LONG-TERM HYDROLOGIC AND SEDIMENT DYNAMICS: DISENTANGLING EFFECTS OF FOREST HARVEST, WESTERN CASCADES, OREGON

The hydrologic and geomorphic effects of forest harvest have been the focus of scientific research for over a century. Since the relation between sediment transport and discharge typically follows a power law, small increases in discharge can translate into large increases in sediment transport. But interpreting the geomorphic effects of streamflow increases is confounded by the fact that forest treatment also influences the sediment supply of a watershed, making it difficult to isolate the streamflow effect. We explored this problem using 50 years of discharge and sediment data from two small, well-studied, paired-watersheds in the H.J. Andrews Experimental Forest in Oregon. We used regressions of paired storm peak streamflows to predict synthetic hydrographs that represent streamflow response of the clearcut watershed in the absence of treatment as a function of both flow magnitude and time. Using various combinations of measured or synthetic hydrographs with pre- or post-treatment sediment rating curves, we calculated 1) measured sediment yield, 2) background sediment yield, and 3) sediment yield due to flow increases alone, allowing us to estimate the relative importance of changes in discharge and increased sediment supply. The increased supply of sediment that accompanied the clearcutting accounted for the majority of the post-treatment sediment yield, while flow increases produced only minor suspended sediment and bedload transport above background levels. Both discharge and sediment relationships recover exponentially, with suspended sediment yield returning to background levels after less than 20 years. Continued elevated bedload, especially in wetter years, suggests that forest harvest may stimulate new and persistent coarse sediment sources that continue to contribute to enhanced sediment yield even 40+ years following harvest.