Geology
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 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 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 formed a "moisture channel" running as far inland as the headwaters of the Snake River. Rainclouds from the Pacific Ocean blown into the moisture channel travel eastwards over 1,000 miles (1,600 km). When the Teton Range uplifted about 9 million years ago along a detachment fault running north-south through the central Rockies, rainclouds began to encounter a barrier at the eastern end of the channel, gorging the headwaters of the Snake River with frequent rainfall. These rains fed the Snake River, helping it to cut through the Tetons, 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, and as the river cut through the rising mountains, the ancestral Hells Canyon was formed. 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 (64 km)-long 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 away with only the 50-foot (15 m)-high American Falls left in the end. The flood waters of Lake Bonneville, approximately twenty times the flow of the Columbia River or 5,300,000 cubic feet per second (150,000 m3/s), swept down the Snake River leaving debris and sediment deposits across southern Idaho. For miles on either side of the Snake flood waters stripped away soils and scoured the underlying basalt bedrock, in the process creating Shoshone Falls, Twin Falls, Crane Falls, and Swan Falls, while cutting and deepening gorges and canyons along the way. The Bonneville flood waters continued through Hells Canyon. 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 farther north. The Missoula Floods, which took place over 40 times in the time span from 15,000 to 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. The Palouse River canyon was the largest of the many gorges cut through the Palouse Hills, and could not have become as large as it now is if it were not for the Missoula Floods. 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 100,000,000 acre feet (120 km3) of water, underlying about 10,000 square miles (26,000 km2) in a plume 1,300 feet (400 m) thick. In places, water exits from rivers at rates of nearly 600 cubic feet per second (17 m3/s). 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.
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