The huge lake that used to be where Boise now stands

The Snake River Plain of Southwest Idaho and Southeast Oregon is a large, flat, and arid region. Interstate 84 travels through it en route from the Pacific Northwest toward Utah. Boise sits on the northern edge of the plain, with it also hosting smaller cities like Mountain Home and Ontario.

Today, Southern Idaho is one of the driest places in the United States with Boise only observing 11.5 inches (300 mm) of precipitation in an average year. In the past, however, the area was flooded under a huge lake.

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Lake Idaho was formed by the Snake River when mountain development cut off the river’s access to the sea. Sediments in the former lake bed lead geologists to interpret its history as having two periods of existence – the earliest being from 10.1 to 6.4 million years ago and the late phase encompassing from about 4.3 to 2.5 million years ago.

This was a dynamic period in Southern Idaho’s geologic history. Faulting around what is now Boise led to the creation of the valley where water would pool to form the lake. Around the same time, the Yellowstone hotspot underlied the region, creating massive volcanic eruptions and raising the land sitting above it.

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The western half of the Snake River Plain is geologically distinct from the eastern half. The western half is oriented along other Pacific Northwest faulting like the Olympic-Wallowa Lineament. The Yellowstone hotspot did not pass through the western half of the plain. To the eastern plain is oriented along the path of the Yellowstone hotspot, indicating the volcano was an important driver in the formation of that valley.

At its greatest extent, Lake Idaho was 150 miles (240 km) long and 40 miles (65 km) wide. It predates major lake-creating flood events in the Pacific Northwest like the Ice Age Floods and the Lake Bonneville Flood.

The geomorphology of the Yellowstone hotspot is quite interesting when we look at waterflow during this period. In the early phase of Lake Idaho, the highland area was closer to Twin Falls. This caused the upper Snake River to be cut off from the lower valley.

Resent geological research, based on sediments from the Grand Teton region, Idaho Panhandle, and Columbia Basin, shows that during this period the Upper Snake may have been routed through Western Montana and pass near Spokane before reaching the Columbia River.

At the same time, the Lower Snake River likely flowed southwestward into Nevada and eventually California. It has variously been described as a tributary to the Sacramento River or the Klamath River.

Eventually, uplift in the Great Basin led to the Snake being cut off from whatever river it was a tributary to. During this period, Lake Idaho transitioned from a freshwater lake to a salty one. At some point the lake largely dried up but formed again around four million years ago.

Toward the end of Lake Idaho’s existence, the Yellowstone hotspot continued to migrate1 toward its present location under Northwestern Wyoming. Heat from the hotspot leads land under it to rise, but when it moves away that land sinks as it cools.

Lowering over time between Twin Falls and Pocatello combined with uplift in the mountains north of Idaho Falls led to the Upper Snake being captured by the Lower Snake and flowing into Lake Idaho.

This brought a lot of additional water into the lake, raising its level to about 3600 feet (1100 meters) above sea level – high enough to flow over a natural barrier north of Ontario. The lake slowly drained and the erosion from the new Snake River flow created Hells Canyon.2

Eventually the new Snake River dug the natural barrier low enough that by two million years ago the lake was gone. Hells Canyon remains one of the deepest river gorges in the world. At its deepest point, the vertical distance from the bottom to the top of Hells Canyon is 7993 feet (2436 meters).

Much of the evidence for former flow patterns along the Snake River is found using fish fossils in sediments the rivers (and Lake Idaho) laid down. These fossils indicate that most of the Snake River watershed (excluding the Upper Snake) was disconnected from the Columbia River.

Fossil fish show the Snake River being connected to warmer regions in the south. Most evidence points toward a Northern California outlet though a paper published in 2004 also demonstrated DNA evidence that the system was connected to the Colorado River watershed for some period of time.

DNA evidence is important here because a lot of the sediment laid down by the Ancestral Snake River is likely buried in downstream areas. Northern Nevada, Southern Oregon, and Northern Utah all hosted large lakes during the Last Glacial Maximum.3 These lakes produced their own sedimentary layers on top of those left behind by the Snake. In Northern California, they’re probably buried under more recent river deposits.

Even today, fish populations are different above and below Shoshone Falls.4 Speckled dace above Shoshone Falls have more in common with those of the Great Salt Lake watershed (especially the Bear River) than below, which are more common to the Columbia River watershed. This is part of the interdisciplinary evidence used to paint the picture of the Snake River’s past.

Sediments from Lake Idaho are found in the Chalk Hills and Glenns Ferry geologic formations in Southern Idaho and Eastern Oregon.

The featured image is a view of the Snake River Canyon south of Boise, Idaho. (Bob Wick)

1. Technically it’s North America that migrates over the Yellowstone hotspot. ↩︎
2. Some northward-flowing canyons already existed in the Hells Canyon region before Lake Idaho spilled into it. The new Snake River flow deepened and widened what was already present. ↩︎
3. The Last Glacial Maximum was around 26,000 to 20,000 years ago – well after Lake Idaho ceased to exist. ↩︎
4. Shoshone Falls near Twin Falls is a significant natural barrier for fish in the Snake River. ↩︎