Turlough_(lake)

Turlough (lake)

A Turlough, or Turlach, is a unique type of disappearing lake found mostly in limestone areas of Ireland, west of the River Shannon. The name comes from the Irish "tuar", meaning dry, with the suffix "lach", meaning a place (in an abstract sense). The "lach" suffix is often mistakingly spelled and/or thought to refer to the word "loch", the Irish for lake. They are found in Irish karst (exposed limestone) areas. The features are almost unique to Ireland although there is one example in Britain in Llandeilo. They are of great interest to many scientists: geomorphologists are interested in how turloughs were formed, hydrologists try to explain what makes turloughs flood, botanists study the unusual vegetation which covers the turlough floor and zoologists study the animals associated with the turloughs.

Seasonal pools

Turloughs are mostly found on the central lowlands west of the Shannon, in counties Galway, Clare, Mayo and Roscommon, although a few are also found elsewhere, e.g. in - Limerick, Sligo and Longford. Most turloughs flood in the autumn, usually some time in October, and then dry up some time between April and July. However, some turloughs in the Burren can flood at any time of year in a matter of a few hours after heavy rainfall and they may empty again a few days later. A few turloughs are affected by the tide: in the summer, Caherglassaun Lough, which is 5 kilometres from Galway Bay, can be seen to flood and empty again twice every 24 hours. Most turloughs flood to a depth of about 2 metres but some are much deeper: for example, some of the turloughs near Gort are about five metres (16 feet) deep in midwinter. Turloughs are variable in size: the largest turlough in Ireland, Rahasane, which lies to the west of Craughwell in Co. Galway, covers about 2.5 square kilometres.

The cause of turloughs

All of the turloughs are found in limestone areas. This is because limestone is a unique rock in that it can be dissolved away by rainwater, particularly rainwater that has become acidic by picking up carbon dioxide as it passes through the soil. The cracks or joints in the rock become widened to such an extent that eventually all of the rain falling on the limestone disappears underground and the water moves through the rock openings ranging from cracks a few millimetres wide to large cave passages. The limestone is then said to be karstified. To the east of the Shannon, the limestone is often covered by great thicknesses of glacial drift deposited during the Ice Age but in many areas to the west of the Shannon where the limestone is pure and the drift cover is thin, there is no proper surface river network. In these areas, rainfall disappears underground, flows through openings in the rock and then rises at springs: large springs are found to the west of the area, flowing into Lough Corrib and Galway Bay. In winter, when the underground water level (or water table) rises, and when the underground flow rises, and when the underground flow routes to the springs are not capable of dealing with the amount of water entering them, groundwater may appear temporarily at the surface in the form of a turlough. Many of the rivers seen in these areas today are largely artificial, constructed by drainage engineers from the nineteenth century to the present day, often linking a series of turloughs. For example, much of the River Clare is artificial and the middle section of its course used to be a huge turlough, the largest in Ireland at 6.5 square kilometres.

Swallow holes

Turloughs usually fill and empty at particular places on the floor: sometimes an actual hole or passage is visible but more often a hollow with stones in the bottom is all that can be seen and it may not be easy to recognise when it is dry in midsummer. Some turloughs have a spring at one place and a swallow hole somewhere else on the floor where water drains away, but many turloughs fill and empty through the same hole. A few turloughs are filled by rivers and streams flowing into them as well as by water rising from underground.

The water sinking in the swallow hole travels underground to a spring, which may be several kilometres away. In most rock types, groundwater flows very slowly (from just a few centimetres to a few metres per day), but in karstified limestone the flow rate can be quite rapid: water from the turlough may flow underground to a spring at a rate of 100 metres per hour or more.

Calcium carbonate

Limestone is made up of the mineral calcium carbonate and as water passes through limestone, it dissolves the calcium carbonate - this is what makes hard water and causes furring on the inside of kettles, as the calcium carbonate comes out of solution when the water is heated. Something rather similar happens in turloughs - water which has picked up a lot of calcium carbonate during its underground travel rises in the turloughs and then some of the calcium carbonate comes out of solution and forms a white deposit. If a turlough has emptied recently, a whitish coating on the vegetation on the turlough floor may be visible. When water comes to the surface in a turlough, it loses carbon dioxide back to the atmosphere and to plants which use it for photosynthesis and this loss causes a deposit of calcium carbonate on the surface.

Sometimes a special whitish deposit which has the appearance of sheets of paper is found in the turloughs when they dry up. This "algal paper" is made up of filaments of an alga which grows abundantly in warm weather and is then left to dry out in sheets when the turlough empties.

In drainage ditches in a turlough, or in holes made with a soil auger, one may find a white- or cream-coloured deposit beneath the vegetation cover, or beneath a layer of peat. This is often called "white marl"; again, it is made of calcium carbonate. About half of the turloughs contain marl: it was deposited at a time several thousand years ago when these turloughs were not seasonal lakes but were flooded all year round.

Plant and animal life

Most turloughs have a springy, short-cropped turf of grasses, sedges and herbs. In the Burren, the highwater mark is often shown by the shrubby cinquefoil (potentilla fruticosa) with its attractive yellow flowers, and meadow rue. Just below, dog violets are abundant and in some turloughs there may be a dense sward of the rare sky-blue turlough violet about one metre further down. Other characteristic plants of turlough sides include Marsh orchids and speedwell. About half way down the sides, and across the bottom of shallow turloughs, silverweed (Potentilla anserina) may blanket almost all other plants.

If the turlough has a marshy zone near the swallow hole there may be mint, water cress, pondweeds, aquatic buttercups and knotgrass living a semiterrestrial existence. But most swallow holes (also known as swallets) when dry are represented by a jumble of rocks, clothed with blackish and dried aquatic mosses (Cinclidotus, the turlough moss and Fontinalis, usually found in streams).

Many people think that turloughs have no animal life. However, frogs and newts may spawn there and sticklebacks may survive in the larger turloughs, retreating into underground cracks in the rock when waters are low. Shrimp and water lice do the same and where fish are absent there may be a rich fauna of delicate water fleas and fairy shrimp, some unknown elsewhere in Ireland. These hatch and grow fast, finding safety in the warm fishless waters. Flatworms and snails are also often abundant; these pass the dry periods in springmouths or marshy areas.

When turloughs retain some water all year, they may be important bird haunts. Rahasane in Galway is famous for its white-fronted geese, whooper swans, widgeon, teal and many waders in winter.

Draining of turloughs

Turloughs provide good summer grazing for cattle, sheep and horses, partly because of the annual deposition of lime rich silt. However, for many years, farmers have seen the winter flooding as a waste of potential and they have attempted to find some means of draining the turloughs so that they can be used all year round. This has usually been achieved by digging an artificial channel through the turlough, which is capable of carrying away any water entering the turlough from surface or groundwater - such channels have often been constructed as part of major arterial drainage schemes. At least a third of the turloughs in Ireland have already been drained and more are being drained each year. This has very serious consequences from the point of view of the environmentalist - the unique flora and fauna of the turlough cannot survive in the absence of seasonal flooding. Even for the farmer, the benefits are not always as great as anticipated - the stopping of the annual limey silt deposition means that the soil may become impoverished and fertilisers must be used. Also, the poorly developed, delicate soil may not be able to withstand the presence of animals through the winter.

To suggest that no more turloughs should be drained is a rather extreme view but the case for conserving at least some of them is very strong.