A LATE-GLACIAL OUTBURST FLOOD FROM GLACIAL LAKE CARBON, WASHINGTON STATE, USA

Blockage of the Carbon River by the Vashon Stade Puget Ice Lobe (as mapped by Crandell, 1963) northwest of Mount Rainier, Washington, created Glacial Lake Carbon. The lake lowered progressively as the ice retreated through successive stages, from about 1500’ asl at the maximum, to 1470’ and then 1300’ for the first two closely spaced recessional stages, which are marked by kame terraces. This recession was probably due to a combination of climatic forcing and subglacial fluvial erosion by water in hydraulic continuity with the lake that was escaping through the permeable kame terrace sediments. We hypothesize that the rapid release of the majority of the lake waters between the first and second recessional positions created a catastrophic flood as the Puget Lobe retreated. We estimate that the lake contained about 5x10⁷ m³ of water at its maximum depth, and that the peak discharge was on the order of 10³ m³/sec. This flood may have been much larger than other floods from similar Cascade Range drainages blocked by the Puget Lobe because the Carbon River valley was being fed by runoff from Mount Rainier. Portions of the andesite boulder-rich Lily Creek Formation (1 Ma) were undercut and remobilized by the flood. The discontinuous train of boulders deposited by the flood extends from the Fox Creek valley, northeast of Electron, to the southwest, across, and along the general trend of the Ohop Creek valley. The present day Tanwax Valley, marked by Tanwax Creek and a line of lakes, and extending from slightly east of Lake Kapowsin southwest to its confluence with the Nisqually River, marks the southernmost extent of the receding Puget Lobe at the time of the outburst flood. This ice margin, therefore, acted to constrain the northernmost extent of the floodwaters. The flood had a gradient of .007 from its source to this region, 34 km downstream, and carried boulders up to 2m across at least this far downstream.