The Duck River is a remarkable national resource, boasting outstanding biodiversity (Ahlstedt et al. 2017). It contains 151 fish species, over 60 mussel species, and 22 snail species, many of which are federally listed as threatened or endangered (Etnier and Starnes 1993; Johnson et al. 2013; Ahlstedt et al. 2017; Womble and Rosenberger 2025a). However, this natural legacy is under increasing urban and agricultural pressures, including providing drinking water for 250,000+ people (Webbers 2003; USACE 2018). Surface water withdrawals, mainly for public water supply, are anticipated to increase significantly with regional population growth (USACE 2018). High water demands during drought conditions could significantly alter an otherwise hydrologically and ecologically resilient system (Knight et al. 2012). This situation is further complicated by a backdrop of climate projections that portend greater variability in hydroclimate and water availability (Naz et al. 2016; U.S. Global Change Research Program 2014). With multiple state and federal agencies involved in managing the Duck River’s water resources (USACE 2018), access to quality-controlled, regionally and topically relevant information will assist in understanding how the unique biodiversity of the Duck River may respond to changing water availability.
Despite improvements in the region that contributed to increases in mussel ranges and numbers (e.g., the Tennessee Valley Authority [TVA]’s Reservoir Release Improvement program in 1991; Ahlstedt et al. 2017; USACE 2018), water management remains a critical factor for maintaining the unique species richness and abundance of Duck River freshwater mussels. This report summarizes the current state of knowledge on how changes in riverine flow regimes affect freshwater mussels, with a primary focus on droughts and sustained low flows. We emphasize factors most likely to affect the ongoing diversity, abundance, and recovery of mussel assemblages in the Duck River. Relevant studies were identified through searches of databases including Web of Science, Scopus, and Google Scholar using targeted combinations of the following search terms: ("freshwater mussel*" OR "unionid*" OR "bivalve*") AND ("flow*" OR "discharge" OR "hydrolog*") AND ("drought" OR "low flow*" OR "drying"). We also reviewed studies investigating the effects of upstream flow changes on hyporheic or groundwater processes that may be important for delivering mineral resources and nutrients to downstream mussel populations in the Duck River. These terms included: ("hyporheic" OR "groundwater" OR “interstitial”) AND ("flow" OR "discharge" OR "hydrolog*" OR “drying” “drought”, “low flow*”) AND ("nutrient*" OR "mineral*" OR "geochem*") AND ("mussel*" OR "bivalve*" OR “unionid*”) AND (“upstream” OR “downstream”). We searched reference lists of highly relevant papers for additional studies. Titles and abstracts of records returned by searches were screened for inclusion based on relevance to the effects of flow alterations on mussels. Studies focused on species, families, and river systems relevant to the Duck River were prioritized.
We begin the review by covering the significance of freshwater mussels to aquatic ecosystems, including their taxonomic uniqueness, life cycle, habitat requirements, and the ecosystem services they provide. This is followed by a summary of what is understood about mussels and environmental flows, starting with existing reviews in the literature that suggest anthropogenic water management practices have led to the current conservation status of most freshwater mussel species. Alterations include isolation or inundation of centers of biological diversity through the construction of dams, river channelization, changed flow regimes, and poor or untargeted management of water quality in tailwaters and other parts of the river. This is followed by a more detailed review of our current state of knowledge on how low flow conditions affect freshwater mussels, whether human-induced through water withdrawals, low water releases, or natural drought.
Our focus then shifts to the Duck River in particular, where we discuss the significance of this river system for freshwater mussel diversity worldwide and in Tennessee, a state renowned for its extraordinary biodiversity. We describe its current fauna and spatial distribution, as well as historical and current threats and potential synergistic interactions. We conclude this section by summarizing the overall effects of flow alteration on this system, including the operations of Normandy Dam, ongoing and proposed water withdrawals, future droughts, and potential interactions between water management and land-use changes. Considering the effects of changing flow regimes on the Duck River mussel fauna, we highlight data gaps and unknowns that may inform evidence-based, adaptive, and flexible water management to achieve desired management outcomes. Our review ends with a proposed outline and structure for a decision support model for environmental flows, highlighting data gaps and remaining research questions. This may serve as a conceptual blueprint for approaching management and withdrawals in the Duck River in an evidence-based and adaptable manner, serving as a transferable approach for Tennessee and beyond.