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In steep headwater basins of the Oregon Coast Range (OCR), debris flows episodically deliver material from low-order tributary basins to larger catchments. Much of this material is stored in valleys and gradually removed by fluvial processes. Quantifying the transfer of material from hillslopes to mainstem channels is essential in understanding the routing of sediment through these headwater systems. This study employs a dense radiocarbon sampling strategy to characterize the transit times of material delivered from tributaries to Cedar Creek and Golden Ridge Creek in the central OCR. As proxies for transit-times, sixty-eight age estimates from 14C were removed from randomly assigned locations in the banks of tributary and mainstem channels at the channel confluences. Transit time distributions inferred from these age estimates have characteristic double-exponential shapes for both tributary deposits and imply mean transit times of 1240 14C yrs B.P. for Cedar and 1510 14C yrs B.P. for Golden Ridge Creek sites. This type of distribution indicates that younger deposits are preferentially evacuated such that, while most material moves through the tributary basins rapidly, there is a slower-moving component that can be stored in tributary deposits for millennia. Reservoir flux estimates derived
from the inferred mean transit times indicate that most (>66%) of the sediment yield of the Cedar Creek tributary is stored in the fan for some time but, for the Golden Ridge Creek deposit, only a small part (3%) of the basin tributary basin yield is stored there. These results indicate that debris fans similar to that at Cedar Creek play an important role in the transition between debris-flow and fluvial processes by buffering higher-order streams from episodic debris-flow inputs. In both cases, however, these depositional features retain effects of disturbance lasting for millennia. |
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