What the Glacier Left

Stand anywhere in the Missoula valley and you are standing in the bed of a vanished lake. Between roughly fifteen thousand and thirteen thousand years ago, a lobe of glacial ice pushed south through a valley in northern Idaho and blocked the Clark Fork River. The ice dam that resulted was roughly two thousand feet high. Behind it, water backed up across western Montana until it reached the level of the surrounding mountains. Geologists estimate the lake held as much water as Lakes Erie and Ontario combined, more than five hundred cubic miles of it.

The ice dam failed, probably many times. Water seeping into cracks and fissures would weaken the base until the whole structure gave way, releasing that five-hundred-cubic-mile reservoir in roughly two days. The floods that resulted were among the largest in the geological record. They scoured eastern Washington into the channeled scablands, deposited giant ripple marks on hillsides hundreds of feet above the Columbia River, and carved the Columbia Gorge. Then the glacier would advance again, the dam would re-form, and the lake would slowly fill once more. This cycle repeated as many as forty times over roughly two thousand years.

What you see in Missoula today, the flat valley floor and the broad terraced benches rising on either side, is the landscape that lake made. The sediment that settled in the still water, the shorelines it carved on the surrounding hills, the gentle grade from east to west: all of it is the signature of a lake that no longer exists in a climate that no longer produces glaciers of that scale. Norman Maclean mentions the lake in A River Runs Through It. He does not linger on it, but he knows it is there underneath everything.