Linking Non-Linear Decay and Rapid, Threshold Responses to Long Term Sediment-Transport Rates at Mount St. Helens

The 1980 catastrophic eruption of Mount St. Helens buried the valley of the North Fork Toutle River (NFT) to a depth of up to 140 m. Integration of a new drainage network took place episodically by the “filling and spilling” of depressions formed during emplacement of the debris avalanche. Channel incision of 20-30 m occurred in the debris avalanche and pyroclastic flow deposits. Headward migration of the channel network resulted in complete integration within 2.5 years. Downstream reaches were converted from gravel-cobble streams with step-pool sequences to smoothed channels dominated by sand-sized materials. Subsequent channel evolution was dominated by channel widening with changes in channel width 60 to 100 times greater than changes in channel depth. Widening resulted in significant adjustment of hydraulic variables. For a given discharge, flow depths were reduced, relative roughness increased and flow velocity and boundary shear stress decreased non-linearly. These changes, in combination with coarsening of the channel bed with time resulted in systematically reduced rates of degradation upstream, aggradation downstream and sediment-transport rates through the 1990s. Vertical and lateral adjustments were characterized with non-linear decay functions.

High magnitude events generated from the upper part of the mountain, however, can cause rapid (threshold) morphologic changes. A rain-on-snow event in November 2006 caused up to 9 m of incision along a 6.5 km reach of Loowit Creek and the upper NFT. The event triggered a debris flow which cutoff tributary channels, redirected Step and Loowit Creeks (the latter was temporarily blocked), and forced enhanced flow volumes through the main channel. Coarse, armored bed materials were mobilized allowing for deep incision. Moderate flows still undercut 30 m-high pyroclastic flow deposits causing significant erosion. As the channels continue to widen, incision will attenuate non-linearly. Flows in the enlarged and over-widened sections have lower shear stresses, resulting in reductions in bank-toe erosion, associated mass failures and ultimately, non-linear attenuation of widening and sediment-transport rates. Empirically derived predictions of erosion trends are supported by numerical modeling using the Bank-Stability and Toe Erosion Model (BSTEM).