ice dam

Ice dam is sometimes used as a synonym for ice jam, a clog of fractured river ice at spring breakup. A different sort of ice dam can develop out of sight beneath a glacier, where it may back up a large lake and release it suddenly in a glacial outburst. A third and larger variety produced epic floods in the Pacific Northwest only 14,000 to 16,000 years ago. A lobe of the Cordilleran ice sheet periodically plugged the canyon of the Clark Fork River in northern Idaho, giving rise to ancient Lake Missoula, which covered three thousand square miles of western Montana to a depth of nineteen hundred feet. When the ice plug melted and broke down—or popped up like a cork—surging waters scoured out the channeled scablands of eastern Washington, swelled the Columbia River a thousand feet deep at Wallula Gap, overtopped Crown Point in the Columbia Gorge, and repeatedly made a deep mud puddle out of Oregon’s Willamette Valley, bequeathing three hundred feet of rich, exotic silts that Oregon Trail pioneers would declare the firmament of a New Eden.

John Daniel