RIVER INCISION AND STRATH TERRACE FORMATION AS A RESULT OF SHIFTS IN SEDIMENT RETENTION

The pace of river incision is thought to be fundamentally controlled by the interplay between river slope, water discharge, and sediment supply. Observations in the Pacific Northwest suggest that not only does the supply of sediment affect river incision rates, but the retention of sediment on the riverbed also controls the tempo of river incision. In this study, we date and map strath terraces in Pacific Northwest rivers to investigate the role of sediment retention. Straths are beveled during periods of lateral planation when incision rates are low and abandoned when fluvial incision rates increase; as a result, strath terraces are often used to interpret past changes in river incision. We observed several strath terraces less than 5 meters above the current bedrock channel in the central Cascade Range, Olympic Mountains, and Willapa Hills in Washington State. Radiocarbon ages on these terraces indicate they are less than 100 ybp, suggesting a dramatic increase in river incision rates at this time. Documented rapid removal of woody debris is coincident with the timing of increased incision rates and subsequent strath abandonment. After wood removal, sediment once held in place by log jams and deposited in low velocity zones was flushed from the system, exposing channel bedrock to abrasion. Subsequent rapid lowering of the bedrock surface in these rivers is observed by Stock et al. (2005) to be on the order of 10-100 mm yr^-1. We predict that changes in sediment retention would also occur due to valley-blocking or constricting landslides or changes in the abundance and type of bank-stabilizing vegetation, and that these changes can result in rapid river incision and strath terrace abandonment. Our results suggest that sediment retention is an important control on the tempo of river incision, and thus should be evaluated alongside the magnitude of sediment supply, water discharge, and river slope when interpreting past changes in river incision and landscape evolution.