The Snake river , United States:

The Snake is a major river of the greater Pacific Northwest in the United States. At 1,078 miles long, it is the largest tributary of the Columbia River, the largest North American river that empties into the Pacific Ocean.Rising in western Wyoming, the river flows through the Snake River Plain then rugged Hells Canyon and the rolling Palouse Hills to reach its mouth at the Tri-Cities of the state of Washington. Its drainage basin encompasses parts of six U.S. states, and its average discharge is over 54,000 cubic feet per second.Rugged mountains divided by rolling plains characterize the physiographically diverse watershed of the Snake River. The Snake River Plain was created by a volcanic hotspot which now lies underneath Yellowstone National Park, the headwaters of the Snake River. Gigantic glacial-retreat flooding episodes that occurred during the previous Ice Age carved out many topographical features including various canyons and ridges along the middle and lower Snake. Two of these catastrophic flooding events significantly affected the river and its surrounds.More than 11,000 years ago, prehistoric Native Americans lived along the Snake. Salmon from the Pacific Ocean spawned in the millions in the river. These fish were central to the lives of the people along the Snake below Shoshone Falls. By the time Lewis and Clark crossed the Rockies and sighted the valley of a Snake tributary, the Nez Perce and Shoshone were the most powerful people in the region. Contact with Europeans introduced horses to some tribes, reshaping their lifestyles for the next few hundred years before outside settlement. Later explorers and fur trappers further changed and utilized the resources of the Snake River basin. At one point, a hand sign made by the Shoshones representing fish was misinterpreted to represent a snake, giving the Snake River its name.By the middle 19th century, the Oregon Trail, a pioneer trail of which a major portion followed the Snake River, had been established. Steamboats and railroads moved agricultural products and minerals along the river throughout the 19th and early 20th centuries. The powerful, steep flow of the Snake has been utilized since the 1890s to generate hydroelectricity, enhance navigation and provide irrigation water from fifteen major dams that have transformed the lower river into a series of reservoirs, several of which have been proposed for removal to restore some of the river's once-tremendous salmon runs.The Snake River was once one of the most important rivers for the spawning of anadromous fish—which are born in the headwaters of rivers, live in the ocean for most of their lives, and return to the river to spawn—in the United States.The river supported species including chinook salmon, coho salmon, and sockeye salmon, as well as steelhead, white sturgeon, and Pacific lamprey. It is known that before the construction of dams on the river, there were three major chinook salmon runs in the Snake River; in the spring, summer and fall, totaling about 120,000 fish, and the sockeye salmon run was about 150,000. The historical barrier to fish migration on the Snake River was Shoshone Falls, a waterfall that occurs as the Snake River passes through the Snake River Plain.Since the early 20th century, when Swan Falls Dam was constructed on the middle Snake River upstream of Hells Canyon, the fifteen dams and reservoirs on the river have posed an increasing problem for migrating salmon. Agricultural lands and their resulting runoff have also had a significant impact on the success rate of migrating fish. Salmon can travel up the Snake River as far as Hells Canyon Dam, using the fish passage facilities of the four lower Snake River dams, leaving the Clearwater, Grande Ronde and Salmon Rivers to sustain spawning salmon. Rising in several forks in the Clearwater Mountains of central Idaho, the Clearwater and Salmon River watersheds are nearly undeveloped with the enormous exception of Dworshak Dam on the North Fork Clearwater River. The watershed of the Grande Ronde in northeastern Oregon is also largely undeveloped. The four reservoirs formed by the lower Snake River dams—Lake Sacagawea, Lake Herbert G. West, Lake Bryan, and Lower Granite Lake—have also formed problems, as the downstream current in the pools is often not enough for the fish to sense, confusing their migration routes.At the confluence of the Snake and Clearwater Rivers, young salmon that swim down from spawning gravels in the headwaters of the Clearwater River often delay their migrations because of a significant temperature difference. Agricultural runoff and water held in reservoirs higher upstream on the Snake warm its waters as it flows through the Snake River Plain, so as the Snake meets the Clearwater, its average temperature is much higher. Directly below the confluence, the river flows into Lower Granite Lake, formed by Lower Granite Dam, the uppermost dam of the Lower Snake River Project. Paradoxically, the combination of these factors gives the young salmon further time to grow and to feed in Lower Granite Lake, so when they begin the migration to the Pacific Ocean, they often have a higher chance at survival, compared to those salmon who migrate to the ocean earlier.

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