Fine-time-scale DEM analysis illuminates long-term geomorphic legacy of 1980 Mount St. Helens eruption in North Fork Toutle River watershed

The 18 May 1980 eruption of Mount St. Helens reset the fluvial landscape of upper North Fork Toutle River (NFTR) valley. A massive landslide deposit filled the valley (45 m mean thickness over 60 km²) and hydrologically disconnected upper NFTR basin on several scales. It severed connection between the upper and lower basin, isolated Spirit Lake, and disconnected volcano drainages from the valley. As a result, a new drainage network evolved. Network development began soon after disturbance as the landslide deposit dewatered locally, but took nearly three years to fully integrate. Although the drainage system was well-established by the mid-1980s, channel corridors have since enlarged and beds coarsened. Channel evolution, and influencing factors, are revealed by fine-time-series DEM analysis.

Channel positions in upper NFTR basin are influenced chiefly by topography and secondarily by geology. Present drainage-network position bears a strong resemblance to the pre-eruption drainage network. Despite thick valley fill, a topographic low abuts a bounding, steep, valley-parallel ridge; both the pre- and post-eruption courses of upper NFTR occupy that declivity. Channel extensions, which drain basin headwaters, have been influenced by topography, geology, hazard mitigation, and happenstance. Differential resistance to erosion among landslide and assorted pyroclastic-flow deposits, location of a pre-1980 lava flow, emergent springs, emergency pumping of Spirit Lake, and random shifting across a debris fan at the base of the volcano's north flank have influenced proximal channel position.

Channel development initially evolved during a period of enhanced runoff when widely ranging discharges had high transport capacity; channels incised swiftly by tens of meters and widened by hundreds of meters. By the late-1980s, rates and magnitudes of erosion diminished, and geomorphic response evolved toward an event-driven regime, requiring moderate- to large-magnitude daily mean streamflow (>150 m³/s, as measured 40 km downstream) to do much geomorphic work beyond nibbling channel margins. These observations highlight the rapid and dramatic landscape changes that can accompany eruptions, provide insight on response process and timescale, and underscore the importance of monitoring long-term response.