Toutle River debris flows initiated by atmospheric rivers: November 2006

In early November, 2006, an atmospheric river brought heavy rainfall and high freezing levels to the Pacific Northwest. Without snowpack to buffer the hydrologic response, the storm caused widespread landslides and debris flows in drainages sourced from every central Cascades volcano. At Mount St. Helens, in southwestern Washington State, intense rainfall in the crater of the volcano caused at least two mass failures with an aggregate volume of 4.5 million m3, which spawned a series of debris flows with velocities as great as 11 m/s. The debris flows incised Loowit Creek as much as 9 m and traveled 16 river km before transforming to hyperconcentrated flow via rapid dilution at the confluence of North Fork Toutle River (NFT), Castle Creek, and Maratta Creek (Figure 1). Reduced channel gradient downstream lowered velocity enough to transform to sediment-laden streamflow less than 40 km from the source, where suspended-sediment concentration (SSC) peaked at 177,000 mg/L. Heavy rain persisted for several days, producing over 100 cm of rainfall in upper NFT basin which caused immediate remobilization of surface deposits; half of the annual suspended-sediment discharge (SSQ) for water year (WY) 2007 was transported in 5 days, despite a relatively low 7-year flood recurrence interval. This analysis expands upon Major et al. (2005), Pitlick et al. (2007), and Olsen (2011) to document the largest suite of rainfall-triggered debris flows identified to date at Mount St. Helens. We investigate debris flow initiation, velocity, deposition, and downstream sediment flux in the NFT basin by integrating data from near real-time monitoring stations, remote sensing, terrestrial surveying, and fluvial sediment samples.