Multidecadal geomorphic evolution of a profoundly disturbed gravel-bed river system—a complex, nonlinear response and its impact on sediment delivery

Journal of Geophysical Research: Earth Surface
By: , and 

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Abstract

A 2.5-km3 debris avalanche during the 1980 eruption of Mount St. Helens reset the fluvial landscape of upper North Fork Toutle River valley. Since then, a new drainage network has formed and evolved. Cross-section surveys repeated over nearly 40 years at 16 locations along a 20-km reach of river valley document channel evolution, geomorphic processes, and their impacts on sediment delivery. We analyzed spatial and temporal changes in channel morphology using two new metrics: 1) a shape index that defines the degree of U-shaped or V-shaped valley geometry; and 2) an alluvial phase-space diagram that relates bed degradation or aggradation between consecutive surveys to increases or decreases in cross-section area. Metric relations reveal more diverse channel evolution than originally described by a simple, linear-response model of sequential channel initiation and incision; aggradation and widening; and subsequent episodic scour and fill with little change in bed elevation. Instead, vertical and lateral adjustments have been crucial processes intertwined throughout channel evolution. Channel evolution has followed a distinctly nonlinear and non-sequential trajectory, migrating through several phase spaces and involving varied 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 channel widening and reworking of the channel bed presently drive elevated sediment delivery from this basin. Elevated sediment delivery is likely to persist until valley-floor widths greatly exceed that of the channel-migration corridor, and/or channel banks and valley walls stabilize.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Multidecadal geomorphic evolution of a profoundly disturbed gravel-bed river system—a complex, nonlinear response and its impact on sediment delivery
Series title Journal of Geophysical Research: Earth Surface
DOI 10.1029/2018JF004843
Volume 124
Issue 5
Year Published 2019
Language English
Publisher American Geophysical Union
Contributing office(s) Volcano Science Center
Description 29 p.
First page 1281
Last page 1309
Country United States
State Washington
Other Geospatial North Fork Toutle River Valley