RIVER NON-RESPONSE TO A LARGE SEDIMENT PULSE: TIMESCALES OF SEDIMENT DELIVERY AND RIVER ADJUSTMENT BELOW MOUNT RAINIER (Invited Presentation)

Understanding and predicting physical river response to disturbance hinges on a knowledge of the timescales over which bed material is delivered and routed through a watershed. However, such timescales are often not well constrained. In 2006, an unprecedented rain event triggered major (~10⁶ m³) debris flows in many deglaciating headwater basins on the flanks of Mount Rainier, a stratovolcano in western Washington. These debris flows often eroded more proglacial sediment than all other events over the past 60 years combined. The expected response to this scale of sediment pulse included downstream aggradation, as rivers processed the newly delivered sediment. In contrast, we found that 1) material from the 2006 debris flows was predominately deposited within several kilometers of proglacial source areas, and largely remained there or was remobilized only a short distance over the subsequent decade; 2) nearly all headwater rivers experienced significant net erosion in the decade after 2006, with most erosion occurring well downstream of 2006 debris flow deposits. Rivers also experienced substantial (10⁵-10⁶ m³) net erosion over 10-30 km reaches during the 2006 storm. Together, results imply that upland rivers around Mount Rainier are still actively unloading valley fill deposits emplaced by glacial and volcanic processes over past millennia. Results further suggest that decadal-scale variability in coarse sediment delivery from Mount Rainier is unlikely to have significant impacts on upland river channels much downstream of debris flow deposition zones, both because abundant accommodation space and high thresholds for motion limit the remobilization of deposited sediment, and because even ‘extreme’ contemporary pulses are small in comparison to the glacial and volcanic pulses emplaced over millennial timescales.