Morphological evolution of the North Fork Toutle River following the eruption of Mount St. Helens, Washington

The North Fork Toutle River (NFTR) has undergone extensive morphological changes following the catastrophic eruption of Mount St. Helens, Washington, in 1980, especially the upper reaches affected by a 2.5-km3 debris-avalanche deposit caused by the eruption. This paper reports analysis and interpretation of vertical adjustments to the thalweg long-profile at some 33 km river reaches redeveloped on the debris-avalanche deposit during the 30-year period since the eruption. The results confirm that adjustments in the upper part of the study reaches have generally been led by degradation, while that in the lower reaches have been led by aggradation, with the middle reaches acting as a hinge zone. Trends of change in the thalweg long profile and bedslope reveal that channel gradients have decreased nonlinearly through time and with distance downstream from the volcano. Values of stream power have decreased with time commensurately owing to reductions in slope and channel widening (while the bed has coarsened) so that rates of erosion of the debris-avalanche deposit in the upper NFTR have slowed to the point that the long profile, now perched and slightly steeper, is relaxing toward a new equilibrium or graded condition. Thirty-year relaxation paths for thalweg elevation were simulated at seven key cross sections using newly developed, comprehensive rate law models based on nonlinear decay in rates of morphological response to perturbation. The results indicate that both single- and multistep rate law models can simulate the observed records. Consequently, the rate law approach provides an effective method for studying and simulating morphological response of the fluvial system to a major, instantaneous disturbance, such as a volcanic eruption.