Thomas E. Lisle | USGS Science

USGS2018

Geomorphic response of the Muddy River Basin to the 1980 eruptions of Mount St. Helens, 1980–2000

The 18 May 1980 eruption of Mount St. Helens produced a mosaic of primary landscape disturbances that decreased in intensity with distance from the volcano across the headwaters of Muddy River and its tributaries. Subsequent geomorphic responses were influenced by evolving hillslope and channel conditions that affected fluxes of water, sediment, and wood, as well as by an exceptional storm in February 1996. Sediment fluxes have generally decreased, but downed wood in channels remains episodically mobile. Geomorphic change and biotic activity in the basin continue to interact in terrestrial, riparian, and aquatic ecosystems and in many cases diversify ecosystem conditions.

Washington↗

USGS2000

Variability of bed mobility in natural, gravel‐bed channels and adjustments to sediment load at local and reach scales

Local variations in boundary shear stress acting on bed‐surface particles control patterns of bed load transport and channel evolution during varying stream discharges. At the reach scale a channel adjusts to imposed water and sediment supply through mutual interactions among channel form, local grain size, and local flow dynamics that govern bed mobility. In order to explore these adjustments, we used a numerical flow model to examine relations between model‐predicted local boundary shear stress (т j ( and measured surface particle size ( D 50 ) at bank‐full discharge in six gravel‐bed, alternate‐bar channels with widely differing annual sediment yields. Values of т j and D 50 were poorly correlated such that small areas conveyed large proportions of the total bed load, especially in sediment‐poor channels with low mobility. Sediment‐rich channels had greater areas of full mobility; sediment‐poor channels had greater areas of partial mobility; and both types had significant areas that were essentially immobile. Two reach‐mean mobility parameters (Shields stress and Q *) correlated reasonably well with sediment supply. Values which can be practicably obtained from carefully measured mean hydraulic variables and particle size would provide first‐order assessments of bed mobility that would broadly distinguish the channels in this study according to their sediment yield and bed mobility.

Water Resources Research↗