LATE HOLOCENE FLOOD HISTORY OF THE TANANA RIVER, ALASKA, USA

The Tanana River basin in central Alaska drains both the north slope of the Alaska Range and the south slope of the Yukon-Tanana upland. A sequence of historic and prehistoric flood deposits of the Tanana River is preserved in a small bedrock-sheltered slough near Fairbanks. Examination of these deposits using a suite of radiometric dates, microstratigraphic observations, and granulometric statistics suggests that large changes in flood frequency occurred during the late Holocene. Three major lithostratigraphic units are observed: (1) thick crossbedded, pedogenically unaltered alluvial silty sands which were deposited between 3000 and 2000 yr BP, recording an interval of large floods; (2) a series of thin silty beds and paleosols formed after 2000 yr BP during an interval when large floods were uncommon; and (3) a sequence of sand units recording large floods during the last several hundred years. Flood frequencies appear to have changed in response to regional climate changes, with more frequent flooding occurring during times of widespread alpine glaciation and increased storminess.