A multidecade analysis of fluvial geomorphic evolution of the Spirit Lake blockage, Mount St. Helens, Washington

Volcanic eruptions can affect landscapes in many ways and consequently alter erosion and the fluxes of water and sediment. Hydrologic and geomorphic responses to volcanic disturbances are varied in both space and time, and, in some instances, can persist for decades to centuries. Understanding the broad context of how landscapes respond to eruptions can help inform how they may evolve, and therefore provides context for managing and mitigating hazards associated with future volcanic and hydrologic events. Here, we assess the geomorphic evolution of the upper North Fork Toutle River valley, the valley most heavily affected by the Mount St. Helens May 18 and later 1980s eruptions. By doing so, we provide context for the landscape changes caused by the eruptions as they relate to potential hydrological hazards associated with Spirit Lake, an iconic landform at the northern foot of the volcano. The Spirit Lake basin was transformed by the cataclysmic 1980 eruption and had its outlet blocked. The analyses presented provide context for considerations of potential outlets for Spirit Lake, a landform which might be viewed as a “sleeping giant” on this landscape: a giant capable of causing catastrophic downstream consequences if water is released uncontrollably from the lake.