DID THE LATE WISCONSIN GLACIAL LAKE COLUMBIA FILL WITH SEDIMENTS GENERATED BY GLACIAL LAKE MISSOULA OUTBURST FLOODS?

Glacial Lake Columbia was formed by the late Wisconsin Okanogan ice lobe which blocked the Columbia River, with a maximum lake surface elevation between 715 and 730 meters. A study of the lake and flood deposits from Spokane Valley westward to the Sanpoil Arm of Lake Roosevelt revealed a complex history of deposition and erosion within the confines of the Lake Columbia basin. We contend that Lake Columbia was filled by outburst flood sediments from glacial Lake Missoula and that subsequent removal of these sediments occurred by a combination of floods and downcutting as a result of the spasmodic breaching of the upper Grand Coulee.

Lacustrine varved sediments and flood sand-silt rhythmites occur above the surface of Lake Roosevelt from 393 to 433 meters. Pebbly granule gravels filled the central portion of Lake Columbia from 433 to 715 meters. Contemporaneously, sand rhythmites and varved sediments were deposited within the quieter tributaries peripheral to the main channel of Lake Columbia.

We also propose that the Okanogan ice lobe and the Purcell ice lobe that dammed Lake Missoula were contemporaries. The initial deposition of varved sediments in Lake Columbia occurred during the initial filling of the lake. The classic rhythmic flood sands interspersed with varved layers, attributed to the relatively frequent outburst floods from Lake Missoula, was probably due in part to Lake Columbia waters floating the Purcell lobe ice dam. However, the filling of Lake Columbia with numerous layers of pebbly granule gravels has not been fully appreciated until now.

Spasmodic breaching of the upper Grand Coulee drainage during late flood and post-flood times caused entrenchment of river systems through the flood-lacustrine sediments. River meanders formed the prominent paired terraces capped by riverine gravels, in response to the ever-lowering Lake Columbia surface level. If our contentions are correct, then almost 90% of the unconsolidated flood-lacustrine sediments that filled the Lake Columbia basin were erosionally removed over the last 13,000 years.