RELATIVE INFLUENCE OF LANDSCAPE VARIABLES, DISCHARGE, AND FOREST MANAGEMENT ON SEDIMENT YIELDS IN TEMPERATE MOUNTAIN CATCHMENTS

Suspended sediment is important for aquatic ecology, but is difficult to predict and regulate. This difficulty is due to the high spatial and temporal variability in suspended sediment yield, which is partly influenced by streamflow and catchment characteristics. We quantified the relative influence of annual discharge and site physiography on suspended sediment yields for ten temperate mountainous watersheds of the US Pacific Northwest (H.J. Andrews LTER) ranging in size from 8.5 to 6,242 ha over a 50-year period. We found that cumulative annual discharge (partial R² = 0.30) and setting (partial R² = 0.62) explained the majority of variation in suspended sediment yield (R² = 0.71). Despite forest management history in over half of the watersheds, this factor did not explain differences in yield. Instead yield was a function of the degree of variability of the watershed slope derived from high-resolution LiDAR. There were, however, large-yield outliers not explained by the linear models. Outliers typically followed large storm events and were more frequent in watersheds with specific physiographic characteristics. Watersheds with the highest yields and most vulnerable to either anthropogenic or natural disturbance had both high mean and high standard deviation of watershed slope, which should therefore be considered in sediment yield predictions and management.