Effective hydrological events in an evolving mid-latitude mountain river system following cataclysmic disturbance—a saga of multiple influences

Abstract:

A 2.5 km³ landslide at Mount St. Helens (USA) reset 30 km of upper North Fork Toutle River valley to a zero-state fluvial condition in 1980. Consequently, a new channel system evolved. Initially, a range of streamflows effectively eroded channels (to tens of meters incision, hundreds of meters widening) and transported immense sediment loads. Now, single, infrequent, large-magnitude or multiple moderate-magnitude events within a year are needed to accomplish substantial geomorphic change. More than a decade after emplacement, three large floods (two ≥100-year events; one ~10–25-year event) affected the valley within a twenty-year span (1996, 2006, 2015). The largest and smallest of these (1996, 2006) transported the most sediment by single floods since 1982, shortly after landslide emplacement; curiously, erosion and sediment transport in 2015 was not exceptional. The 1996 flood incised and widened 20 km of channel; the 2006 event incised 5 km of headwater channel by as much as 10 m. In contrast, the 2015 flood induced only modest local bank erosion.

Flood effectiveness on North Fork Toutle River is affected by both geomorphic and environmental factors. Strong coupling between the river and its tall bounding banks, local geologic and hydraulic conditions promoting threshold erosion of gravel-bed armor, and possibly a longitudinal gradient in stream power are important. But broader environmental factors also play a role. Principal differences among the three large flood events are: (1) the amount of snowpack on the volcano and valley floor; (2) storm path trajectories and rainfall distributions; and (3) generation of debris flows. Variations in snowpack and storm conditions affected rainfall and snowmelt runoff as well as sediment mobilization from slopes in channel headwaters. These variations in runoff affected flood magnitudes and durations in the upper valley. In 2006, abundant and intense rainfall on the snow-free volcano triggered debris flows that exceeded a channel erosion threshold leading to disproportionate channel incision. Factors invoked to explain effectiveness elsewhere—such as vegetation anchoring, strong channel–hillslope coupling, disparities between flood frequencies and perturbation relaxation times, and large variations in flood duration—have not been critical influences.