Missoula Flood Rhythmites
Lake Missoula filled many times and emptied catastrophically in many Missoula Floods. Rhythmite sequences [a series of repeated beds of similar origin] at numerous localities provide this evidence: slack-water rhythmites in backflooded tributary valleys below the dam indicate multiple floods, and varved rhythmites in Lake Missoula attest to multiple fillings of the lake. Below the dam, most slack-water rhythmites are graded beds deposited by flood bores surging up tributary streams. They grade upward from coarse sand and gravel to silt, with occasional ice-rafted erratics. The tops of some rhythmites are marked by thin paleosols, or buried soil horizons, which indicate a period of subaerial exposure. Thus, each rhythmite represents a separate flood event, and each deposit records multiple floods. The most complete record occurs at Sanpoil Valley, an embayment on the north side of Lake Columbia, where varved rhythmites document 89 flood events, with the period of time between floods initially increasing to a maximum of about 50 years and then decreasing to less than 10 years. Thousands of varves were deposited in Lake Missoula. At the best-known Ninemile locality near Missoula, about 40 rhythmites consist of varves overlain by a sand/silt layer. The varves were deposited on the floor of Lake Missoula, and the sand/silt layers represent subaerial exposure and deposition in a stream. The number of varves in each rhythmite varies from 9 to 40, decreasing regularly upward, and the total number of varves is just less than one thousand. An interpretation of these data would suggest: [1] Lake Missoula filled and emptied [in a catastrophic flood] about 40 times, [2] it took 9 to 40 years to fill the lake, each successive lake requiring less time, and [3] the process was repeated over a period of about one thousand years. Because Ninemile is about in the middle of the very long lake, the record here would not provide a complete history of the lake. Correlating Ninemile with the downstream record would suggest these events were in the latter half of the entire flood history.
Glacial Lake Missoula
This feature-filled video by Tom Foster and Nick Zentner explores the evidence for Glacial Lake Missoula, and provides a treasure trove of places to visit and sights to see when you plan your field trip to the area.
Ice Age Flood Simulation Video
An interesting, 4 minute captioned video from UC Santa Cruz (ingomar200) of a satellite-view computer simulation illustrating flood paths and transient lakes of an Ice Age Flood. The video shows a physics-based computer simulation of the Great Flood from Glacial Lake Missoula about 15,000 years ago. At the time, an ice dam blocked the Clark Fork River near the Idaho-Montana border and backed up a lake about equal in volume to Lake Huron. When the ice dam broke a cataclysmic flood scoured much of central Washington State leaving a vast region covered with erosional remnants.