Idaho’s Snake River Plain: A Tale of Two Basins

The Snake River Plain is a prominent river drainage that puts a big “smile” on southern Idaho and is easy to spot on satellite imagery. The geological history of the Eastern Snake River Plain and the Yellowstone Hotspot Track are closely intertwined, but the Western Snake River Plain has a different story to tell.

The Snake River flows more than 1,000 miles from its headwaters in Yellowstone, through Jackson Hole, through the Snake River Plain in southern Idaho, and through Hells Canyon before joining the Columbia River in southern Washington. The course of the river encompasses an incredibly diverse spectrum of geology and topography. The Snake River also carried high water from the massive floods of the Bonneville and Missoula Ice Ages.

Today, the Snake River Plain is home to eight of Idaho’s ten most populous cities and supports much of the state’s agricultural industry. While the Snake River flows seamlessly through southern Idaho today, the Eastern Snake River Plain and Western Snake River Plain were formed by very different geological processes.

The Eastern Snake River Plain is a northeast-southwest trending topographical depression in southeastern Idaho, extending from the Wyoming border to approximately the town of Twin Falls. As the North American tectonic plate moved southwest, the (mostly) stationary Yellowstone hotspot plume heated the crust and produced a significant amount of melt. This resulted in a series of volcanic calderas that are progressively aging along the northeast-southwest hotspot stretch.

Map of southern Idaho and the Snake River Plain
This is a map of southern Idaho and the Snake River Plain showing the eastern (Eastern Snake River Plain) and western (Western Snake River Plain) portions of the Geologic Province. (Zach Lifton/Idaho Geological Survey)

As dense magma generated by the hotspot accumulated in the middle crust, the added weight caused the crust to sag. The subsidence of the crust accumulated additional sediments and volcanic rocks at the surface, which caused further subsidence.

Ultimately, the rocks along the hotspot stretch subsided about 2.8 miles compared to the surrounding rocks. Faulting is a common way for crustal rock to move, but in the case of the eastern Serpent River Plain, subsidence occurred through flexing and warping of the crust. The rock faults surrounding the eastern Snake River Plain are clearly seen in sloping rock strata on the margins of the plain, where the Lost River, Lemhi and Beaverhead mountain ranges end on the northwest side and the Albion, Sublett, Deep Creek, Bannock, Pocatello and Portneuf Ranges end on the southeast side.

Cross-section of the Western Snake River Plain
This is a cross-section of the Western Snake River Plain in southern Idaho. (USGS Groundwater Atlas of the United States/https://pubs.usgs.gov/ha/ha730/ch_h/)

The Western Snake River Plain is a southeast-northwest trending topographical depression that extends approximately from the town of Twin Falls to the Oregon border. It is roughly perpendicular to the Eastern Snake River Plain and the Yellowstone Hotspot Track.

While the eastern Snake River Plain was warped directly along the Yellowstone hotspot route, the western Snake River Plain was faulted. The passage of the hotspot about 12 million years ago triggered crustal expansion north of the hotspot track. Normal faults formed on both sides of the Western Snake River Plain: a southwest-dipping fault on the northeast side (now known as the Boise Front Fault) and a northeast-dipping fault on the southwest side (now known as the Owyhee Mountains Fault). . .

Movement on these faults caused the intervening block of crust, called a graben, to fall downward relative to the surrounding rocks. The Western Snake River Plain fault was most active from about 11 million years ago to about 9 million years ago. Since then, the faults that bound the Western Snake River Plain graben have been moving very slowly. There is no evidence of movement on the Boise Front fault for the last ~2.6 million years; However, the Owyhee Mountains fault shows evidence of fault movement over the last ~500,000 years. The 2020 Stanley earthquake and its aftershocks (which are still ongoing!) are not directly related to the Western Snake River Plain or the Yellowstone hotspot, but the same expansive forces played a role in the sequence.

The fault-induced subsidence of the Western Snake River Plain created space for water, sediment and lava flows to accumulate. In fact, from about 10 million years ago to about 2.5 million years ago, the Western Snake River Plain was occupied by the massive Lake Idaho. Lake Idaho varied greatly in size during this period.

Hagerman horse fossil
Here is a reconstructed fossil horse skeleton found at Hagerman Fossil Beds National Monument in Idaho. (Courtesy of the National Park Service)

It left deposits of fine-grained sediments about 5,500 feet thick that covered much of southwest Idaho. These sediments include spectacular assemblages of fossils found at Hagerman Fossil Beds National Monument, including horses, peccaries, and otters. About 2.5 million years ago, the lake overtopped a drainage divide near present-day Huntington, Oregon and was captured by the lower Snake River. The lake drained north, forming Hells Canyon, the deepest river canyon in North America.

The origin and development of the Snake River Plain illustrates the diverse and far-reaching geological impact of the Yellowstone hotspot. That “smile” that characterizes southern Idaho’s topography is more than just a pretty faceā€”er, a geomorphic feature. It also has an amazing geological story to tell!

Yellowstone Caldera Chronicles is a weekly column written by scientists and staff at Yellowstone Volcano Observatory.

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