The Western Snake River Plain is a desert now. Sagebrush, bitterbrush and rabbitbrush growing on layers of ash and loess, which in turn lie on thousands of feet of basalt, congealed lava. It’s a desert, and a few minutes there in the summer will remove any doubts you might have that it is a desert.
But not that long ago, at least in geologic terms, it was a lake. It was Lake Idaho. That was a Lake Ontario-sized body of water, some 1,500 feet deep. It existed from about 12 million years ago (MYA) to about 2 MYA, a very long time for a lake. Lakes fill up with wind-blown dust, organic material and river sediments. A lake can’t last that long unless it is deepening through subsidence as fast as it is filling with sediment.
But Lake Idaho was over then-geologically recent path of the Yellowstone Hot Spot, and the eruptions associated with that series of calderas blasted a lot of rock into ash. The surface subsides after that. As the post-eruption rock cools, it contracts and subsides still more. So there was a geological basis for subsidence, to preserve a lake for 10 million years.
Lake Idaho finally drained when the Snake River cut its present channel through Hells Canyon to the Columbia River. Both before and after Lake Idaho drained, the Basin and Range system began to develop, stretching the continental crust. As a result, there was local volcanism, some of the fairly extensive. In some places, the exposed lava from those later eruptions clearly ran into water, creating pillow basalts. In other places, there’s just a lot of basalt laid over old lake bed.
Much of the sediment from Lake Idaho are buried under those subsequent lava flows and rhyolite deposits. But you can find exposures in the Snake River Canyon, and in the steep-sided stream cuts incising the Boise Front.
When Charles Lyell laid down the principles of geology, he focused on uniformitarianism, rejecting the Biblical Flood and catastrophic events as driving geology. Like all generalizations, that one was partially false. Alaska’s Lake Ahtna, Montana’s Lake Missoula and Utah’s Lake Bonneville and the epic floods associated with them; the extraordinary volcanism of Idaho, Eastern Oregon and Washington all demonstrate that.
That’s the difference between science and faith, of course. The whole idea of science is to see if a theory can be proven wrong.