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:''This article is about the river in the northwestern United States. For other uses, see Snake River (disambiguation).'' The Snake River is a major river of the greater Pacific Northwest in the United States. At 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 of southern Idaho then through the rugged Hells Canyon area via northeastern Oregon and the rolling Palouse Hills to reach its mouth near the Washington Tri-Cities area, where it continues as part of the Columbia. Its drainage basin encompasses parts of six U.S. states, and its average discharge is over . 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 used 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 River 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. ==Course== Formed by the confluence of three tiny headstreams on the southwest flank of Two Oceans Plateau in western Wyoming and Yellowstone National Park, the Snake starts out as a small river flowing west and south into Jackson Lake. Its first run through the valley of Jackson Hole, which cuts between the Teton Range and the Continental Divide. The Snake takes a large bend northwest through Snake River Canyon, cutting through the Snake River Range and into eastern Idaho, receiving first the Hoback and Greys Rivers before entering Palisades Reservoir where it is also met by the Salt River at the mouth of Star Valley. After passing through Palisades Dam, the Snake River empties onto the Snake River Plain, a vast physiographic province extending through southern Idaho across the massif of the Rocky Mountains and underlain by the Snake River Aquifer, one of the most productive aquifers in the United States. Southwest of the city of Rexburg, the Snake receives from the right the Henrys Fork, sometimes called the North Fork of the Snake River. The confluence with the Henrys Fork takes the river southwards through downtown Idaho Falls, rounding the Fort Hall Indian Reservation and into American Falls Reservoir, receiving 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 Plain, carving out many topographic features and significantly altering the Snake River landscape. The Snake River resumes its journey westwards, then enters the Snake River Canyon of Idaho, where it drops over Shoshone Falls, a waterfall that marks the historical upriver limit of migrating salmon,〔 and passing under the Perrine Bridge.〔〔(【引用サイトリンク】title=I.B. Perrine Bridge )〕 Close to Twin Falls, the Snake approaches the southernmost point in its entire course, after which it starts to flow generally northwest.〔〔〔〔 Shortly after it passes within of the Idaho state capital of Boise, the river surges past the state border into Oregon, close to where it meets the Owyhee River, Boise River and Payette River. The Snake River then begins to define the roughly Idaho-Oregon state border, which follows the river into Hells Canyon, a steep and spectacular 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, which pioneers on the Oregon Trail and steamboat operators in the 19th century had great difficulty negotiating. There were hundreds of rapids in Hells Canyon, some of which have been stilled by the three dams of the Hells Canyon Hydroelectric Project: Hells Canyon, Oxbow, and Brownlee.〔〔〔 The Salmon River, the largest tributary of the Snake River, meets the river in one of the remotest areas of its entire course, nearly at the halfway point in Hells Canyon. From there, the Snake crosses into Washington and Idaho, receiving the Grande Ronde River from the west before receiving the Clearwater River at Lewiston, the uppermost major city on the navigable stretch of the Snake. As the Snake leaves Hells Canyon and spreads into the low-lying Palouse Hills of eastern Washington, the Lower Snake River Project's four dams have transformed the Snake River into a series of reservoirs. The confluence of the Snake and Columbia rivers has been submerged in Lake Wallula, the reservoir of McNary Dam. The Columbia River then flows about further to the Pacific Ocean, cutting through the Cascade Range by way of the Columbia River Gorge.〔〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Snake River」の詳細全文を読む スポンサード リンク
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