Seasonal Shear Stress Explains Presence/Absence of Endangered Freshwater Mussels: Implications for Stream Restoration Design on a Large River
Bob Hawley
Sustainable Streams
Louisville, KY
Authors: Bob Hawley, Kurt Cooper, Abi Raetz, Monte McGregor, Nora Korth, Jessie Boles, Mitchell Boles, Bethany Mulhall, Rob Lewis, and Andrew Stump
Centuries of beaver extirpation, deforestation, mill dams, floodplain drainage, and channelization have made North American rivers more disconnected from their floodplains, trapping more energy in their channels, degrading aquatic habitat, and making the streambed more prone to erosion. Rivers naturally self-adjust to excessively erosive hydraulics via systematic downcutting, bank erosion, channel widening, bar building, and the gradual recovery of geomorphic equilibrium with well-connected vegetated benches that dissipate hydraulic energy and restore a more natural disturbance regime. The channel evolution process can last centuries depending on the geologic and climatic setting, and the extents and durations of anthropogenic disturbances.
Life histories of two endangered freshwater mussels, the snuffbox and fanshell, suggest they depend on the natural disturbance regime. Early juveniles release from their host fish in early summer by attaching to suspended sand particles that settle out in the streambed. From that point the young juveniles need approximately four months of streambed stability prior to growing large enough to migrate to more stable substrate such as cobbles or large wood. In their native ranges, summer through early autumn typically coincides with mild flows and a relatively stable streambed.
Combining the life history of these endangered mussels with the channel evolution process, we hypothesize that a potential prerequisite to supporting fanshell and snuffbox populations is a geomorphically recovered channel/floodplain corridor that sufficiently dissipates its hydraulic energy to maintain a seasonally stable streambed during typical (non-hurricane) summer/autumns. To test this hypothesis, we conducted mussel and geomorphic surveys, and hydrologic, hydraulic, and shear stress analyses in a ~3-mile reach of the Rolling Fork River in Kentucky, just upstream of the formerly documented range extents of these species.
Our analyses documented increasing mussel diversity, including the presence of endangered fanshell/snuffbox, with increasing width of seasonally-stable streambed and bench habitat that remains moist during low flows.
These results have directly informed our stream restoration design approach: preserving geomorphically-recovered low-energy reaches that support fanshell and snuffbox, and laying back banks in chronically unstable reaches that lack T&E mussels to restore the low-energy environment that mimics the geomorphic equilibrium and seasonal shear stresses of reference-like reaches.
About Bob Hawley
Robert J. Hawley is a licensed Professional Engineer in six states and the Principal Scientist at Sustainable Streams in Louisville, KY, a company dedicated to stream and watershed science, service, and solutions. Dr. Hawley uses both his professional and research projects to advance the state of the stream industry by making stream restoration and stormwater management more ecologically and socioeconomically sustainable. He has more than 120 miles of stream restoration experience and watershed-scale stormwater management experience in watersheds that collectively drain over 70,000 square miles, with more than $100M in constructed projects. Dr. Hawley has authored ~30 peer-reviewed publications with ~900 citations and thousands of reads and downloads.
https://www.linkedin.com/in/bob-hawley-81a37612/
Google scholar: https://scholar.google.com/citations?user=hsdwLr4AAAAJ&hl=en
Resources: https://www.sustainablestreams.com/resources