I need to make it crystal clear that none of this is my own research. The pictures on this and associated pages were taken on a GSA field trip in 1994 led by Richard B. Waitt of the U.S. Geological Survey, and the interpretations presented here are largely those of Dr. Waitt as presented on the field trip and its guidebook. I thank Dr. Waitt and his colleagues for a stupendous field experience.
|This is what started it all. This irrigation ditch got out of control and carved the ravine below, exposing dozens of backflood deposits.|
|Closeup of finely laminated sediments|
|13,000 Ka ash layer from Mount St. Helens. The presence of a well-defined ash layer argues strongly for subaerial conditions between rhythmite layers.|
|Left and below: sunset on the Blue Mountains.|
What caused the backflooding at Burlingame Canyon was the constriction at Wallula Gap. The floodwaters could not get through a gap a mile wide. Maximum flood waters reached elevations of about 380 meters. It's fairly simple to estimate flow through a constriction, but the picture is complicated by flooding of the gorge further downstream. When that complication is taken into account, the peak flow is estimated to have been about 10 million cubic meters per second (the volume of Grand Coulee Dam every second). With a channel 2000 meters wide and water 250 meters deep, that translates to a velocity of 20 meters per second or 45 miles an hour. 10 million cubic meters per second is about 50 times the flow of the Amazon River and 300 times as big as the largest historic flood along the Columbia River.
Created 7 April 2003, Last Update 01 July 2012
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