JÖKULHLAUPS FROM GLACIAL LAKE PUYALLUP, PIERCE COUNTY, WASHINGTON

Multiple glacial outburst flood events issued from glacial Lake Puyallup about 13,500 BP during deglaciation of the Puget Lowland. The jökulhlaups created a series of channels and deposited open-work cobbles across an extensive plain. The deposit, originally defined as flood gravels, was named the Steilacoom Gravel by Willis in 1898. Remnants of these channels and plains now cover an area of approximately 500 km2. Large Gilbert-type deltas formed at the mouths of these channels as the floods discharged into the lower elevation glacial Lake Russell near Steilacoom (67 m in elevation) and Sequalitchew (67 m) in the main trough of Puget Sound. Glacial Lake Puyallup initially discharged southwest through the Ohop channel, but lower gaps in the valley wall west of Orting (Muck Creek and Kirby channels) were uncovered as the ice front retreated. Additional retreat uncovered the Bradley (145 to 120 m), Clover Creek (115 to 88 m), and South Tacoma (105 to 73 m) channels southeast, south, and then west of Puyallup. Stepped terraces in some channels suggest sequential floods. Discharge from glacial Lake Puyallup must have been in the form of outburst floods to have caused the extensive scouring of the former fluted till upland visible in the modern landscape. Numerous remnants of the former fluted till upland are present across the Steilacoom Gravel plain. High-energy discharge events, such as those that formed the Clover Creek channel, scoured through Vashon till into Esperance Sand, leaving only a veneer of Steilacoom Gravel. Remnants of till-capped uplands are enclosed by the Bradley channel complex, most notably at Pierce College. Extensive drilling in the McCord Air Force and Ft. Lewis Army Base areas confirm that some of the remnant uplands are rooted drumlins surrounded by on-lapping Steilacoom gravel deposits. Numerous kettles and swales present on the Steilacoom Gravel surface presumably record melting of ice blocks carried in floodwaters. Lidar topography and computer-enhanced mapping allow recognition of more channels and better delineation of these channels. Recognition of jökulhlaups in Vashon glacial history suggests the likelihood that there could be other such open-work deposits and buried kettle topography in the stratigraphic record.