ALLUVIAL STRATH TERRACES DEVELOPED ALONG THE SPOKANE AND COLUMBIA RIVER DRAINAGES IN RESPONSE TO LOWERING LEVELS OF THE POST-MISSOULA FLOOD GLACIAL LAKE COLUMBIA

One of the most conspicuous geomorphic features along the Spokane and Columbia River drainage basin are the multiple terraced landforms which postdate the 13,400 ¹⁴C BP (16,130 calibrated) Missoula outburst megaflood. Six prominent late Wisconsin terraces are preserved downriver from Spokane at the elevations of 730-740 m, 610-670 m, 510-535 m, 475-485 m, 430-450 m, and 405-415 m. The highest terrace, at 735-745, corresponds to the upper surface water elevation of glacial Lake Columbia and reflects ongoing wave erosion during a residence period of many thousands of years prior to the Missoula outburst flood. The others are typical strath terraces developed in response to a series of base-level drops in the surface levels of glacial Lake Columbia. Intracanyon bedrock knickpoints are not a factor in terrace development below Spokane Falls.

There are several possible explanations for the lowering lake elevations, but the most probable cause may have to do with modifications of the constraining blockages, either at the Okanogan ice lobe or erosion of the head of the Grand Coulee. If the glacial lake was flowing across the gap at the head of the Grand Coulee, then headward erosion and sudden removal of each individual lava flow along the less resistant contact zone could account for terraces developed above 430 m.

The alluvial stratigraphy associated with the strath terraces is distinctive and fairly repetitious, with a sharp lithologic break between the underlying glaciolacustrine sediment, including rhythmites. The basal deposit consists of an unsorted to poorly sorted, subrounded, clast-supported polymictic cobble to boulder gravel several meters thick. The clasts are dominated by plutonic and metamorphic quartz-rich clasts and a minor basalt component, as opposed to flood lacustrine boulder beds which are wholly dominated by basalt clasts within the confines of GLC. The alluvial gravel is often overlain by unlayered to poorly layered sheet sand and occasional dunes. Slope wash of flood/lacustrine pebble granule gravel blankets some of the terraces, giving the impression of a flood sequence rather than an alluvial one. Strath terrace deposits may have been influenced by floods during river incision, but they are not part of the Missoula flood sequence in which they are superimposed.