names often overlap; e.g., "water lily" is used for most species of the family and even for other unrelated aquatic plants with similar flowers. Most species of Nymphaea in cultivation are tropical, but some of the hardy kinds are native to the United States and to the corresponding temperate areas of the Southern Hemisphere. Both day- and night-blooming species open at fairly definite hours. Included in the genus is the blue or white Egyptian lotus (Nymphaea caerulea or N. lotus, respectively), sacred from remote times and the national emblem of Egypt. The lotus flower is traditional in Egyptian art and architecture, as in the lotus capital. The genus Nelumbo contains two species: the American, or yellow, lotus, also called water chinquapin, is found in E North America; the Indian lotus, also called the sacred lotus or Egyptian bean, is sacred to Hinduism and to several other Asian religions, e.g., Buddhism. Its large pink blossom is used symbolically in religion and art. The seeds of the sacred lotus can remain viable after long periods of dormancy (see seed). Most species of Nuphar are native to North America. Many members of the water lily family have seeds or tubers that have been used for food; however, the fruit of the lotus-eaters of classical literature has been most often identified as that of the jujube of the buckthorn family or the nettle-tree of the elm family. Lotus is also the botanical name for a genus of the pea family. Water lilies are classified in the division Magnoliophyta, class Magnoliopsida, order Nymphaeales, family Nymphaeaceae.

water hyacinth: see pickerelweed.

water hog: see capybara.

water hemlock: see poison hemlock.

water glass or soluble glass, colorless, transparent, glasslike substance available commercially as a powder or as a transparent, viscous solution in water. Chemically it is sodium silicate, potassium silicate, or a mixture of these. It is prepared by fusing sodium or potassium carbonate with sand or by heating sodium or potassium hydroxide with sand under pressure. Water glass is very soluble in water, but the glassy solid dissolves slowly, even in boiling water. Water glass has adhesive properties and is fire resistant. It is used as a detergent; as a cement for glass, pottery, and stoneware; for fireproofing paper, wood, cement, and other substances; for fixing pigments in paintings and cloth printing; and for preserving eggs (it fills the pores in the eggshell, preventing entrance of air).

water gas, colorless poisonous gas that burns with an intensely hot, bluish (nearly colorless) flame. The gas is a mixture of carbon monoxide and hydrogen with very small amounts of other gases, e.g., carbon dioxide, and is almost entirely combustible as a result. Water gas is so named because of the use of water (steam) in its preparation. This process involves treating white-hot hard coal or coke with a blast of steam; carbon monoxide and hydrogen are formed. The gas is manufactured in vast quantities for commercial use. It is of much importance in the preparation of hydrogen and as a fuel in the making of steel and in other industrial processes, e.g., the Fischer-Tropsch process.

water gardening: see hydroponics.

water flea: see crustacean.

water fern, common name for various species of aquatic ferns.

water clover, common name for various species of aquatic ferns.

water clock: see clepsydra.

water chestnut: see sedge.

water bug, name for a large number of water-living bugs, comprising several families of the order Hemiptera (true bugs). All have jointed, sharp, sucking beaks, breathe air, and undergo gradual metamorphosis (see insect). They are found on or below the surface of almost all quiet streams and ponds; a few forms live in rapidly flowing water. The water boatmen (family Corixidae) are abundant in lakes and ponds throughout most of the world. They are ovalbodied, with flattened, oarlike hind legs used for propulsion; the short front legs are used for gathering food and for anchoring the bug to aquatic vegetation. Water boatmen store air in a concavity beneath the wings and are thus able to remain submerged for long periods. They feed on algae and other small aquatic organisms and, unlike the predaceous water bugs, do not bite humans. The other water bugs are carnivorous and prey, according to their size, on young fishes, snails, crustaceans, and the adults and larvae of other insects. The backswimmers (family Notonectidae) resemble the water boatmen in appearance, but swim upside down. The water striders, or water skaters (family Gerridae), have two pairs of long, slender legs that enable them to move over the surface film of quiet waters, where they often congregate in large numbers. They also have a pair of short, grasping forelegs, used for catching insects on the surface. All live in freshwater except those of the genus Halobates, which are found in oceans. The giant water bugs (family Belostomatidae), with wide, flat bodies and grasping forelegs, are the world's largest bugs and among the largest of the insects. Members of some North American species grow 2 in. (5 cm) long, while one South American form attains a length of more than 4 in. (10 cm). They fly well and are attracted to lights at night, hence their other common name, electric-light bug. In some species the female glues her eggs to the back of the male, where they remain until they hatch. The water scorpions (family Nepidae) are named for the breathing tube that protrudes from the rear of the abdomen. There are several other water bug families. The term water bug is also sometimes applied to the various water beetles. True water bugs are classified in the phylum Arthropoda, class Insecta, order Hemiptera.

water buffalo: see buffalo.

water boatman: see water bug.

water beetle, name for aquatic beetles of several families. They should not be confused with water bugs, which are true bugs (order Hemiptera). The predaceous diving beetles (family Dytiscidae) are a large group, widespread in quiet streams and ponds. They are black, brown, or greenish, .08 to 1.57 in. (3-40 mm) long, with smooth oval bodies and hairy, oarlike hind legs. There is a cavity beneath the wings for holding an air supply, so the diving beetle can remain submerged for long periods; however, the insect often hangs head down from the surface, exposing the openings (spiracles) of the last two of its breathing tubes, located at the tip of the abdomen. Both the adults and the long, conspicuously segmented larvae prey on a variety of animals, including snails and fish much larger than themselves. Adults of most species are strong fliers, and many are attracted to lights at night. The water scavenger beetles (family Hydrophilidae), of similar appearance, are abundant in marshy places in warm parts of the world and feed on water plants and decaying matter. Their larvae are predaceous. The whirligig beetles (family Gyrinidae) are oval, shiny, blue-black to dark brown beetles, about 3/4 in. (19 mm) long. They are found in groups in sheltered places, spinning around on the surface of the water. They feed on small insects on the surface and seldom dive. There are several other groups of water beetles, all classified in the phylum Arthropoda, class Insecta, order Coleoptera.

water, odorless, tasteless, transparent liquid that is colorless in small amounts but exhibits a bluish tinge in large quantities. It is the most familiar and abundant liquid on earth. In solid form (ice) and liquid form it covers about 70% of the earth's surface. It is present in varying amounts in the atmosphere. Most of the living tissue of a human being is made up of water; it constitutes about 92% of blood plasma, about 80% of muscle tissue, about 60% of red blood cells, and over half of most other tissues. It is also an important component of the tissues of most other living things.

Chemical and Physical Properties

Chemically, water is a compound of hydrogen and oxygen, having the formula H₂O. It is chemically active, reacting with certain metals and metal oxides to form bases, and with certain oxides of nonmetals to form acids. It reacts with certain organic compounds to form a variety of products, e.g., alcohols from alkenes. Because water is a polar compound, it is a good solvent. Although completely pure water is a poor conductor of electricity, it is a much better conductor than most other pure liquids because of its self-ionization, i.e., the ability of two water molecules to react to form a hydroxide ion, OH^-, and a hydronium ion, H₃O⁺. Its polarity and ionization are both due to the high dielectric constant of water.

Water has interesting thermal properties. When heated from 0°C;, its melting point, to 4°C;, it contracts and becomes more dense; most other substances expand and become less dense when heated. Conversely, when water is cooled in this temperature range, it expands. It expands greatly as it freezes; as a consequence, ice is less dense than water and floats on it. Because of hydrogen bonding between water molecules, the latent heats of fusion and of evaporation and the heat capacity of water are all unusually high. For these reasons, water serves both as a heat-transfer medium (e.g., ice for cooling and steam for heating) and as a temperature regulator (the water in lakes and oceans helps regulate the climate).

Structure of the Water Molecule

Many of the physical and chemical properties of water are due to its structure. The atoms in the water molecule are arranged with the two H-O bonds at an angle of about 105° rather than on directly opposite sides of the oxygen atom. The asymmetrical shape of the molecule arises from a tendency of the four electron pairs in the valence shell of oxygen to arrange themselves symmetrically at the vertices of a tetrahedron around the oxygen nucleus. The two pairs associated with covalent bonds (see chemical bond) holding the hydrogen atoms are drawn together slightly, resulting in the angle of 105° between these bonds. This arrangement results in a polar molecule, since there is a net negative charge toward the oxygen end (the apex) of the V-shaped molecule and a net positive charge at the hydrogen end. The electric dipole gives rise to attractions between neighboring opposite ends of water molecules, with each oxygen being able to attract two nearby hydrogen atoms of two other water molecules. Such hydrogen bonding, as it is called, has also been observed in other hydrogen compounds. Although considerably weaker than the covalent bonds holding the water molecule together, hydrogen bonding is strong enough to keep water liquid at ordinary temperatures; its low molecular weight would normally tend to make it a gas at such temperatures.

Various other properties of water, such as its high specific heat, are due to these hydrogen bonds. As the temperature of water is lowered, clusters of molecules form through hydrogen bonding, with each molecule being linked to others by up to four hydrogen bonds, each oxygen atom tending to surround itself with four hydrogen atoms in a tetrahedral arrangement. Hexagonal rings of oxygen atoms are formed in this way, with alternate atoms in either a higher or lower plane than their neighbors to create a kinked three-dimensional structure.

Liquid Water

According to present theories, water in the liquid form contains three different molecule populations. At the highest temperatures single molecules are the rule, with little hydrogen bonding because of the high thermal energy of the molecules. In the middle range of temperatures there is more hydrogen bonding, and clusters of molecules are formed. At lower temperatures aggregates of clusters also form, these aggregates being the most common arrangement below about 15°C;. On the basis of these three population types and the transitions between them, many aspects of the anomalous behavior of water can be explained. For example, the tendency of water to freeze faster if it has been cooled rapidly from a relatively warm temperature than if it has been cooled at the same rate from a lower temperature is explained in terms of the greater number of irregularly shaped cluster aggregates in the cooler water that must find a suitable means of fitting together with a neighboring aggregate.

The discovery in the late 1960s of "superwater," or "polywater," helped to shed light on some aspects of the structure of water. This substance was thought by some to be a giant polymer of water molecules, 40 times denser and 15 times more viscous than ordinary water. Studies showed, however, that these new and unexplained properties were connected with the presence of contaminants in the water. Even so, the interaction of the water molecules with these other substances may be helpful in understanding the way in which water molecules interact with each other.

Ice

In ice, each molecule forms the maximum number of hydrogen bonds, resulting in crystals composed of open, hexagonal columns. Because these crystals have a number of open regions and pockets, normal ice is less dense than water. However, other forms of ice also exist at conditions of higher pressure, each of these different forms (designated ice II, ice III, etc.) having greater density and other distinct physical properties that differ from those of normal ice, or ice I. As many as eight different forms of ice have been distinguished in this manner. The higher pressures creating such forms cause rearrangements of the hexagonal columns in ice, although the basic kinked hexagonal ring is common to all forms.

When ice melts, it is thought that the fragments of these structures fill many of the gaps that existed in the crystal lattice, making water denser than ice. This tendency is the dominant one between 0°C; and 4°C;, at which temperature water reaches its maximum density. Above this temperature, expansion due to the increased thermal energy of the molecules is the dominant factor, with a consequent decrease in density.

soda water, a solution of carbon dioxide in water.

seltzer water, mineral water containing free carbon dioxide, obtained originally from springs at Niederselters, Germany. Reputed to have curative value in treating several diseases, it became very popular in the 19th cent. In the 20th cent. it was produced synthetically and, as soda water, is widely used as a beverage, often lightly flavored, as well as a component of other beverages.

salinity of ocean water: see ocean.

navigable water, in the broadest sense, a stream or body of water that can be used for commercial transportation. When, as in the early common law, the term is restricted to waters affected by tides, it denotes only the open sea and tidal rivers. In most U.S. jurisdictions the definition tends to include any body of water that may be put to public use, e.g., streams that can be used only for logging and for small pleasure boats would still be considered navigable. In the United States each state determines what private use may be made of wholly intrastate navigable waters (see water rights), but the federal government alone has authority over navigable interstate and international waters. In general, if the water is of restricted navigability, the right of public use is strictly confined to transporting goods; use of the water for irrigation, power, and the like is limited to the abutting landowners.

mineral water, spring water containing various mineral salts, especially the carbonates, chlorides, phosphates, silicates, sulfides, and sulfates of calcium, iron, lithium, magnesium, potassium, sodium, and other metals. Various gases may also be present, e.g., carbon dioxide, hydrogen sulfide, nitrogen, and inert gases. Ordinary well or spring water, in contrast, contains far fewer substances, mostly dissolved sulfates and carbonates, and calcium and other alkali and alkaline earth metals. Many mineral waters also contain trace elements that are thought to have therapeutic value. Spa therapy, widely practiced in Europe, advocates bathing in and drinking mineral waters as a cure for a variety of diseases. Many authorities believe that the success of such therapy really results from the beneficial effects of rest and relaxation. Famous European resorts include Bath, Spa, Aix-les-Bains, Aachen, Baden-Baden, and Karlovy Vary (Carlsbad). Prominent among resorts in the United States are Poland, Maine; Saratoga Springs, N.Y.; Berkeley Springs and White Sulphur Springs, W.Va.; Hot Springs, Ark.; French Lick, Ind.; Waukesha, Wis.; and Las Vegas Hot Springs, N.Mex. Many mineral waters are now prepared synthetically, the various mineral ingredients being added to ordinary water in proportions determined by careful chemical analysis of the original ingredients. See spring.

holy water, in Christian churches, water blessed to symbolize spiritual cleansing. In Roman Catholic churches there is a bowl (stoup or font) of holy water near the doors, so that the faithful may bless themselves with it on entering. Holy water is a sacramental and is used in formal blessings, including the asperges.

heavy water: see deuterium.

desalination of water: see water, desalination of.

Water Bearer, The, English name for Aquarius, a constellation.

Wast Water, lake, 3 mi (4.8 km) long and 1/2 mi (.8 km) wide, in the Lake District, NW England, SW of Keswick. It is the deepest lake in England (maximum depth 258 ft/79 m). The ravine of Wastdale Head is nearby.

Javelle water or Javel water, Fr. eau de Javelle, aqueous solution of sodium or potassium hypochlorite. It was originally made near the French town of Javelle (now part of Paris) and was the first chemical bleach, a use first demonstrated by C. L. Berthollet in 1785. It was produced by passing chlorine gas through a water solution of potash (potassium carbonate). After the invention of bleaching powder Javelle water was sometimes produced by reacting the bleaching powder with potash or soda ash (sodium carbonate). Now usually sodium hypochlorite solution, it is used in bleaching and as a disinfectant.

Irish water spaniel, breed of large sporting dog developed in Ireland in the 19th cent. The tallest of the spaniels, it stands about 23 in. (58.4 cm) high at the shoulder and weighs between 45 and 65 lb (20.4-29.5 kg). Its dense, liver-colored coat is tightly curled and water-repellent. The tapering, smooth-coated tail and the topknot of long, loose curls that hang from a peak between the eyes are characteristic of the breed. A natural water dog with great endurance, the Irish water spaniel is often used to retrieve ducks. However, its coat, which may snag in heavy underbrush, makes it less suitable for hunting upland game. See dog.

Derwent Water, lake, 3 mi (4.8 km) long and 1 mi (1.6 km) wide, Cumbria, NW England, formed by a widening of the River Derwent. It is surrounded by wooded hills, with the Lodore (c.90 ft/27 m) and Barrow (c.108 ft/33 m) waterfalls at its upper end. Its islands include Lord's Isle, site of the former mansion of the earls of Derwent; Isle of St. Herbert (a 7th-century hermit); and Floating Island, a mass of vegetation, sometimes inundated. The River Derwent flows NW through the Lake District to Solway Firth.

Delaware Water Gap, scenic gorge, 2 mi (3.2 km) long, cut by the Delaware River through Kittatinny Mt., on the N.J.-Pa. line; located in a mountain resort area around Stroudsburg, Pa. The gap, parts of wooded Kittatinny Mt., several islands, and c.40 mi (64 km) of river bank (the Delaware National Scenic River) are included in Delaware Water Gap National Recreation Area (see National Parks and Monuments, table).

American water spaniel, breed of medium-sized sporting dog developed in the American Midwest. It stands about 17 in. (43.2 cm) high at the shoulder and weighs between 30 and 40 lb (13.6-18.1 kg). Its dense and closely curled coat ranges in color from solid liver to dark chocolate. A sturdy, muscular dog, the American water spaniel is a versatile scent hunter, flushing, or springing, game birds rather than pointing them. It is a strong swimmer and retrieves both on land and water. It is also used for hunting rabbits and other small animals. See dog.

Facilities for the collection, treatment, storage, and distribution of water. Ancient systems included wells, storage reservoirs, canals and aqueducts, and water-distribution systems. Highly advanced systems appeared circa 2500 BC and reached their peak in the Roman aqueduct system. In the Middle Ages, water supplies were largely neglected and epidemics caused by waterborne organisms were common. In the 17th–18th century, distribution systems utilizing cast-iron pipes, aqueducts, and pumps began to be installed. The link between polluted water and disease came to be understood in the 19th century, and treatment methods such as slow sand filtration and disinfection with chlorine were introduced. Modern reservoirs are formed usually by constructing dams near the collection point of mountain-water runoff or across rivers. After the water reaches collection points, it is treated to improve its quality; it is then pumped either directly into a city or town's distribution system or to an elevated storage location, such as a water tank. Seealso plumbing.

Learn more about water-supply system with a free trial on Britannica.com.

Principle that subatomic particles possess some wavelike characteristics, and that electromagnetic waves, such as light, possess some particlelike characteristics. In 1905, by demonstrating the photoelectric effect, Albert Einstein showed that light, which until then had been thought of as a form of electromagnetic wave (see electromagnetic radiation), must also be thought of as localized in packets of discrete energy (see photon). In 1924 Louis-Victor Broglie proposed that electrons have wave properties such as wavelength and frequency; their wavelike nature was experimentally established in 1927 by the demonstration of their diffraction. The theory of quantum electrodynamics combines the wave theory and the particle theory of electromagnetic radiation.

Learn more about wave-particle duality with a free trial on Britannica.com.

or abrasion platform

Gently sloping rock ledge that extends from the high-tide level at a steep cliff base to below the low-tide level. It develops as a result of wave abrasion; beaches protect the shore from abrasion and therefore prevent the formation of platforms. A platform is broadened as waves erode a notch at the base of the sea cliff, causing overhanging rock to fall. As the sea cliffs are attacked, weak rocks are quickly eroded, leaving the more resistant rocks as protrusions.

Learn more about wave-cut platform with a free trial on Britannica.com.

Variable quantity that mathematically describes the wave characteristics of a particle. It is related to the likelihood of the particle being at a given point in space at a given time, and may be thought of as an expression for the amplitude of the particle wave, though this is strictly not physically meaningful. The square of the wave function is the significant quantity, as it gives the probability for finding the particle at a given point in space and time. Seealso wave-particle duality.

Learn more about wave function with a free trial on Britannica.com.

Imaginary surface that represents corresponding points of waves vibrating in unison. As identical waves from the same source travel through a homogeneous medium, corresponding crests and troughs are in phase at any instant; that is, they have completed the same fraction of their periodic motion. Any surface drawn through all points of the same phase constitutes a wave front.

Learn more about wave front with a free trial on Britannica.com.

In oceanography, a ridge or swell on the surface of a body of water, normally having a forward motion distinct from the motions of the particles that compose it. Ocean waves are fairly regular, with an identifiable wavelength between adjacent crests and with a definite frequency of oscillation. Waves result when a generating force (usually the wind) displaces surface water and a restoring force returns it to its undisturbed position. Surface tension alone is the restoring force for small waves. For large waves, gravity is more important.

Learn more about wave with a free trial on Britannica.com.

Vibration generated by an earthquake, explosion, or similar phenomenon and propagated within the Earth or along its surface. Earthquakes generate two principal types of waves: body waves, which travel within the Earth, and surface waves, which travel along the surface. Seismograms (recorded traces of the amplitude and frequency of seismic waves) yield information about the Earth and its subsurface structure; artificially generated seismic waves are used in oil and gas prospecting.

Learn more about seismic wave with a free trial on Britannica.com.

or snakebird

Any fish-eating bird of the family Anhingidae (order Pelecaniformes), sometimes considered a single species (Anhinga anhinga) with geographical variants. Anhingas are about 35 in. (90 cm) long, slender, and long-necked. They are mostly black, with silvery wing markings. Males, glossed with green, develop pale head plumes and a dark “mane” in breeding season. Anhingas live in small colonies along lakes and rivers in tropical to warm temperate regions except in Europe. They swim nearly submerged; the head and neck show above water, darting snakelike from side to side.

Learn more about anhinga with a free trial on Britannica.com.

or groundwater table

Surface of a body of underground water below which the soil or rocks are permanently saturated with water. The water table separates the groundwater zone (zone of saturation) that lies below it from the zone of aeration that lies above it. The water table fluctuates both with the seasons and from year to year because it is affected by climatic variations and by the amount of precipitation used by vegetation. It also is affected by withdrawing excessive amounts of water from wells or by recharging them artificially. Seealso aquifer.

Learn more about water table with a free trial on Britannica.com.

Any of about 200 snake species of about 38 genera, especially Natrix and Nerodia, classified within the family Colubridae, found worldwide except in South America. Most species have a stout body with dark blotches or streaks and ridged scales. Some are similar in appearance to venomous species. They kill fishes and amphibians with a nonvenomous bite. The New World species live in or near water and bear live young; European species are less water-dependent and lay eggs. In defense they inflate the head, strike, and release a foul secretion. Average length is about 3 ft (1 m); some Old World species reach 6 ft (1.8 m).

Learn more about water snake with a free trial on Britannica.com.

Sport of planing and jumping on water skis, broad skilike runners that a rider wears while being towed by a motorboat. The sport originated in the U.S. in the 1920s. International competitions have been held since 1946. Single-ski slalom competition is held on a course consisting of a specified number of buoys around which the skier must negotiate. Jumping competitions employ a ramp; skiers are judged for distance and style. Barefoot and trick skiing are also part of some competitions. A later development in waterskiing, wakeboarding began in the U.S. in the 1980s when surfers began riding their boards as they were pulled behind boats. Since the mid 1990s wakeboarding has been an event at both the Gravity Games and the X Games (see extreme sports) and has become the fastest-growing water sport worldwide.

Learn more about waterskiing with a free trial on Britannica.com.

or Archimedes' screw

Machine for raising water, said to have been invented by Archimedes for removing water from the hold of a large ship. One form consists of a circular pipe enclosing a helix and inclined at an angle of about 45°, with its lower end dipped in the water; rotation of the device lifts the water in the pipe. Other forms consist of a helix revolving in a fixed cylinder or a helical tube wound around a shaft.

Learn more about Archimedean screw with a free trial on Britannica.com.

Any of the entire range of natural waters (vapour, liquid, or solid) that occur on the Earth and that are of potential use to humans. These resources include the waters of the oceans, rivers, and lakes; groundwater and deep subsurface waters; and glaciers and permanent snowfields. Continuing increase in water use has led to growing concern over the availability and quality of water supplies.

Learn more about water resource with a free trial on Britannica.com.

Power produced by a stream of water as it turns a wheel or similar device. The waterwheel, probably invented in the 1st century BC, was widely used throughout the Middle Ages and into modern times for grinding grain, operating bellows for furnaces, and other purposes. The more compact water turbine, which passes water through a series of fixed and rotating blades, was introduced in 1827. Water turbines, used originally for direct mechanical drive for irrigation, now are used almost exclusively to generate hydroelectric power.

Learn more about waterpower with a free trial on Britannica.com.

Sport played in a swimming pool by teams of seven with a buoyant ball resembling a football (soccer ball). The ball may be carried or thrown, and a point is scored when the ball is placed in the opposing team's goal. The name derives from a mid-19th-century version of the sport in which players rode barrels and struck the ball with sticks. A rough and demanding game, it is played by both men and women. Modern water polo was introduced as an Olympic sport in 1900.

Learn more about water polo with a free trial on Britannica.com.

or water ouzel

Any of five songbird species in the genus Cinclus (family Cinclidae), noted for hunting insects by walking underwater in rushing streams. The species are widely distributed in Asia, Africa, Europe, and North and South America. Dippers are plump, stub-tailed birds, about 7 in. (18 cm) long, with a thrushlike bill and legs. They are commonly blackish brown or dull gray. They nest in a dome of moss built in a crevice, often behind a waterfall. Seealso ouzel.

Learn more about dipper with a free trial on Britannica.com.

Accumulation of cerebrospinal fluid (CSF) in the ventricles (cavities) of the brain. Hydrocephalus is caused by overproduction of CSF, congenital blockage that prevents drainage (see neural tube defect), or complications of head injuries or infections. Normally, CSF circulates through the brain and spinal cord and drains into the circulation. In infants and young children, hydrocephalus causes the brain and skull to enlarge because the fontanels have not yet closed. Without surgery to divert the excess fluid into the blood or abdomen, accumulating fluid eventually compresses the brain, causing convulsions, intellectual disability, and death.

Learn more about hydrocephalus with a free trial on Britannica.com.

Alcoholic beverage obtained by distillation from wine or other fermented fruit juice or from various cereal grains that have first been brewed. The essential ingredient is usually a natural sugar or a starchy substance that may be easily converted into a sugar. The distillation process is based on the different boiling points of water (212 °F [100 °C]) and alcohol (173 °F [78.5 °C]). The alcohol vapours that arise while the fermented liquid boils are trapped and recondensed to create a liquid of much greater alcoholic strength. The resultant distillate is matured, often for several years, before it is packaged and sold. Seealso aquavit; brandy; gin; liqueur; rum; vodka; and whiskey.

Learn more about distilled liquor with a free trial on Britannica.com.

Coarse annual grass (Zizania aquatica) of the family Poaceae (or Gramineae) whose grain, now often considered a delicacy, has long been an important food of American Indians. Despite its name, the plant is not related to rice. Wild rice grows naturally in shallow water in marshes and along the shores of streams and lakes in northern central North America. Cultivated varieties are now grown in Minnesota and California. The plant, about 3–10 ft (1–3 m) tall, is topped with a large, open flower cluster. The ripened grains, dark brown to purplish-black, are slender rods 0.4–0.8 in. (1–2 cm) long.

Learn more about wild rice with a free trial on Britannica.com.

Machine for tapping the energy of running or falling water (hence a prime mover) by means of a set of paddles or buckets mounted around a wheel. The force of the moving water against the paddles, or the weight of water poured into the buckets, rotates the wheel. The resulting power is transmitted to machinery via the shaft of the wheel. The waterwheel was perhaps the earliest source of mechanical energy to replace that of humans and animals, and it was first used for such tasks as raising water and grinding grain. Seealso waterpower.

Learn more about waterwheel with a free trial on Britannica.com.

Any of the freshwater plants in eight genera that make up the family Nymphaeaceae, native to temperate and tropical regions. All are perennial except those in the genus Euryale. Most have rounded, floating, waxy-coated leaves growing atop long stalks that contain many air spaces. Thick, fleshy, creeping underwater stems are buried in the mud. In some species the leaves are also submerged. Showy, solitary, cuplike flowers with numerous spirally arranged petals are borne at or above the water surface on the long stalks. The genus Nymphaea includes the water lilies proper (or water nymphs). The common North American white water lily, pond lily, or toad lily is N. odorata. The lotus of ancient Egyptian art was usually the blue lotus (N. caerulea). The largest water lilies are two species that make up the tropical South American genus Victoria; the Santa Cruz water lily (V. cruziana) has leaves 2–6 ft (60–180 cm) in diameter. Water lilies provide food for fish and wildlife but sometimes cause drainage problems because of their rapid growth. Many varieties have been developed for ornamental use in garden pools and conservatories.

Learn more about water lily with a free trial on Britannica.com.

Any of about five species of aquatic plants that make up the genus Eichhornia of the pickerelweed family (Pontederiaceae). They are native mainly to the New World tropics. Some species float in shallow water; others are rooted in muddy streambanks and lakeshores. All have slender rootstocks, feathery roots, rosettes of stalked leaves, and flowers arranged in spikes or clusters. The common water hyacinth (E. crassipes) is the most widespread. The leafstalk is spongy and inflated, and the upper lobes of its purple flowers have blue and yellow markings. It reproduces quickly, often clogging slow-flowing streams. It is used as an ornamental in outdoor pools and aquariums.

Learn more about water hyacinth with a free trial on Britannica.com.

Semiaquatic rodent (Hydrochoerus hyrdrochaeris) of Central and South America. Classified as the only species in its family, it is related to the cavy and the guinea pig. Capybaras are the largest living rodents, growing as large as 50 in. (1.25 m) long and weighing 110 lbs (50 kg) or more. They are sparsely haired and brownish, with a blunt snout, short legs, small ears, and almost no tail. Capybaras are shy and associate in groups along the banks of lakes and rivers. Herbivorous, they can become pests when they eat cultivated melons, grain, and squash. They swim and dive readily and commonly enter water to elude predators.

Learn more about capybara with a free trial on Britannica.com.

Any of about 450 species (order Anomopoda) of microscopic, mostly freshwater crustaceans distributed worldwide. Species in the genus Daphnia are ubiquitous in Europe and North America. Water fleas have a discrete head that bears antennae. The carapace (shell) encloses all or most of the body, except on the predatory giant Leptodora (up to 0.7 in. [18 mm] long), whose carapace is just a small brood sac. Most species swim by powerful strokes of the antennae, sometimes producing a hopping-and-sinking motion. All but a few predatory species use specialized thoracic limbs to filter organic matter from the water. Seealso copepod.

Learn more about water flea with a free trial on Britannica.com.

Cycle that involves the continuous circulation of water in the Earth-atmosphere system. Water is transferred from the oceans through the atmosphere to the continents and back to the oceans by means of evaporation, transpiration, precipitation, interception, infiltration, subterranean percolation, overland flow, runoff, and other complex processes. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing.

Learn more about hydrologic cycle with a free trial on Britannica.com.

or clepsydra

Ancient device for measuring time by the gradual flow of water. One form, used by North American Indians and some African peoples, consisted of a small boat or floating vessel that shipped water through a hole until it sank. In another form, water escaped through a hole in a vessel marked with graduated lines; specimens from Egypt date from the 14th century BC. The Romans invented a clepsydra consisting of a cylinder into which water dripped from a reservoir; a float provided readings against a scale on the cylinder wall. Galileo used a mercury clepsydra to time his experimental falling bodies. Seealso clock.

Learn more about water clock with a free trial on Britannica.com.

Any of several perennial water plants of the genus Trapa (family Trapaceae), native to Europe, Asia, and Africa, or their edible, nutlike fruit. The water caltrop (T. natans) has two sets of leaves—long, feathery, rootlike, submerged leaves and a loose rosette of floating leaves attached to leafstalks 2–4 in (5–10 cm) long. The small fruit usually has four spiny angles. The Chinese water chestnut (Eleocharis tuberosus or E. dulcis) is a member of the sedge family.

Learn more about water chestnut with a free trial on Britannica.com.

or Indian buffalo

Any of three subspecies of oxlike bovid (species Bubalus bubalis). Two have been domesticated in Asia since the earliest recorded history. The animal is named for its ability to work on waterlogged land and in humid climates. The largest breeds stand 5–6 ft (1.5–1.8 m), is up to 9 ft (2.8 m) long, and may weigh over 2,000 lb (900 kg). The dull black or dark gray body has little hair. The horns spread outward and upward, measuring up to 7 ft (2 m) across. One subspecies, the swamp buffalo, is the principal draft animal of southern China and South and Southeast Asia. Another, the river buffalo, is used for dairy and meat production and draft work in southern and South Asia and Egypt. The third subspecies is the wild water buffalo, of which only a few dozen herds remain. It is larger than domestic buffaloes and is sometimes referred to as a separate species (B. arnee).

Learn more about water buffalo with a free trial on Britannica.com.

Dense aquatic accumulation of microscopic organisms produced by an abundance of nutrients in surface water coupled with adequate sunlight for photosynthesis. The microorganisms or the toxic substances they release may discolour the water, exhaust its oxygen content, poison aquatic animals and waterfowl, and irritate the skin and respiratory tract of humans. Single species of algae, diatoms, or dinoflagellates, reproducing every few hours, may dominate a bloom's population; the number of individuals per quart (litre) of water, normally about 1,000, can increase to 60 million. Blooms of the dinoflagellate genus Gymnodinium cause red tides. The Red Sea is named for the occasional blooms of the alga Trichodesmium erythraeum. Seealso water pollution.

Learn more about water bloom with a free trial on Britannica.com.

White semiaquatic bear (Ursus maritimus) found throughout Arctic regions, generally on drifting oceanic ice floes. A swift, wide-ranging traveler and a good swimmer, it stalks and captures its prey. It primarily eats seal but also fish, seaweed, grass, birds, and caribou. The male weighs 900–1,600 lbs (410–720 kg) and is about 5.3 ft (1.6 m) tall at the shoulder and 7–8 ft (2.2–2.5 m) long. It has a short tail. The hairy soles of its broad feet protect it from the cold and help it move across the ice. Though shy, it is dangerous when confronted.

Learn more about polar bear with a free trial on Britannica.com.

Swimming sport in which the movements of one or more swimmers are synchronized with a musical accompaniment. The sport developed in the U.S. in the 1930s and was admitted as an Olympic event (solo and duet only) in 1984; in 1996 the rules were changed to allow only teams of eight women. Teams are judged on compulsory and optional routines.

Learn more about synchronized swimming with a free trial on Britannica.com.

Inorganic compound composed of hydrogen and oxygen (H₂O), existing in liquid, gas (steam, water vapour), and solid (ice) states. At room temperature, water is a colourless, odourless, tasteless liquid. One of the most abundant compounds, water covers about 75percnt of Earth's surface. Life depends on water for virtually every process, its ability to dissolve many other substances being perhaps its most essential quality. Life is believed to have originated in water (the world's oceans or smaller bodies), and living organisms use aqueous solutions (including blood and digestive juices) as mediums for carrying out biological processes. Because water molecules are asymmetric and therefore electric dipoles, hydrogen bonding between molecules in liquid water and in ice is important in holding them together. Many of water's complex and anomalous physical and chemical properties (high melting and boiling points, viscosity, surface tension, greater density in liquid than in solid form) arise from this extensive hydrogen bonding. Water undergoes dissociation to the ions H⁺ (or H₃O⁺) and OH⁻, particularly in the presence of salts and other solutes; it may act as an acid or as a base. Water occurs bound (as water of hydration) in many salts and minerals. It has myriad industrial uses, including as a suspending agent (papermaking, coal slurrying), solvent, diluting agent, coolant, and source of hydrogen; it is used in filtration, washing, steam generation, hydration of lime and cement, textile processing, sulfur mining, hydrolysis, and hydraulics, as well as in beverages and foods. See also hard water; heavy water.

Learn more about water with a free trial on Britannica.com.

Water that makes up the oceans and seas. Seawater is a complex mixture of 96.5percnt water, 2.5percnt salts, and small amounts of other substances. Much of the world's magnesium is recovered from seawater, as are large quantities of bromine. In certain parts of the world, sodium chloride (table salt) is obtained by evaporating seawater. In addition, desalted seawater can theoretically furnish a limitless supply of drinking water, but the high processing costs are prohibitive. Large desalination plants have been built in dry areas along seacoasts in the Middle East and elsewhere to relieve shortages of fresh water.

Learn more about seawater with a free trial on Britannica.com.

Discoloration of seawater caused by dinoflagellates during periodic blooms (population increases). Toxic substances released by these organisms into the water may be lethal to fish and other marine life, and they irritate the human respiratory system. Coastal resorts sometimes close when breaking waves release the toxic substances into the air. The causes of red tide are uncertain; it may require the confluence of several natural phenomena, in which human influence may or may not play a part.

Learn more about red tide with a free trial on Britannica.com.

or deuterium oxide

Water composed of two atoms of deuterium (D; a heavy isotope of hydrogen) and one atom of oxygen (O), chemical formula D₂O. Water from most natural sources contains about 0.015percnt deuterium oxide; this can be enriched or purified by distillation, electrolysis, or chemical processing. Heavy water is used as a moderator in nuclear power plants, slowing down the fast neutrons so that they can react with the fuel in the reactor. Heavy water is also used in research as an isotopic tracer for chemical reactions and biochemical pathways. Water with tritium (T₂O) rather than deuterium may also be called heavy water.

Learn more about heavy water with a free trial on Britannica.com.

Water that contains mineral salts of calcium and magnesium, principally as bicarbonates, chlorides, and sulfates, and sometimes iron. Hardness caused by calcium bicarbonate is known as temporary, because boiling converts the bicarbonate to the insoluble carbonate; hardness from the other salts is called permanent. The calcium and magnesium in hard waters form a hard, adherent scale on boiler plates, increasing fuel consumption and leading to deterioration through overheating. Home water softeners consist of tanks containing zeolite minerals or ion-exchange resins, which contain sodium ions that change places with the calcium and magnesium.

Learn more about hard water with a free trial on Britannica.com.

Lowermost layer of ocean water that can be distinguished by its characteristic low temperature, high density, and low oxygen content compared with surface waters. Most bottom waters are formed near Antarctica during the southern winter. The partial freezing of seawater over the Antarctic continental shelf produces salt-free ice and residual brine with a high density, which causes it to sink; it then flows northward along the seafloor. The Arctic Ocean is less important as a source of bottom water because it is isolated by barriers such as the Bering Sill and submarine ridges and banks between Greenland and the British Isles.

Learn more about bottom water with a free trial on Britannica.com.

Lake, Cumbria, England. Located in the Lake District, it is about 3 mi (5 km) long and 0.5–1.25 mi (0.8–2 km) wide, with a maximum depth of 72 ft (22 m). The River Derwent enters its southern end and leaves the lake's northern end near the market town of Keswick. Several sites on its shores are National Trust property and are frequented by tourists. Lords Island, one of several in the lake, was once the residence of the earls of Derwentwater.

Learn more about Derwent Water with a free trial on Britannica.com.