CHANNEL-FLOODPLAIN HYDROGEOMORPHOLOGY DOWNSTREAM OF AN EARLY HOLOCENE DAM-OUTBURST FLOOD DEPOSIT: LONG-TERM EFFECTS OF EXTREME SEDIMENT LOADING IN THE SYCAN AND SPRAGUE RIVERS, SOUTHERN OREGON

About 7,580 years ago, the Sycan River of southern Oregon experienced a dam-outburst flood that was equivalent to 25 times the flood of record. The flood emplaced sandy terrace and fan deposits totaling approximately 13.6 x 106 m3. Subsequent erosion and/or reworking of the deposits has removed an estimated half of their original surface area, resulting in increased loading of sediment to the Sprague River. Modern sediment transport is generally much greater in the Sycan River than in the Sprague upstream of its confluence with the Sycan, evidenced by higher rates of bedload (~ 5x) discharge in a reach actively eroding the flood deposits. Mapping and analysis of 0.5-m LIDAR bare-earth data show that in areas proximal to the fan, the floodplain is wider and contains more channels, and bars are larger and more numerous, than in other areas along the river corridor. The abandonment of channels occupied after the outburst flood has promoted a high degree of topographic dissection of the floodplain and a variety of floodplain habitats (spring channels, oxbows) with differing types and levels of hydrologic connection to the main channel. Many of these habitats support rare riparian plant communities, endangered fish species, or both. Generation of these habitats is linked to high rates of post-flood lateral channel movement, avulsion, and meander cutoff, although human impacts are also a major factor over the last 100 years. Therefore, through its interrelated effects on sediment loading, lateral channel movement, floodplain dissection, and bar accretion, the Sycan outburst flood has been a sustained, broad-scale control on channel-floodplain hydrogeomorphology and habitat quality of the Sprague and lower Sycan Rivers.