LOCATION, EVOLUTION, AND MOVEMENT OF SEDIMENT BULGES IN THE MUDDY FORK OF THE COWLITZ RIVER, MOUNT RAINIER NATIONAL PARK

Sediment “bulges” at Mount Rainier National Park are aggradational expressions of glacial retreat and subsequent release of glacially transported sediment into park rivers. These convex zones of sediment accumulation initiate by rapid deposition in the form of mass wasting such as debris flows or hyperconcentrated flows, then are fluvially reworked downstream. Series of first, second, and third order bulges are emplaced by debris flows, fluvial processes, and large wood, respectively. Following the introduction of sediment bulges, braided river channels evolve at decadal scales by periods of unstable fanning and eventual main stem anchoring within or around bulges. This response to high influx of sediment liberated by glacial retreat contributes to complexity in the management of park resources by either causing river avulsion paths that often detrimentally coincide with road and trail settings, or alternately by creating more stable river corridors with anchored channel main stems. For example, Carbon River Road has experienced repeated problems that may be more productively addressed within the geomorphic context of sediment bulges. In 2016, geomorphologists at Mount Rainier completed a bulge mapping study and provided hazard mitigation strategies in the Carbon River area. We extend this study by examining sediment bulges in the Muddy Fork of the Cowlitz River in the southeastern section of the park. This study aims to contribute to the temporal perspectives that “bulge-ology” can add to applied resource protection. GPS mapping, aerial imagery studies, and GIS methods for analyzing cross channel gradients give insight to potential methods for tracking bulge movement over time. Understanding the gradual organization of near-source, transport-limited reaches as well as their interaction with old growth forests will help preserve visitor access and improve efforts to protect natural and cultural resources in Mount Rainier National Park.