HOLISTIC SPATIAL AND TEMPORAL PATTERNS OF POST-ERUPTION EROSION AND DEPOSITION, SOUTH FORK TOUTLE RIVER, MOUNT ST. HELENS, WASHINGTON

The 18 May 1980 eruption of Mount St. Helens heavily impacted river valleys around the volcano. In general, post-eruption sediment loads declined swiftly; some returned to pre-eruption levels, others remain elevated in a state of quasi-equilibrium. In the South Fork Toutle River (SFT), however, sediment loads remain equivalent to those measured in the early 1980s. To understand this anomalous response, we used post-eruption, high-resolution (~ 3m), custom digital elevation models of the 50-km-long river valley from 1980, 2000, and 2003 to create elevation-change maps of the valley floor that show long-term spatial extents and amounts of post-eruption erosion and deposition. Between 1980 and 2000, the high, steep flanks of the volcano, composed of unconsolidated, readily erodible sediment prone to mass wasting, contributed significant sediment volume to the river’s headwaters. Within 15 km of the summit, long-term channel deposition dominated in response to a persistent influx of sediment from the volcano. In contrast, long-term channel erosion dominated along the valley’s distal 10 km, where hillslope sediment sources capable of channel replenishment are more limited and sediment removal outpaced upstream sediment input. Between 2000 and 2003, system-wide erosion dominated the fluvial response. The areas of greatest channel deposition and erosion were in the system’s headwaters and distal 10 km, respectively; in both reaches the valley is relatively wide and characterized by prominent pre-existing terraces. The valley’s middle 25-km reach is chiefly confined by bedrock, and sediment appeared to pass through with fewer areas of erosion and deposition. Persistent sediment input to the SFT headwaters from the volcano’s highly erodible flank may explain the river’s anomalous sediment load behavior. Unlike the North Fork Toutle River, having headwaters that are largely the surface of a massive debris avalanche and the volcano’s crater floor, and the Muddy River, having headwaters that abruptly flatten and flow across a broad plateau before reaching the main channel, the SFT headwaters convey sediment directly from the mountain’s flanks into the steep, upper reaches of the river valley and recharge easily accessed sediment sources.