Quiet Water and Great Floods: A Story of the Steamboat Rock Silt, Grand Coulee, Washington

Kate Logan

Whitman College, Walla Walla WA 99362

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The Okanagon Lobe of the Cordilleran ice sheet advanced as far south as upper Grand Coulee, Washington during the late Wisconsinan, but the timing of glacial advance and retreat in eastern Washington remains largely unknown. Maximal glaciation is constrained in the Okanagon valley between 17.24 and 13.5 14C B.P. by the radiocarbon dating of organics in glacial outwash, preservation of tephra, and timing of the Missoula Floods. Portions of upper Grand Coulee’s floor were previously mapped as Quaternary sands and gravels associated with the Missoula floods. More recent investigation has shown that the veneer of gravel is discontinuously underlain by a series of silty to sandy beds. The rhythmite outcrops, dubbed the Steamboat Rock Silt in honor of the basaltic monolith at the northern end of the coulee, are between 3 and 10 meters in height with individual normally graded beds ranging between 1 and 25 cm. The outcrop’s appearance varies between locations—some are more varve-like, while others display wavy bedding with sudden influxes of cross-stratified sand. A radiocarbon age of 16,680 +/- 70 14C B.P. was determined for the Steamboat Rock Silt from an organic-rich clay layer. Using this date in conjunction with the rhythmites’ apparent depositional environment, we believe that the Steamboat Rock Silt was deposited in a slack-water lacustrine setting created by the advance of the Okanagon Lobe into upper Grand Coulee before 16,680 14C B.P. Sometime after the glacier’s retreat from the coulee, we suppose one or more of the last Missoula Floods partially dissected the Steamboat Rock Silt resulting in the soft-sediment deformation and deposition of the gravel bars preserved along the edges of the coulee.