Channel change and flooding, Skokomish River, Washington

Abstract

Analysis of 45 years of cross-section data documents changes in channel width and bed elevation along the Skokomish River, Washington. The bed of the South Fork Skokomish incised over 1 m between 1940 and 1964, although both prior and subsequent to this period the mean bed elevation oscillated as much as 0.8 m with almost no cumulative change. In contrast, the mainstem Skokomish channel bed aggraded nearly 0.5 m between 1939 and 1944, oscillated at amplitudes up to 1 m with little net change from 1945 to 1964, and aggraded over 1.3 m between 1965 and 1997. In the late 1920s, prior to the onset of gaging station records, damming of the North Fork significantly reduced flow in the mainstem Skokomish. From the 1930s to the 1990s, peak discharge data for both the South Fork and the mainstem indicate no net increase in peak flows. Despite the reduced discharge from dam construction in the 1920s and no increase in peak flows during the following years, the frequency of overbank flooding in recent decades has increased on the floodplain of the mainstem. Systematic written descriptions and aerial photographs of the catchment from 1929 to 1992 document land use, including timber harvesting, road construction, and in-channel debris removal. The timing of changes in channel width and elevation imply that debris removal may have triggered periods of degradation and that near-channel and headwater land use potentially elevated the sediment supply to both reaches. Although direct land use causality is difficult to constrain, progressive reduction of channel conveyance in the mainstem as observed in USGS gage height trends does indicate that increased flooding on the mainstem Skokomish River resulted from aggradation, without an increase in peak discharges.