Sediment reservoirs at mountain stream confluences: Dynamics and effects of tributaries dominated by debris-flow and fluvial processes

Radiocarbon age estimates (N = 68) from bank, terrace riser, and in-channel materials sampled from random locations near two channel confluences, a debris-flow-dominated tributary to Cedar Creek, and a fluvially dominated tributary to Golden Ridge Creek in the Oregon Coast Range, are proxies for sediment transit times through tributary and mainstem sediment reservoirs separated from one another by incised bedrock risers. Geomorphic, volumetric, stratigraphic, and sedimentologic data aided reservoir characterizations. Inferred transit-time distributions for tributary deposits are right-skewed and heavy-tailed, indicating preferential evacuation of younger deposits. The debris-flow fan is much larger than fluvial terraces on the other tributary, but mean transit times (+/-sigma) in both reservoirs are similar: 1370 +/- 2240 yr and 1660 +/- 2130 yr for fan and terrace deposits, respectively. Whereas tributary deposits are much larger than mainstem deposits at both sites, mainstem deposits adjacent to the fan have a relatively short mean transit time of 442 +/- 491 yr, but mean transit time in mainstem deposits adjacent to the fluvial terrace is much greater: 3870 +/- 6720 yr. Reservoir flux estimates indicate that most (>60%) of the debris-flow fan tributary's sediment yield enters fan storage, but only a small part (3%) of the fluvial tributary's yield enters storage at the confluence. Debris flows from the debris-flow fan tributary apparently promote both greater storage of mainstem sediments and more rapid unbiased evacuation of mainstem deposits, whereas old mainstem deposits adjacent to the fluvial tributary have a much greater probability of preservation.