Snake River

The Snake River is a major river of the greater Pacific Northwest region in the United States. At long, it is the largest tributary of the Columbia River, in turn, the largest North American river that empties into the Pacific Ocean. The Snake River rises in western Wyoming, then flows through the Snake River Plain of southern Idaho, the rugged Hells Canyon on the Oregon–Idaho border and the rolling Palouse Hills of Washington, emptying into the Columbia River at the Tri-Cities in the Columbia Basin of Eastern Washington.

The Snake River drainage basin encompasses parts of six U.S. states (Idaho, Washington, Oregon, Utah, Nevada, and Wyoming) and is known for its varied geologic history. The Snake River Plain was created by a volcanic hotspot which now lies underneath the Snake River headwaters in Yellowstone National Park. Gigantic glacial-retreat flooding episodes during the previous Ice Age carved out canyons, cliffs, and waterfalls along the middle and lower Snake River. Two of these catastrophic flooding events, the Missoula Floods and Bonneville Flood, significantly affected the river and its surroundings.

Native Americans have lived along the Snake for more than 11,000 years. Salmon from the Pacific Ocean spawned by the millions in the river and were a vital resource for people living on the Snake downstream of Shoshone Falls. By the time Lewis and Clark explored the area, the Nez Perce and Shoshone were the dominant Native American groups in the region. Later explorers and fur trappers further changed and used the resources of the Snake River basin. At one point, the sign language used by the Shoshones representing weaving baskets was misinterpreted to represent a snake, giving the Snake River its name.

By the middle 19th century, the Oregon Trail had become well established, bringing numerous settlers to the Snake River region. Steamboats and railroads moved agricultural products and minerals along the river throughout the 19th and early 20th centuries. Starting in the 1890s, fifteen major dams were built on the Snake River to generate hydroelectricity, enhance navigation, and provide irrigation water. However, these dams blocked salmon migration above Hells Canyon and have led to water quality and environmental issues in certain parts of the river. The removal of several dams on the lower Snake River has been proposed in order to restore some of the river's once-tremendous salmon runs.

Course

Formed by the confluence of three tiny streams on the southwest flank of Two Oceans Plateau in Yellowstone National Park, western Wyoming, the Snake starts out flowing west and south into Jackson Lake. Its first run through Jackson Hole, a wide valley between the Teton Range and the Gros Ventre Range. Below the tourist town of Jackson, the river turns west and flows through Snake River Canyon, cutting through the Snake River Range and into eastern Idaho. It receives the Hoback and Greys Rivers before entering Palisades Reservoir, where the Salt River joins at the mouth of Star Valley. Below Palisades Dam, the Snake River flows through the Snake River Plain, a vast arid physiographic province extending through southern Idaho southwest of the Rocky Mountains and underlain by the Snake River Aquifer, one of the most productive aquifers in the United States.

Southwest of Rexburg, Idaho, the Snake is joined from the north by Henrys Fork. The Henrys Fork is sometimes called the North Fork of the Snake River, with the main Snake above their confluence known as the "South Fork". From there, it turns south, flowing through downtown Idaho Falls, then past the Fort Hall Indian Reservation and into American Falls Reservoir, where it is joined by the Portneuf River. The Portneuf River Valley is an overflow channel that in the last glacial period carried floodwaters from pluvial Lake Bonneville into the Snake River, significantly altering the landscape of the Snake River Plain through massive erosion. From there, the Snake resumes its journey west, entering the Snake River Canyon of Idaho. It is interrupted by several major cataracts, the largest being Shoshone Falls, which historically marked the upriver limit of migrating salmon. A short distance downstream it passes under the Perrine Bridge. Near Twin Falls, the Snake approaches the southernmost point in its entire course, after which it starts to flow west-northwest.

The Snake continues through its canyon, receiving the Malad River from the east near Bliss and then the Bruneau River from the south in C.J. Strike Reservoir. It passes through an agricultural valley about southwest of Boise and flows briefly west into Oregon, before turning north to define the Idaho–Oregon border. Here the Snake River almost doubles in size as it receives several major tributaries – the Owyhee from the southwest, then the Boise and Payette rivers from the east, and farther downstream the Malheur River from the west and Weiser River from the east. North of Boise, the Snake enters Hells Canyon, a steep, spectacular, rapid-strewn gorge that cuts through the Salmon River Mountains and Blue Mountains of Idaho and Oregon. Hells Canyon is one of the most rugged and treacherous portions of the course of the Snake River, posing a major obstacle for 19th-century American explorers. Here the Snake is also impounded by Hells Canyon, Oxbow, and Brownlee Dams, which together make up the Hells Canyon Hydroelectric Project.

At the halfway point in Hells Canyon, in one of the most remote and inaccessible sections of its course, the Snake River is joined from the east by its largest tributary, the Salmon River. From there, the Snake begins to form the Washington–Idaho border, receiving the Grande Ronde River from the west before receiving the Clearwater River from the east at Lewiston, which marks the head of navigation on the Snake. The river leaves Hells Canyon and turns west, winding through the Palouse Hills of eastern Washington. The Lower Snake River Project's four dams and navigation locks have transformed this part of the Snake River into a series of reservoirs. The confluence of the Snake and Columbia rivers at Burbank, Washington is part of Lake Wallula, the reservoir of McNary Dam. The Columbia River flows about farther west to the Pacific Ocean near Astoria, Oregon.

Geology

Most of western North America was drowned by shallow seas off and on from around 800–200 million years ago. This was the accumulation phase of the mountain building cycle, where mostly sedimentary rocks accumulate as the seafloor subsides. The western edge of North America was a passive continental margin and therefore was not near a plate boundary. 180 million years ago, the tectonic setting of western North America changed to an active continental margin, and a convergent plate boundary formed, which crumpled the deposits from the accumulation phase into fold-thrust mountain ranges. Plate convergence gave rise to the Coastal Ranges and then to the Rocky Mountains due to friction between the Farallon Plate sinking beneath the North American Plate. Another tectonic player is the Yellowstone hotspot, which is a large plume of partially molten material lodged in the mantle. As the North American Plate moved west–southwest over the hot spot, massive amounts of basaltic lava were extruded onto the crust. A load of these dense lava flows bent down the crust immediately below it, and the mountains formed earlier to make the WSW-ENE oriented trough known as the Snake River Plain. The river did not carve the Snake River Plain, but rather the Snake River took the course of least resistance and followed the path formed by the hot spot through the Rocky Mountains. The first lava flows started pouring out in what is now the western part of the Snake River Plain and is still extruding in the eastern part of the plain. Even larger lava flows of Columbia River basalts issued over eastern Washington, forming the Columbia Plateau southeast of the Columbia River and the Palouse Hills in the lower Snake. Separate volcanic activity formed the northwestern portion of the plain, an area far from the path of the hotspot The hot spot now resides beneath Yellowstone National Park. The heat from the molten rock beneath the park domes the crust up to form the Yellowstone Plateau

The Snake River Plain and the gap between the Sierra Nevada and the Cascade Range together formed a "moisture channel," opening the way for Pacific storms to travel more than inland to the headwaters of the Snake River. When the Teton Range uplifted about 9 million years ago along a detachment fault running north–south through the central Rockies, the river maintained its original course and cut through the southern end of the mountains, forming the Snake River Canyon of Wyoming. About 6 million years ago, the Salmon River Mountains and Blue Mountains at the far end of the plain began to rise; the river cut through these mountains as well, forming Hells Canyon. Lake Idaho, formed during the Miocene, covered a large portion of the Snake River Plain between Twin Falls and Hells Canyon, and its lava dam was finally breached about 2 million years ago.

Lava flowing from Cedar Butte in present southeast Idaho blocked the Snake River at Eagle Rock about 42,000 years ago, near the present-day site of American Falls Dam. A lake, known as American Falls Lake, formed behind the barrier. The lake was stable and survived for nearly 30,000 years. About 14,500 years ago, pluvial Lake Bonneville in the Great Salt Lake area, formed in the last glacial period, spilled catastrophically down the Portneuf River into the Snake in an event known as the Bonneville flood. This was one of the first in a series of catastrophic flooding events in the Northwest known as the Ice Age Floods.

The deluge caused American Falls Lake to breach its natural lava dam, which was rapidly eroded with only the American Falls left in the end. The flood waters of Lake Bonneville, approximately twenty times the flow of the Columbia River or 5 million ft³/s (140,000 m³/s), swept down the Snake River and across the entirety of southern Idaho. For miles on either side of the river, flood waters stripped away soils and scoured the underlying basalt bedrock, transforming the region into channeled scablands forming the Snake River Canyon and creating Shoshone Falls, Twin Falls, Crane Falls, Swan Falls and other waterfalls along the Idaho section of the river. The Bonneville flood waters continued through Hells Canyon and eventually reached the Columbia River. The flood widened Hells Canyon but did not deepen it.

As the Bonneville Floods rushed down the Snake River, the Missoula Floods occurred in the same period, but originating farther north. The Missoula Floods, which occurred more than 40 times between 15,000 and 13,000 years ago, were caused by Glacial Lake Missoula on the Clark Fork repeatedly being impounded by ice dams then breaking through, with the lake's water rushing over much of eastern Washington in massive surges far larger than the Lake Bonneville Flood. These floods pooled behind the Cascade Range into enormous lakes and spilled over the northern drainage divide of the Snake River watershed, carving deep canyons through the Palouse Hills including the Palouse River canyon and Palouse Falls. The Lake Bonneville Floods and the Missoula Floods helped widen and deepen the Columbia River Gorge, a giant water gap which allows water from the Columbia and Snake rivers to take a direct route through the Cascade Range to the Pacific.

The massive amounts of sediment deposited by the Lake Bonneville Floods in the Snake River Plain also had a lasting effect on most of the middle Snake River. The high hydraulic conductivity of the mostly-basalt rocks in the plain led to the formation of the Snake River Aquifer, one of the most productive aquifers in North America. Many rivers and streams flowing from the north side of the plain sink into the aquifer instead of flowing into the Snake River, a group of watersheds called the lost streams of Idaho. The aquifer filled to hold nearly of water, underlying about in a plume thick. In places, water exits from rivers at rates of nearly . Much of the water lost by the Snake River as it transects the plain issues back into the river at its western end, by way of many artesian springs.

Watershed

The Snake River is the thirteenth longest river in the United States. Its watershed is the 10th largest among North American rivers, and covers almost in portions of six U.S. states: Wyoming, Idaho, Nevada, Utah, Oregon, and Washington, with the largest portion in Idaho. Most of the Snake River watershed lies between the Rocky Mountains on the east and the Columbia Plateau on the northwest. The largest tributary of the Columbia River, the Snake River watershed makes up about 41% of the entire Columbia River Basin. Its average discharge at the mouth constitutes 31% of the Columbia's flow at that point. Above the confluence, the Snake is slightly longer than the Columbia— compared to —and its drainage basin is slightly larger—4% bigger than the upstream Columbia River watershed.

The mostly semi-arid, even desert climate of the Snake River watershed on average, receives less than of precipitation per year. However, precipitation in the Snake River watershed varies widely. At Twin Falls, in the center of the Snake River Plain, the climate is nearly desert, with an annual rainfall of just , although the average snowfall is . This desert climate occupies the majority of the basin of the Snake River, so although it is longer than the Columbia River above the Tri-Cities, its average discharge is significantly less. However, in the high Rockies of Wyoming, in the upper Jackson Hole area, the average precipitation is over , and snowfall averages . Most of the Snake River basin consists of wide, arid plains and rolling hills, bordered by high mountains. In the upper parts of the watershed, however, the river flows through an area with a distinct alpine climate. There are also stretches where the river and its tributaries have incised themselves into tight gorges. The Snake River watershed includes parts of Yellowstone National Park, Grand Teton National Park, Hells Canyon National Recreation Area, and many other national and state parks.

Much of the area along the river, within a few miles of its banks, is irrigated farmland, especially in its middle and lower course. Irrigation dams include American Falls Dam, Minidoka Dam, and C.J. Strike Dam. Aside from water from the river, water is also pulled from the Snake River Aquifer for irrigation. Major cities along the river include Jackson in Wyoming, Twin Falls, Idaho Falls, Boise, and Lewiston in Idaho, and the Tri-Cities in Washington (Kennewick, Pasco and Richland). There are fifteen dams in total along the Snake River, which aside from irrigation, also produce electricity, maintain a navigation channel along part of the river's route, and provide flood control. However, fish passage is limited to the stretch below Hells Canyon.

The Snake River watershed is bounded by several other major North American watersheds, which drain both to the Atlantic or the Pacific, or into endorheic basins. On the southwest side, a divide separates the Snake watershed from Oregon's Harney Basin, which is endorheic. On the south, the Snake watershed borders that of the Humboldt River in Nevada, and the watershed of the Great Salt Lake (the Bear, Jordan and Weber rivers) on the south.
The Snake River also shares a boundary with the Green River to the southeast; the Green River drains parts of Wyoming and Utah and is the largest tributary of the Colorado River. On the western extremity, for a short stretch, the Continental Divide separates the Snake watershed from the Bighorn River, a tributary of the Yellowstone River, which the Snake begins near. On the north the Snake River watershed is bounded by the Red Rock River, a tributary of the Beaverhead River, which flows into the Jefferson River and into the Missouri River, part of the Gulf of Mexico drainage basin.

The rest of the Snake River watershed borders on several other major Columbia River tributaries - mostly the Spokane River to the north, but also Clark Fork in Montana to the northeast and the John Day River to the west. Of these, the Clark Fork (via the Pend Oreille River) and the Spokane join the Columbia above the Snake, while the John Day joins downstream of the Snake, in the Columbia River Gorge. It is of note that the northeastern divide of the Snake River watershed forms the Idaho-Montana boundary, so the Snake River watershed does not extend into Montana.

Mountain ranges in the Snake watershed include the Teton Range, Bitterroot Range, Clearwater Mountains, Seven Devils Mountains, and the extreme northwestern end of the Wind River Range. Grand Teton is the highest point in the Snake River watershed, reaching in elevation. The elevation of the Snake River is when it joins the Columbia River.

Pollution

Agricultural runoff from farms and ranches in the Snake River Plain and many other areas has severely damaged the ecology of the river throughout the 20th century. After the first irrigation dams on the river began operation in the first decade of the 20th century, much of the arable land in a strip a few miles wide along the Snake River was cultivated or turned to pasture, and agricultural return flows began to pollute the Snake. Runoff from several feedlots was dumped into the river until laws made the practice illegal. Fertilizer, manure and other chemicals and pollutants washed into the river greatly increase the nutrient load, especially of phosphorus, fecal coliforms and nitrogen. During low water, algae blooms occur throughout the calm stretches of the river, depleting its oxygen supply.

Much of the return flows do not issue directly back into the Snake River, but rather feed the Snake River Aquifer underneath the Snake River Plain. Water diverted from the river for irrigation, after absorbing any surface pollutants, re-enters the ground and feeds the aquifer. Although the aquifer has maintained its level, it has become increasingly laced with contaminants. Water in the aquifer eventually travels to the west side of the Snake River Plain and re-enters the river as springs. Throughout much of the Snake River Plain and Hells Canyon, excessive sediment is also a recurring problem. In December 2007, the U.S. Environmental Protection Agency (EPA) issued a permit requiring owners of fish farms along the Snake River to reduce their phosphorus discharge by 40%. Pollutant levels in Hells Canyon upstream of the Salmon River confluence, including that of water temperature, dissolved nutrients, and sediment, are required to meet certain levels.

Discharge

The Snake River's average flow is . The United States Geological Survey recorded the river's discharge from a period of 1963–2000 at a stream gauge below Ice Harbor Dam. In that period, the largest average annual flow recorded was in 1997, and the lowest was in 1992. The lowest recorded daily mean flow was on February 4, 1979. On August 27, 1965, there was temporarily no flow as a result of testing at Ice Harbor Dam. The highest recorded flow was on June 19, 1974. The highest flow ever recorded on the Snake River was at a different USGS stream gauge near Clarkston, which operated from 1915 to 1972. This gauge recorded a maximum flow of —more than the Columbia's average discharge—on May 29, 1948. An even larger peak discharge, estimated at , occurred during the flood of June 1894.

The river's flow is also measured at several other points in its course. Above Jackson Lake, Wyoming, the discharge is about from a drainage area of . At Minidoka, Idaho, about halfway through the Snake River Plain, the river's discharge rises to . However, at Buhl, Idaho

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