THE NATURE AND RATE OF ALLUVIAL FAN AGGRADATION IN A HUMID TEMPERATE ENVIRONMENT, NORTHWEST WASHINGTON

Abstract

Comparatively little is known about net aggradation on alluvial fans, despite fan construction wherever sediment-delivery rates from uplands exceed sediment-removal rates from receiving basins. In January 1983, 20 alluvial fans in the forested Cascade foothills, northwest Washington, experienced net aggradation in response to debris torrents and stream floods triggered by intense warm rains falling on antecedent snow. Five trenches were excavated to 5 m depth on the Mills Creek fan to place the 1983 event in temporal perspective. The deposits reflect normal streamflow, hyperconcentrated streamflow and debris torrent (flow) conditions. One trench revealed residues of 7 events since 1720 BP. Net rates of Holocene aggradation, based on sediments overlying late Pleistocene fluvioglacial deposits, average 0.42 m ka-1. Net rates for later Holocene time range from 2.17 m ka-1 since 1720 BP to 2.36 m ka-1 since 430 BP. These recent rates exceed the local value for the entire Holocene and rates for humid temperate...