Geology of the Foothills

Lake Idaho
Fire and water built the western Snake River Plain below the foothills. Fire came in the form of volcanic eruptions, which spread red-hot lava and rained searing ash across southern Idaho. Today the Plain is world famous for its vast, stark volcanic landscape.
Once the volcanic rock was laid down, water began to take a dominant role. Between 9 and 2 million years ago a series of lakes formed, then dried up and formed again as the climate shifted. At times, these lakes joined and filled the valley. This enormous body of water is known as Lake Idaho.
Lake Idaho used to be where the Treasure Valley is now.. Imagine its shimmering water stretching before you for 200 miles, from present-day Weiser, Idaho to Twin Falls! The rivers that fed Lake Idaho also slowly filled the valley with sediment from the surrounding mountains. The lake sediments, which in places accumulated to more than 4,000 feet thick, underlie most of the Treasure Valley today.
Lake Idaho existed for more than 6.5 million years. Where did it go? Geologists aren’t certain, but many think that, between 2-4 million years ago, melting glaciers caused the lake to overflow and drain west in a massive flood that gouged Hell’s Canyon, the deepest canyon in North America.