Frenchman Coulee is a dual coulee and cataract system. Scabland floods created Frenchman Coulee when they spilled westward over a low divide across Evergreen Ridge, out of the southwest corner of the Quincy Basin. During initial stages of flooding, the difference in water levels between the flood-filled Quincy Basin and the Columbia River immediately west of Evergreen Ridge approached 700 feet over just a few miles. This incredible difference in water levels caused floodwaters to relentlessly eat away the underlying rock layers. Erosion continued for at least as long as it took for the water level in the Columbia Valley to rise to about 1200 feet, or until the floodwater supply was exhausted.
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