MULTI-DECADE HYDROGEOMORPHIC EVOLUTION OF THE SPIRIT LAKE BLOCKAGE, MOUNT ST. HELENS, WA

The 1980 eruption of Mount St. Helens profoundly disturbed the hydrology and geomorphology of the upper North Fork Toutle River, disrupting hydrological connection within the watershed and effectively resetting the drainage network. Current focus on present and future management of Spirit Lake has renewed interest in determining how channel networks may develop and respond in the un-incised portions of the 1980 deposits that currently dam Spirit Lake from drainage into the North Fork Toutle River (the blockage). Past work documenting post-eruption discharge patterns and channel cross-sections has shown an initial period of severe hydrologic disequilibrium, channel incision, and exceptional sediment mobility followed by a sediment-discharge regime that is now more event-driven and strongly influenced by lateral erosion of the existing network. Here, we integrate discharge records, channel cross-sections, previously published decadal digital elevation models of difference, and new photogrammetry-derived digital surface models from 1980–2018 to examine the relationships between post-eruption hydrologic and geomorphic trajectories. Our work verifies past findings of an initial pulse of disequilibrium related to network initiation and integration and a subsequent, less active hydrogeomorphic period, punctuated by large changes related to hydrologic extremes (e.g., 1996, 2006, and 2015 regional floods). In the early period of network initiation and integration, we document the influence of gross valley topography for setting overall spatial distribution of the network, and the potential for geologic variation in the blockage to influence channel positions; later channel refinement appears less sensitive to geology. Our observations have important implications for future management decisions, pointing to risks in routing water over previously uneroded deposits, even 39 years after the 1980 eruption.