The Snake River is a major tributary of the Columbia River in the U.S. states of Wyoming, Idaho, Oregon, and Washington. The river's length is , its watershed drains , and the average discharge at its mouth is . The river flows from its source in Yellowstone National Park through a series of mountain ranges, canyons, and plains.
The Snake is the largest tributary of the Columbia River, with a mean discharge of , or according to the USGS, the 12th largest in the United States.
For much of its course the Snake River flows through the Snake River Plain, a physiographic province extending from eastern Oregon through western and central Idaho into northwest Wyoming. Much of the Snake River Plain is high desert and semi-desert at elevations averaging around . Many of the rivers in this region have cut deep and meandering canyons. West of Twin Falls, the plain is mainly covered with stream and lake sediments. During the Miocene, lava dams created Lake Idaho, which covered a large portion of the Snake River Plain between Twin Falls and Hells Canyon. This large lake expanded and contracted several times before finally receding in the early Pleistocene. In more recent geologic time, about 14,500 years ago, glacial Lake Bonneville spilled catastrophically into the Snake River Plain. The flood carved deep into the land along the Snake River, leaving deposits of gravel, sand, and boulders, as well as a scabland topography in places. Results of this flood include the falls and rapids from Twin Falls and Shoshone Falls to Crane Falls and Swan Falls, as well as the many "potholes" areas.
The Snake River Aquifer, one of the most productive aquifers in the world, underlies an area of about in the Snake River Plain. Differences in elevation and rock permeability result in many dramatic springs, some of which are artesian. The groundwater comes from the Snake River itself as well as other streams in the region. Some streams on the northern side of the Snake River Plain, such as the Lost River are completely absorbed into the ground, recharging the aquifer and emerging as springs that flow into the Snake River in the western part of the plain. The hydraulic conductivity of the basalt rocks that make up the aquifer is very high. In places water exits the Snake and Lost rivers into ground conduits at rates of nearly . Due to stream modifications and large-scale irrigation, most of the water that used to recharge the aquifer directly now does so in the form of irrigation water drainage.
Below the Palisades Reservoir, the Snake River flows northwest through Swan Valley to its confluence with Henrys Fork near Rigby. The region around the confluence is a large inland delta. Above the juncture, the Snake River is locally called the South Fork of the Snake River, since Henrys Fork is sometimes called the North Fork of the Snake River.
The Snake River then swings south and west in an arc across southern-central Idaho, following the Snake River Plain. It passes through the city of Idaho Falls and by Blackfoot in a region of irrigated agriculture. North of the Fort Hall Indian Reservation the river is impounded by the American Falls Dam. The dam and reservoir are part of the Minidoka Irrigation Project managed by the United States Bureau of Reclamation. The Portneuf River joins the Snake at the reservoir. Downriver from the dam is Massacre Rocks State Park, a site on the path of the old Oregon Trail.
After receiving the waters of Raft River, the Snake River enters another reservoir, Lake Walcott, impounded by Minidoka Dam, run by the Bureau of Reclamation mainly for irrigation purposes. Another dam, Milner Dam and its reservoir, Milner Reservoir, lie just downriver from Minidoka Dam. Below that is the city of Twin Falls, after which the river flows into Idaho's Snake River Canyon (the site of Evel Knievel's stunt) over Shoshone Falls and under the Perrine Bridge.
In Hells Canyon the Snake River is impounded by three dams, Brownlee Dam, Oxbow Dam, and Hells Canyon Dam (which completely blocks the migration of anadromous fish), after which the river is designated a National Wild and Scenic River as is flows through Hells Canyon Wilderness. In this section of the river, the Salmon River, one of the largest tributaries of the Snake, joins. Just across the Washington state line, another large tributary, the Grande Ronde River joins the Snake.
As the Snake flows north out of Hells Canyon, it passed the cities of Lewiston, Idaho and Clarkston, Washington, where it receives the Clearwater River. From there the Snake River swings north, then south, through southeast Washington's Palouse region, before joining the Columbia River near the Tri-Cities. In this final river reach there are four large dams, Lower Granite Lock and Dam, Little Goose Lock and Dam, Lower Monumental Lock and Dam, and Ice Harbor Lock and Dam. These dams, built by the United States Army Corps of Engineers serve as hydroelectric power sources as well as ensuring barge traffic navigation to Lewiston, Idaho.
Variant names of the river have included: Great Snake River, Lewis Fork, Lewis River, Mad River, Saptin River, Shoshone River, and Yam-pah-pa.
People have been living along the Snake River for at least 11,000 years. Daniel S. Meatte divides the prehistory of the western Snake River Basin into three main phases or "adaptive systems". The first he calls "Broad Spectrum Foraging", dating from 11,500 to 4,200 years before present. During this period people drew upon a wide variety of food resources. The second period, "Semisedentary Foraging", dates from 4,200 250 years before present and is distinctive for an increased reliance upon fish, especially salmon, as well as food preservation and storage. The third phase, from 250 to 100 years before present, he calls "Equestrian Foragers". It is characterized by large horse-mounted tribes that spent long amounts of time away from their local foraging range hunting bison. In the eastern Snake River Plain there is some evidence of Clovis, Folsom, and Plano cultures dating back over 10,000 years ago. By the protohistoric and historic era, the eastern Snake River Plain was dominated by Shoshone and other "Plateau" culture tribes.
Early fur traders and explorers noted regional trading centers, and archaeological evidence has shown some to be of considerable antiquity. One such trading center in the Weiser area existed as early as 4,500 years ago. The Fremont culture may have contributed to the historic Shoshones, but it is not well understood. Another poorly understood early cultural hearth is called the Midvale Complex. The introduction of the horse to the Snake River Plain around 1700 helped in establishing the Shoshone and Northern Paiute cultures.
On the Snake River in southeastern Washington there are several ancient sites. One of the oldest and most well-known is called the Marmes Rockshelter, which was used from over 11,000 years ago to relatively recent times. The Marmes Rockshelter was flooded in 1968 by Lake Herbert G. West, the Lower Monumental Dam's reservoir.
Many dams have been built on the Snake River and its tributaries, mainly for purposes of providing irrigation water and hydroelectric power and ranging in size from small diversion dams to major high dams.
Large dams include four on the lower Snake, in Washington, built and operated by the U.S. Army Corps of Engineers: Ice Harbor, Lower Monumental, Little Goose, and Lower Granite. These dams were built from 1962 to 1975 for hydroelectric power and navigation. They are equipped with locks, making the river as far as Lewiston an extension of the Columbia River's barge navigation system. The four dams were modified in the 1980s to better accommodate fish passage.
Upriver, in the Hells Canyon region, there are three large hydroelectric dams, operated by Idaho Power, a private utility company. Collectively named the Hells Canyon Project, the three dams are, in upriver order: Hells Canyon Dam, Oxbow Dam, and Brownlee Dam. Not having fish ladders, they are the first total barrier to upriver fish migration.
In southwestern Idaho there are several large dams. Swan Falls Dam, built in 1901, was the first hydroelectric dam on the Snake as well as the first total barrier to upriver fish migration. It was rebuilt in the 1990s by Idaho Power. Upriver from Swan Falls is another hydroelectric dam operated by Idaho Power, the C. J. Strike Dam, built in 1952. This dam also serves irrigation purposes. Continuing upriver, Idaho Power operates a set of three hydroelectric dam projects collectively called the Mid-Snake Projects, all built in the 1940s and 1950s. They are: Bliss Dam, Lower Salmon Falls Dam, and the two dams of the Upper Salmon Falls Project, Upper Salmon Falls Dam A and Upper Salmon Falls Dam B.
Near the city of Twin Falls two waterfalls have been modified for hydropower, Shoshone Falls and Twin Falls. Collectively called the Shoshone Falls Project, they are old and relatively small dams, currently operated by Idaho Power. Above Twin Falls is Milner Dam, built in 1905 for irrigation and rebuilt in 1992 with hydroelectric production added. The dam and irrigation works are owned by Milner Dam, Inc, while the powerplant is owned by Idaho Power.
Above Milner Dam, most of the large dams are projects of the U.S. Bureau of Reclamation, built mainly for irrigation, some are hydroelectric as well. All part of the Bureau's Minidoka Project, the dams are: Minidoka Dam (built 1909), American Falls Dam (1927), Palisades Dam (1957), and Jackson Lake Dam on Jackson Lake (1911). These dams, along with two others and numerous irrigation canals, supply water to about 1.1 million acres (4,500 km²) in southern Idaho.
There are many other dams on the tributaries of the Snake River, built mainly for irrigation. They are mainly operated by the Bureau of Reclamation or local government and private owners.
While the many dams in the Snake River basin have transformed the region's economy, they have also had an adverse environmental effect on wildlife, most notably on wild salmon migrations. Since the 1990s, some conservation organizations and fishermen are seeking to restore the lower Snake River and Snake River salmon and steelhead by removing four federally-owned dams on the lower Snake River.
The shipping channel is authorized to be at least deep and wide. Where river depths were less than , the shipping channel has been dredged in most places. Dredging and redredging work is ongoing and actual depths vary over time.
With a channel about deeper than the Mississippi River System, the Columbia and Snake rivers can float barges twice as heavy.
Agricultural products from Idaho and eastern Washington are among the main goods transported by barge on the Snake and Columbia rivers. Grain, mainly wheat, accounts for more than 85% of the cargo barged on the lower Snake River. In 1998, over of grain were barged on the Snake. Before the completion of the lower Snake dams, grain from the region was transported by truck or rail to Columbia River ports around the Tri-Cities. Other products barged on the lower Snake River include peas, lentils, forest products, and petroleum.
Among the negative consequences of the lower Snake River's navigational slackwater reservoirs are the flooding of historic and archaeological sites, the stilling of once famous rapids, the slowing of currents and an associated rising of water temperature, and a general decline in the ability of wild fish to migrate up and down the river.