Renewed incision and complex response of the North Fork Toutle River following the eruption of Mount St. Helens in 1980

Abstract

The upper North Fork Toutle River (NFTR) has been evolving in a complex way since being disrupted by a 2.5-km3 debris avalanche deposit during the catastrophic eruption of Mount St. Helens in Washington State, USA in 1980. In this study, we improved the alluvial phase space diagram (APSD), which classifies channel adjustment into different stages based on bed elevation and cross-sectional area changes, and applied it to the NFTR at annual time scale in the 1980s. The results showed that in the early 1980s, the upstream narrow single-threaded reaches were degraded, while the downstream wide anabranching reaches were aggraded, with the middle transitional reach acting as the hinge zone. Gradually, the upstream channel reaches armoured, widened, bed slope and excess stream power decreased in the late 1980s, and quasi-equilibrium occurred locally and during short time periods. Sediment load decreased at the lower reaches, resulting in bed degradation and a transition from sediment sink to source. This reversal seemed to have triggered renewed incision, which migrated upstream. The renewed incision was characterised by Stage 1/2 (bed incision with limited bank erosion) followed by Stage 2 (degradation and widening) in the improved APSD. The degradation rate accelerated in the 1990s, especially due to the big flood in 1996, and then slowed down after ∼2000. Channel adjustment rates of the renewed incision were much smaller compared to those of the initial rapid degradation. The change rates of cross-sectional area and valley width decreased by ∼58% and ∼85%, respectively. The comparison between this study and existing studies highlights the significant role played by the time scale, at which the APSD is applied to study channel morphology adjustment.