Overview of Geology , Hydrology , Geomorphology , and Sediment Budget of the Deschutes River Basin , Oregon Jim

Within the Deschutes River basin of central Oregon, the geology, hydrology, and physiography influence geomoqhic and ecologic processes at a variety of temporal and spatial scales. Hydrologic and physiographic characteristics of the basin are related to underlying geologic materials. In the southwestern part of the basin, Quaternary volcanism and tectonism has created basin fills and covered and deranged the surficial hydrologic system, resulting in a relatively low-relief lava-covered landscape with runoff emerging largely from extensive groundwater systems fed by Cascade Range precipitation. The remarkably steady flows of the entire Deschutes River, as depicted in annual and peak flow hydrographs, are due primarily to buffering by the extensive groundwater system of this part of the basin. The eastern part of the basin is primarily underlain by Tdary volcanic, volcaniclastic, and sedimentary rocks that have weathered into dissected uplands with generally greater slopes and drainage densities than of that of the southwestern part of the basin. Surficial runoff is more seasonal and less voluminous from this more arid part of the basin. The northern part of the basin has been sharply etched by several hundred meters of late Cenozoic incision, resulting in the greatest relief and drainage density of anywhere in the basin. For large floods, such as those of December 1 %4 and February 1996, more than half of the peak flow at the mouth of the Deschutes River is derived from the northern part of the basin. Modem sediment yield for much of the Deschutes River basin, as determined from reservoir surveys, is exceptionally low and is related to regional slope and drainage p r o v e s . Broad-scale sediment budget calculations indicate that more than 50 percent of the sediment produced in the Deschutes River basin produced under modem, pre-impoundment, conditions is h m the northern part of the basin. The= is ample evidence, however, of much greater sediment yields and large pulses of downstream sediment delivery during Quaternary episodes of volcanism and glaciation.