MULTIFACETED GEOMORPHIC EVOLUTION OF A VOLCANICALLY DISTURBED RIVER SYSTEM VIEWED THROUGH THE LENS OF AN ALLUVIAL PHASE-SPACE

Deposition by a 2.5-km³ landslide during the 1980 eruption of Mount St. Helens reset the fluvial landscape of upper North Fork Toutle River (NFTR) valley. Since then, a new drainage network has evolved. An extensive suite of cross-section resurveys at multiple locations along a 20-km reach of river valley document channel evolution and associated geomorphic processes. We analyzed channel evolution using a novel alluvial phase-space metric that relates bed-elevation degradation or aggradation between consecutive surveys to increases or decreases in cross-section area. Our alluvial phase-space depiction is similar to previously proposed channel-stability and geomorphic covariance diagrams. But a key difference is inclusion of phase-space representing uniform degradation or aggradation relative to an initial reference channel condition—a condition of piston-like channel-bed behavior. Inclusion of such behavior relative to a reference condition allows tighter inference of geomorphic processes responsible for channel evolution.

Phase-space relations reveal more diverse patterns of NFTR channel evolution than originally described by a simple, linear-trajectory channel evolution model, which sequentially followed (1) channel initiation and incision, (2) aggradation and widening, then (3) episodic scour and fill with little change in bed elevation. Instead, vertical and lateral channel adjustments have been crucial processes intertwined throughout channel evolution (not occurring sequentially), though lateral adjustments became especially important in the mid-1990s. Channel evolution followed a distinctly nonlinear and non-sequential trajectory, and has migrated through several phase spaces involving various combinations of (1) degradation and aggradation with widening and narrowing, (2) bed-level fluctuations with little change in cross-section area, and (3) changes in cross-section area with little change of bed elevation. Persistent lateral adjustments causing channel and valley widening, and reworking of the channel bed, presently drive elevated sediment delivery from this basin. Until valley-floor width greatly exceeds the active channel-migration footprint, and/or channel banks and valley walls become armored, elevated sediment loads are likely to persist.