Sediment yield from a forested mountain basin in inland Pacific Northwest: Rates, partitioning, and sources

Abstract

Sediment yield estimates, combined with information regarding sediment sources and partitioning (bedload vs. suspended load), can provide important insights into geomorphic character of the landscape of interest. This study reports such an analysis, conducted in a forested mountain basin in inland Pacific Northwest using reservoir deposits, ground surveys, and repeat LiDAR mapping. Our research generated three key conclusions. First, we estimated mean specific yield of clastic sediment from the study basin during the last century as ~51 Mg km−2 a−1. In the context of a data compilation from mountain basins in northwestern North America, this value was among the highest for inland basins, and among the lowest when compared with coastal basins. Second, a sediment source analysis provided valuable clues regarding the relative importance of various geomorphic processes operating in the system under study. These findings were consistent with our hypothesis that anthropogenic disturbances may be an important factor that conditions sediment yield from the basin. A mix of quantitative and qualitative evidence suggested that the estimated sediment yield value reflects a transient increase associated with past timber harvest, road construction, and large wood removal. Legacies of these disturbances appear to have the opposite effect on contemporary processes, limiting lateral activity of the channel as well as hillslope-channel connectivity. Third, our partitioning procedure revealed that bedload constituted approximately a third of the total clastic load exported from the basin. This finding suggests that, in this and similar fluvial systems, sediment yield recovery following major geomorphic disturbances could be protracted, as a considerable portion of mobilized sediment that moves as bedload is routed and evacuated from the basin for years or decades after rapid flushing of suspended material. Moreover, this finding indicates that the common assumption that bedload constitutes 10–20% of the total load can lead to underestimation in mountain basins.