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

Abstract

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 fl ow versus fl uvial transport), distributions of sediment transit times also change. We use fi eld 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 km 2 watershed in the Oregon Coast Range. In the downstream reach, debris fans impound fl uvial deposits; debris-fl ow, fi ne fl uvial, and coarse fl uvial deposits compose nearly equal parts of the valley fi ll; and fl uvial erosion evacuates deposits . Transit times have a sample mean of 1.22 ◊ 10 3 14 C yr and an exponential distribution, indicating uniform probability of evacuation from storage. In the upstream reach, valleyspanning debris jams impound debris-fl ow deposits composing >95% of the valley fi ll, which is routinely scoured by debris fl ows. Transit times have a sample mean of 4.43 ◊ 10 2 14 C yr and, if >100 14 C 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 14 C yr), but signifi cant volumes remain for millennia. Less than 20% of basin-wide denudation passes through these reservoirs, but the latter are still signifi cant buffers between hillslope disturbance and downstream aquatic habitat, especially for coarse sediment.