Moses Coulee is a canyon in the Waterville plateau region of Douglas County, Washington. Moses Coulee is the second-largest and westernmost canyon of the Channeled Scablands, located about to the west of the larger Grand Coulee. This water channel is now dry, but during glacial periods, large outburst floods with discharges greater than carved the channel. While it's clear that megafloods from Glacial Lake Missoula passed through and contributed to the erosion of Moses Coulee, the origins of the coulee are less clear. Some researchers propose that floods from glacial Lake Missoula formed Moses Coulee, while others suggest that subglacial floods from the Okanogan Lobe incised the canyon. The mouth of Moses Coulee discharges into the Columbia River.
Two National Natural Landmarks were established in 1986 for features, notable as the best examples of their kind, in and around Moses Coulee. The Great Gravel Bar of Moses Coulee, on the western edge of Moses Coulee where US Highway 2 crosses the coulee, is described as "the largest and best example of a pendant river bar formed by catastrophic glacial outburst floods that swept across the Columbia Plateau prior to the last Pleistocene glaciation." Withrow Moraine and Jameson Lake Drumlin Field "contains the best examples of drumlins and the most illustrative segment of the only Pleistocene terminal moraine in the Columbia Plateau biophysiographic province. ... They are also the only such glacial features in the world to show a clear geological relationship to catastrophic flooding." The canyon is named for Chief Moses (1829–99).
Geologic history
Geological setting
Moses Coulee cuts into the Waterville Plateau, which lies in the northwest corner of the Columbia River Plateau. The plateau is part of the Columbia River Basalt Group, a large igneous province that lies across parts of the states of Washington, Oregon, and Idaho in the United States of America. During late Miocene and early Pliocene times, one of the larger flood basalts ever to appear on the earth's surface engulfed about 163,700 km2 (63,000 mile2) of the Pacific Northwest, forming a large igneous province with an estimated volume of 174,300 km3. Eruptions were most vigorous from 17—14 million years ago, when over 99% of the basalt was released. Less extensive eruptions continued from 14—6 million years ago. These lava flows have been extensively exposed by the erosion resulting from the Missoula Floods, which laid bare many layers of the basalt flows on the edges of the plateau along the course of Moses Coulee.
The glacial history
Pleistocene glaciers advanced onto the Waterville Plateau, with the Okanogan Lobe of the Cordilleran Ice Sheet reaching as far south as the town of Withrow. Evidence for glaciation on the Waterville Plateau includes polished and striated bedrock, glacial erratics, drumlinoid topography, eskers, moraines, meltwater channels, and glacial till. The Withrow Moraine complex marks the maximum southern extent of the Okanogan Lobe, and a series of recessional moraine complexes represent retreating ice margins. Ice damming of the Columbia River formed Glacial Lake Columbia and Lake Spokane, larger lakes than Lake Roosevelt, which is currently backed up in the same location behind the Grand Coulee Dam.
Creation of Moses Coulee
A precursor to glacial-flood-cut Moses Coulee existed prior to the glacial floods as a drainage basin with a number of side streams, draining the southern portion of the plateau. These streams had combined into a canyon that drained to the Columbia near the current mouth of Moses Coulee. These side drainages are still visible today along the coulee walls as truncated streambeds, interspersed with gable-like highlands.