Snake River
The Snake River is a major river in the interior Pacific Northwest region of the United States. About long, it is the largest tributary of the Columbia River, which is the largest North American river that empties into the Pacific Ocean. Beginning in Yellowstone National Park, western Wyoming, it flows across the arid Snake River Plain of southern Idaho, the rugged Hells Canyon on the borders of Idaho, Oregon and Washington, and finally the rolling Palouse Hills of southeast Washington. It joins the Columbia River just downstream from the Tri-Cities, Washington, in the southern Columbia Basin.
The river's watershed, which drains parts of six U.S. states, is situated between the Rocky Mountains to the north and east, the Great Basin to the south, and the Blue Mountains and Oregon high desert to the west. The region has a long history of volcanism; millions of years ago, Columbia River basalts covered vast areas of the western Snake River watershed, while the Snake River Plain was a product of the Yellowstone volcanic hotspot. The river was further altered by catastrophic flooding in the most recent Ice Age, which created such features as the Snake River Canyon and Shoshone Falls.
The Snake River once hosted some of the largest North American runs of salmon and other anadromous fish. For thousands of years, salmon fishing has played a central role in the culture and diet of indigenous peoples. The Shoshone and Nez Perce were the largest of several tribes that lived along the river by the turn of the 19th century. In 1805, while searching for a route from the eastern US to the Pacific, Lewis and Clark became the first non-natives to see the river. Fur trappers explored more of the watershed, and drove beaver to near extinction as the Americans and British vied for control of Oregon Territory.
Although travelers on the Oregon Trail initially shunned the dry and rocky Snake River region, a flood of settlers followed gold discoveries in the 1860s, leading to decades of military conflict and the eventual expulsion of tribes to reservations. At the turn of the 20th century, some of the first large irrigation projects in the western US were developed along the Snake River. South-central Idaho earned the nickname "Magic Valley" with the rapid transformation of desert into farmland. Numerous hydroelectric dams were also constructed, and four navigation dams on its lower section created a shipping channel to Lewiston, Idaho – the furthest inland seaport on the West Coast.
While dam construction, commercial fishing and other human activities have greatly reduced anadromous fish populations since the late 19th century, the Snake River watershed is still considered important habitat for these fish. The Snake and its tributary, the Salmon River, host the longest sockeye salmon run in the world, stretching from the Pacific to Redfish Lake in Idaho. Since the 1950s, public agencies, tribal governments and private utilities have invested heavily in fishery restoration and hatchery programs, with limited success. The proposed removal of the four lower Snake River dams for fish passage is a significant ongoing policy debate in the Pacific Northwest.
Course
The Snake River starts to the north of Two Ocean Pass near the southern border of Yellowstone National Park, about above sea level in the Rocky Mountains of Wyoming. The river descends west through the high mountains of the Teton Wilderness meeting the Lewis River and continuing south into Jackson Lake in Grand Teton National Park, a natural glacial lake enlarged by Jackson Lake Dam. Joined by Pacific Creek and Buffalo Fork below the dam, it meanders southward through the alpine valley of Jackson Hole situated on the plain in front of the Teton Range to the west and the Gros Ventre Range to the east.Below the town of Jackson it forms the Snake River Canyon of Wyoming, turns west and crosses into Idaho, where the Palisades Dam forms Palisades Reservoir. From there it flows northwest through Swan Valley to join the Henrys Fork on an alluvial plain near Rexburg. The Henrys Fork is sometimes called the "North Fork" of the Snake River, while the section of the main Snake River above their confluence is sometimes called the "South Fork".
Turning southwest, the river begins its long journey across the Snake River Plain, passing through Idaho Falls and receiving the Blackfoot River from the left before entering the -long American Falls Reservoir, formed by American Falls Dam. From American Falls it turns west, flowing through Minidoka Dam and Milner Dam, where large volumes of water are diverted for irrigation. Below Milner Dam it enters the Snake River Canyon of Idaho, where the river narrows, forming rapids and waterfalls. In the stretch between Milner Dam and the confluence with the Malad River near Hagerman Fossil Beds National Monument, the Snake River descends a total of over a series of cataracts and rapids, chief of which include Caldron Linn, Twin, Shoshone, Pillar, Auger, and Salmon Falls. Idaho Power operates several small hydroelectric plants along this stretch of the river. The largest single drop is Shoshone Falls, which in the spring flows with such force that 19th-century writers called it the "Niagara of the West".
Image:Morley Nelson Snake River Birds of Prey NCA.jpg|thumb|left|280px|The Snake River flows through canyons in the Morley Nelson Snake River Birds of Prey National Conservation Area, south of Boise.|alt=Wide view over a river valley surrounded by cliffs and shrub land
The Snake River continues flowing west, through the C. J. Strike Reservoir where it is joined from the left by the Bruneau River, then through the Morley Nelson Snake River Birds of Prey National Conservation Area before entering farmland on the western side of Idaho's Treasure Valley. Passing west of Boise, it crosses briefly into Oregon before turning north to form the Oregon–Idaho border. It is joined by several major tributaries in quick succession – the Boise River from the right, the Owyhee and Malheur Rivers from the left, the Payette and Weiser Rivers from the right near Ontario, Oregon, then the Powder and Burnt Rivers from the left. Continuing north, the river enters Hells Canyon, which slices between the Rocky Mountains of Idaho and the Blue Mountains of Oregon and Washington. The Hells Canyon Hydroelectric Complex includes the Brownlee, Oxbow and Hells Canyon Dams in the upper reaches of the canyon. Since its construction in 1967, Hells Canyon Dam has been the upriver limit for migrating salmon; in the past, salmon swam as far upriver as Shoshone Falls.
Emerging from Hells Canyon Dam, the Snake surges northward through the Hells Canyon Wilderness, where the majority of the river corridor is accessible only by boat and numerous Class III-IV rapids historically posed a major barrier to navigation. Today, the canyon and the surrounding Hells Canyon National Recreation Area are a popular location for whitewater boating, fishing, horseback riding and backpacking. With the adjacent Seven Devils Mountains rising up to above the river, Hells Canyon is one of the deepest canyons in North America, almost one-third deeper than the Grand Canyon. Within the canyon it is joined from the left by the Imnaha River, then from the right by its longest tributary, the Salmon River. Further north, it begins to form the Idaho–Washington border, and receives the Grande Ronde River from the left. From the end of Hells Canyon at Asotin, Washington, it flows north to Lewiston, Idaho, where it is joined from the right by the Clearwater River, its largest tributary by volume. The Snake then turns sharply west to enter Washington.
The final stretch of the Snake River flows through steep-sided valleys in the Palouse Hills of southeast Washington. Near Lyons Ferry State Park, it is joined from the left by the Tucannon River, then from the right by the Palouse River, which forms Palouse Falls about upstream of its confluence with the Snake. The Lower Snake River Project consists of four dams equipped with navigation locks – Lower Granite, Little Goose, Lower Monumental and Ice Harbor – which have transformed the once fast-flowing lower Snake River into a series of lakes, enabling heavy barges to travel between the Columbia River and the Port of Lewiston. About downstream from Ice Harbor Dam, the Snake empties into the Columbia River at Burbank, Washington, southeast of the Tri-Cities. The confluence is located on Lake Wallula, the impoundment behind McNary Dam on the Columbia, above sea level. From there, the Columbia River flows another west to empty into the Pacific Ocean.
Discharge
The U.S. Army Corps of Engineers has measured the discharge, or flow rate, of the Snake River at Ice Harbor Dam since 1962. The mean annual discharge for the 61-year period between 1962 and 2023 was, with a maximum recorded daily mean of on June 19, 1974, and a minimum daily mean of on November 29, 1961. A historic June 1894 flood at the Ice Harbor site reached an estimated peak of. In terms of discharge, the Snake River is the twelfth largest river in the United States, and it contributes about one-fifth of the Columbia's total outflow into the Pacific.The volume of the Snake River peaks in late spring and early summer as snow melts in the Rocky Mountains, and reaches its lowest point in the fall. Despite the numerous dams regulating its flow, its discharge into the Columbia remains highly seasonal. At Ice Harbor Dam, the mean monthly discharge is highest in May and June at over, and lowest in September and October at less than. Mean annual discharge also fluctuates significantly, from a record high of in 1965, to a low of in 1997.
In southern Idaho, Snake River flows are significantly influenced by the Eastern Snake River Plain Aquifer. One of the largest groundwater reserves in the US, the aquifer is founded in porous volcanic rock underneath the plain. It absorbs and stores large volumes of water from the Snake River in the eastern Plain to re-emerge further west as springs in the Snake River Canyon. Water from the lost streams of Idaho, several rivers that disappear underground in the eastern Plain, travels through the aquifer to reach the Snake River, as does excess irrigation water absorbed into the ground. The major spring complexes at American Falls and Thousand Springs keep the river flowing steadily even in the driest of summers. At King Hill, about northwest of Twin Falls, water levels remain about for most of the year, increasing about 20 percent during snowmelt and decreasing about 20 percent with late summer irrigation diversions.
Despite its great length, the Snake River accumulates most of its water in the lower one-fourth of its course. By the time it reaches Hells Canyon Dam, from the mouth, the mean annual discharge is about – just over a third of the discharge at the mouth. Just two downstream tributaries, the Clearwater and Salmon Rivers, contribute about half of the total flow of the Snake.