page_title: Snake River
section_title: Geology
hierarchical_section_title: Snake River / Geology
caption_attribution_description: Snake river in Hagerman Fossil Beds National Monument, Idaho
context_page_description: The Snake River is a major river of the greater Pacific Northwest region in the United States. At 1,078 miles 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, Washington.
  The Snake River drainage basin encompasses parts of six U.S. states 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 that occurred 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.
  Prehistoric Native Americans lived along the Snake starting more than 11,000 years ago.
context_section_description: As recently as 165 million years ago, most of western North America was still part of the Pacific Ocean. The nearly complete subduction of the Farallon Plate underneath the westward-moving North American Plate created the Rocky Mountains, which were pushed up by rising magma trapped between the sinking Farallon plate and the North American plate. As the North American Plate moved westwards over a stationary hotspot beneath the crust, a series of tremendous lava flows and volcanic eruptions carved out the Snake River Plain beginning about 12 million years ago, west of the Continental Divide.  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 which now lies beneath Yellowstone National Park. At this point, the Snake River watershed was beginning to take shape.
  The Snake River Plain and the gap between the Sierra Nevada and Cascade Range together formed a "moisture channel," opening the way for Pacific storms to travel more than 1,000 miles (1,600 km) 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 40-mile-long (64 km) 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 50-foot-high (15 m) 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, 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 
caption: The extraordinarily flat and expansive Snake River Plain was the result of the Lake Bonneville floods and the Yellowstone Hotspot
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