FLUVIAL LANDSCAPE RESPONSE TO AGGRADATION, DEBRIS FLOWS, AND LANDSLIDES: AN EXAMPLE FROM THE NISQUALLY RIVER AND VAN TRUMP CREEK, MOUNT RAINIER, WASHINGTON, USA

The form and function of a braided river is driven by sediment inputs balanced with stream flow. Sediment enters braided rivers through a variety of processes at Mount Rainier, including upland erosion, direct delivery by glaciers, rock fall, bank erosion, and landslides. When sediment production overwhelms stream flow, excess sediment aggrades (raises) the river bed, leading to increased floodplain area. Over time, this process enhances flooding and debris flow threats to infrastructure. Given that much of the park infrastructure is built adjacent to pro-glacial braided rivers, it is critical to understand the complex fluvial landscape in these areas, particularly since increased sediment delivery is a consequence of glacier retreat.

In the past two decades, the Nisqually River at the confluence of Van Trump Creek has seen dramatic changes in channel morphology. The area has been subject to numerous debris flows which deposited ~375,000 m³ of material in the active channel between 2001 and 2006. The debris flow deposit also constricted the river into a narrow channel against the left bank, adjacent to a cliff base. During a ~14 year RI storm in 2008, increased stream flows initiated a landslide that deposited ~18,000 m³ of debris into the river bed. This blocked the active channel for a short time and then concentrated the river into a narrow channel that widened as stream velocities and shear stresses increased. Analysis of repeat LiDAR data shows aggradation zones upstream, incision through the constriction, and aggradation downstream of the debris flow/landslide boundary.

Analysis of landscape response here is important for understanding the complex interactions of braided rivers, debris flows, landslides, and park infrastructure. For example, the 2005 debris flow had the unusual benefit of moving the river away from a primary Park road, decreasing the short-term risk of the road flooding. Incision into the debris flow deposit will increase hazards to the road as sediment is mobilized downstream and the river migrates back toward the road. This research can be used in other locations with similar geomorphic conditions to analyze how rivers route pulse-event sediment downstream, which has important connotations for infrastructure, visitor, and employee safety.