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 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 × 10314C 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 × 10214C 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.

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