Go With the Flow: Impacts of High and Low Flow Conditions on Freshwater Mussel Assemblages and Distribution




Cushway, Kiara C.

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Freshwater mussels are a highly imperiled fauna with a broad diversity in North America. Due to their imperilment, understanding how climate change and natural and anthropogenic flow conditions impact mussels is an important aspect of creating sustainable and effective management and conservation goals. The goal of this study was to understand how high and low flow conditions (i.e., flooding and drought) impact freshwater mussel distribution in a stream in central Texas. This region is facing mounting concerns regarding water scarcity and altered hydrologic regime. Specifically, I sought to understand 1) the capacity of perennial or intermittent pools with various habitat conditions to serve as ecological refuges for freshwater mussels during drought, and 2) the role of hydraulic variables like shear stress, Froude number, stream power, and depth in influencing the composition and distribution of freshwater mussel assemblages during variable flow conditions. For objective one, field surveys in an intermittent segment of the San Saba River in central Texas during 2021 and 2022 indicated that perennial pools with characteristics that buffer the influence of emersion and thermal stress may serve as important refuges for freshwater mussels during drought and drying conditions. However, the low abundances of mussels in this segment may provide a bleak glimpse into the future as climate change and human water usage exacerbate drought conditions in streams in drier regions. For objective two, hydraulic conditions in a 20 km segment of the upper San Saba River were estimated using a 2D HEC-RAS model. Low flow (0.7x median daily flow) and moderate to extreme flood conditions (10 to 50 % exceedance probability) successfully predicted mussel presence across 200 sites between 67 and 79 % of the time using Random Forest modeling. In addition, hotspots of freshwater mussel richness and diversity were observed in flow refuges with lower shear stress, stream power, and Froude number. Species-specific responses to hydraulic conditions also showed that different mussel species are disparately constrained by hydraulic conditions. This is the first study that systematically examined ecological refuges for freshwater mussels during drought, and by combining a hydraulic model with extensive survey data, this study also provides valuable insights into the association of mussel distribution and hydrodynamic conditions in flashflood and drought-prone streams.



freshwater mussels, drought, flooding, hydraulic modeling


Cushway, K. C. (2023). Go with the flow: Impacts of high and low flow conditions on freshwater mussel assemblages and distribution (Unpublished thesis). Texas State University, San Marcos, Texas.


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