EVIDENCE OF MULTIPLE LATE PLEISTOCENE OUTBURST FLOODS, UPPER TANANA RIVER VALLEY, EASTCENTRAL ALASKA

Recent mapping in a proposed gasline corridor through the upper Tanana River valley, Alaska, identified considerable evidence of episodic large-magnitude outburst flooding. Morainal extent and the presence of large granitic erratics on bedrock cliffs along the Tanana River indicate that the Robertson River tributary glacier reached a maximum thickness of ~1400 ft and dammed the Tanana River during the early Delta (oxygen-isotope stage 6?) glaciation. Consequently, a huge lake was impounded upstream in the Tok basin and eventually overflowed through a pass at 1900-2000 ft elevation to re-enter the Tanana River ~2 mi below the ice dam. Outburst flooding produced flood-modified granitic knobs and high-level eddy gravels ~22 mi below the Robertson River ice dam, demonstrating that flood waters were >200 ft deep in that 2-mi-wide reach of the valley. Farther downriver, broad expansion fans invaded lower valleys of clearwater tributary streams in the southern Yukon-Tanana Upland, impounding numerous floodplain-marginal lakes.

Blockage of upper Tanana River during the Donnelly (oxygen-isotope stage 2) glaciation occurred at the junction of the Tanana and Robertson Rivers, where the ice barrier was thin and unstable. Outburst floods released by ice-dam failures produced numerous features downriver, including streamlined and aligned terraces, longitudinal and pendant bars, expansion fans, slackwater basins, flood-scoured bedrock knobs, and lowlands bounded by lengthy flood scarps. Except in proximal outwash or deep gravel pits, large granitic flood boulders were encountered only on a strath terrace of Donnelly age and in one post-Donnelly flood-scoured trough.

We propose that the prominent Clearwater Lake escarpment, which was formerly considered fault related, was initially cut by outburst flooding after the early Delta glaciation and prior to the Donnelly glaciation. Related escarpments extend down the Tanana River valley for ~96 mi.