Geology Snake River

south fork of snake river in blm idaho .

as 165 million years ago, of western north america still part of pacific ocean. complete subduction of farallon plate underneath westward-moving north american plate created rocky mountains, pushed rising magma trapped between sinking farallon plate , north american plate. north american plate moved westwards on stationary hotspot beneath crust, series of tremendous lava flows , volcanic eruptions carved out snake river plain beginning 12 million years ago, west of continental divide. larger lava flows of columbia river basalts issued on eastern washington, forming columbia plateau southeast of columbia river , palouse hills in lower snake. separate volcanic activity formed northwestern portion of plain, area far path of hotspot lies beneath yellowstone national park. @ point, snake river watershed beginning take shape.

locations of yellowstone hotspot passed through snake river plain

the snake river plain , gap between sierra nevada , cascade range formed moisture channel, running far inland headwaters of snake river. rainclouds pacific ocean blown moisture channel travel eastwards on 1,000 miles (1,600 km). when teton range uplifted 9 million years ago along detachment fault running north–south through central rockies, rainclouds began encounter barrier @ eastern end of channel, engorging headwaters of snake river frequent rainfall. these rains fed snake river, helping cut through tetons, forming snake river canyon of wyoming. 6 million years ago, salmon river mountains , blue mountains @ far end of plain began rise, , river cut through rising mountains, ancestral hells canyon formed. lake idaho, formed during miocene, covered large portion of snake river plain between twin falls , hells canyon, , lava dam breached 2 million years ago.

shoshone falls near twin falls, idaho

lava flowing cedar butte in present southeast idaho blocked snake river @ eagle rock, 42,000 years ago, near present-day site of american falls dam. 40-mile-long (64 km) lake, known american falls lake, formed behind barrier. lake stable , survived 30,000 years. 14,500 years ago, pluvial lake bonneville in great salt lake area, formed in last glacial period, spilled catastrophically down portneuf river snake in event known bonneville flood. 1 of first in series of catastrophic flooding events in northwest known ice age floods.

the deluge caused american falls lake breach natural lava dam, rapidly eroded 50-foot-high (15 m) american falls left in end. flood waters of lake bonneville, approximately twenty times flow of columbia river or 5,300,000 cubic feet per second (150,000 m/s), swept down snake river, leaving debris , sediment deposits across southern idaho. miles on either side of snake, flood waters stripped away soils , scoured underlying basalt bedrock, in process creating shoshone falls, twin falls, crane falls, , swan falls, while cutting , deepening gorges , canyons along way. bonneville flood waters continued through hells canyon. flood widened hells canyon did not deepen it.

the extraordinarily flat , expansive snake river plain result of lake bonneville floods , yellowstone hotspot

as bonneville floods rushed down snake river, missoula floods occurred in same period, farther north. missoula floods, took place more 40 times in time span 15,000 13,000 years ago, caused glacial lake missoula on clark fork repeatedly being impounded ice dams breaking through, lake s water rushing on of eastern washington in massive surges far larger lake bonneville flood. these floods pooled behind cascade range enormous lakes , spilled on northern drainage divide of snake river watershed, carving deep canyons through palouse hills. palouse river canyon largest of many gorges cut through palouse hills, , not have become large if not missoula floods. lake bonneville floods , missoula floods helped widen , deepen columbia river gorge, giant water gap allows water columbia , snake rivers take direct route through cascade range pacific.

the massive amounts of sediment deposited lake bonneville floods in snake river plain had lasting effect on of middle snake river. high hydraulic conductivity of mostly-basalt rocks in plain led formation of snake river aquifer, 1 of productive aquifers in north america. many rivers , streams flowing north side of plain sink aquifer instead of flowing snake river, group of watersheds called lost streams of idaho. aquifer filled hold 100,000,000 acre feet (120 km) of water, underlying 10,000 square miles (26,000 km) in plume 1,300 feet (400 m) thick. in places, water exits rivers @ rates of 600 cubic feet per second (17 m/s). of water lost snake river transects plain issues river @ western end, way of many artesian springs.