Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (4) Citing Articles via Google Scholar Google Scholar Articles by Lancaster, S. T. Articles by Casebeer, N. E. Search for Related Content GeoRef GeoRef Citation

Sediment storage and evacuation in headwater valleys at the transition between debris-flow and fluvial processes

¹ Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA

Sediment from landscape disturbance often enters temporary storage in valleys and evacuates over longer times, which in steeplands are poorly delimited. We hypothesize that, across process transitions (e.g., debris flow versus fluvial transport), distributions of sediment transit times also change. We use field surveys and extensive radiocarbon dating to assess the distribution of transit (residence) times through the proxy measurement of ages of bank deposits in two mainstem reaches of a 2.23 km² watershed in the Oregon Coast Range. In the downstream reach, debris fans impound fluvial deposits; debris-flow, fine fluvial, and coarse fluvial deposits compose nearly equal parts of the valley fill; and fluvial erosion evacuates deposits. Transit times have a sample mean of 1.22 x 10^{3 14}C yr and an exponential distribution, indicating uniform probability of evacuation from storage. In the upstream reach, valley-spanning debris jams impound debris-flow deposits composing >95% of the valley fill, which is routinely scoured by debris flows. Transit times have a sample mean of 4.43 x 10^{2 14}C yr and, if >100 ¹⁴C yr, a power-law distribution, indicating preferential evacuation of younger deposits and retention of older deposits. In both reaches, most sediment has short transit times (<600 ¹⁴C yr), but significant volumes remain for millennia. Less than 20% of basin-wide denudation passes through these reservoirs, but the latter are still significant buffers between hillslope disturbance and downstream aquatic habitat, especially for coarse sediment.

Key Words: sediment budget • residence time • debris flows • fluvial transport • radiocarbon dating • stratigraphy