Changing Views of the River
Abstract
Luna Leopold (1915-2006) was a pioneer in quantitative, process-based field studies of how landscapes work. Underlying his research was a question that remains fundamental in fluvial geomorphology: What controls the form and temporal trajectory of a stream channel? The question can be asked in reverse: Is the history of a stream recorded in its form? To answer these questions, geomorphologists have followed a metaphorical braided river with multiple intellectual channels. Beginning with G. K. Gilbert and W. M. Davis in the 19th c., this branching genealogy has been influenced heavily by Leopold and collaborators since the 1950s. Among their best-known works is a compilation of data that identified correlations among channel geometry, hydraulics, and discharge. These scaling relations indicate apparent equilibrium behavior, but don't answer the fundamental question of what controls channel form. Notable progress was made with the finding that gravel bed alluvial rivers are approximately "threshold channels". For threshold channels, if boundary shear stress exceeds a threshold value (critical boundary shear stress), the width and/or depth of the channel are adjusted by erosion such that the ratio of the stresses generally is <1.2.
Continuing along the intellectual strands established by Leopold and others, we investigate how channels form in response to dam breaching and base level fall along mid-Atlantic US streams, a region so heavily impacted by historic damming and now dam removal that it can serve as a "mega-laboratory" to test channel adjustment. Our measurements of post dam-breach erosion rates along streams with mud-rich banks upstream of recently breached dams indicate initial rapid erosion, driven mostly by mass movement, followed by relatively low erosion rates in comparison to dam removal sites along western streams with generally coarser sediment. We propose that critical boundary shear stresses in cohesive silt are sufficiently high to exceed bankfull shear stresses. After decades of post-dam breach bank erosion and channel widening, erosion rates diminish to a low background value determined by frequency of freeze-thaw cycles that weaken banks. Width-depth ratios increase during the decadal period of channel adjustment, such that hydraulic geometry relations do not apply.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP43B..02M
- Keywords:
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- 1824 Geomorphology: general;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY