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 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, and indicate that younger deposits are preferentially evacuated. A tributary debris fan at one confluence is much larger than fluvial terraces on the other tributary, but mean transit times (± σ) are similar for both sediment reservoirs: 1370 ± 2240 a and 1660 ± 2130 a for debris fan and fluvial 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 a, but mainstem deposits adjacent to the fluvial terrace have a relatively long mean transit time of 3870 ± 6720 a. Reservoir flux estimates indicate that most (>60%) of the debris 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 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.