Field Notes: The Late Lake Idaho, Part 2

There are so many mysteries about Lake Idaho. How did it last an incredible 7.8 million years? Where did it go? Why did it go? Geologists are slowly teasing out some of the answers, but as in the case of most good science, each answer just raises more questions. This is Part 2 of 2 looking at what’s known about the last millennia of Lake Idaho.

Hells Canyon, Brownlee Reservoir. About half of Hells Canyon is drowned by Oxbow, Brownlee and Hells Canyon Dams

Meanwhile, the water level of Lake Idaho, in its second form, from about 9.3 millions years ago, continued to rise. Several things contributed to the rise. The Yellowstone hot spot elevates the ground over it. The Snake River likely flowed in to Lake Idaho. In the geomorphology of the region, the migration of the hot spot had the effect of moving the continental divide eastward, increasing the size of the drainage flowing into Lake Idaho. The arrival of the Wisconsonian Ice Age lowered temperatures and increased precipitation. And the volcanic activity under and around Lake Idaho may have displaced enough water to contribute to the rise in Lake Idaho’s water level, much as putting bricks in a bathtub will raise the water level.

Drainage shift of the continental divide, from Wood and Clemens—Geologic and Tectonic History of the Western Snake River Plain

About 3 – 4 million years ago, the waters of Lake Idaho overtopped Dead Indian Ridge at the northwestern corner of Lake Idaho. The lake waters flowed down through what became Hells Canyon, carving the canyon at Cobb Rapids and lowered the surface of Lake Idaho at the rate of about 120 meters per million years. Hells Canyon, in a smaller form, already existed, but the erosion rate increased significantly. But later lava flows into the canyon make any consideration of “average” erosion rate in Hells Canyon misleading.

Hells Canyon, below Hells Canyon Dam

The ongoing volcanism in the Western Snake River Plain had the effect of forcing the course of the Snake River to the southerly side of the plain. Water wells drilled in the Western Snake River Plain suggest that there are older “Snake River Gorges,” filled with basalt flows, to the north of the current channel.

The stretching continued, the thinning of the Plain itself caused still more volcanism along a series of rifts. When you drive Interstate 84 from Boise to Mountain Home, you are roughly following the Kuna-Mountain Home Rift Zone, driving across massive lava flows that erupted from shield volcanoes, burying the Lake Idaho sediments under multiple layers of basalt. Lockman Butte, due north of Mountain Home, is one such shield volcano. Still later, cinder cones developed and spread ash across the Plain.

Cap all that off with the Bonneville Flood, when the catastrophic drainage of Lake Bonneville, the ancestor of the Great Salt Lake, spilled into the Snake River drainage, and you have part at least part of the history of Lake Idaho and the Western Snake River Plain.

But there are so many unanswered questions. The Idaho Batholith includes the Owhyee Mountains on the southerly side of the Plain. Was there Idaho Batholith granodiorite where the Western Snake River Plain is today? If so, where did it go? If not, what explains a 70 mile wide gap? Is there Columbia Flood Basalt under the lake sediments and rhyolite that make up the top layers of the Plain? The flood basalts extend to Weiser, on the northeast side of Lake Idaho. Do they extend further, but are buried? If not, why did the flood basalts stop where they did?

This is why WC loves science.