New field evidence and discharge calculation procedures provide new estimates of maximum late Pleistocene glacial Lake Missoula flood discharges for two important reaches along the flood route. Within the Spokane Valley, near the point of release, the peak discharge probably exceeded 17 ± 3 million m3̇ sec-1. Downstream at Wallula Gap, a major point of flow convergence, peak discharge was about 10 ± 2.5 million m3̇sec-1. Flow duration was on the order of several days. These are the largest known terrestrial fresh-water flows.
Consideration of these discharge values constrains models for the failure of glacial Lake Missoula. The maximum discharges estimated here are larger than theoretical and empirical predictions of maximum subglacial jökulhlaup-style releases for Lake Missoula. We postulate, consistent with geological relations in the glacial Lake Missoula basin and in the Channeled Scabland, that the largest late Wisconsinan Missoula Flood resulted from a cataclysmic failure of the impounding ice dam of glacial Lake Missoula. This large release may have been the result of a complete rupture of the ice dam. Subsequent multiple flows of lesser magnitude may have resulted from repeated subglacial releases from the lake.