## 1 Introduction  
North America supports the greatest number of freshwater bivalves in the world, with about 300 species recorded in the order Unionoida [1]. Mussels are ecologically valuable to freshwater ecosystems [2], influencing nutrient cycling and water filtration [3], stimulating production across trophic levels by transferring nutrients and energy from the water column [4], and providing and improving habitat for other organisms [4, 5]. Unfortunately, mussels are highly imperied due to the numerous threats that face freshwater ecosystems [6], with 28% of native North American mussels federally listed as endangered or threatened, 65% considered imperiled [1], and only 70 species considered stable [7]. Many environmental factors operating at different spatial scales have been shown to strongly influence composition and abundance of freshwater mussel assemblages. At the landscape (catchment) level, land cover, e.g., percent wetland (positive) and percent urban cover (negative; [8, 9, 10, 11]), percent agricultural land use (negative; [12, 13]), percentages of alluvial [14], surficial [15], and aquifer-bearing [16] geologies, watershed slope [14], location within the watershed [17], land use compositional pattern [18], and human density within the watershed (negative; [19]) have all been reported to affect mussel distribution and abundance across the central and mid-central U.S. Although the overall importance of reach-scale variables to mussel density models can be low [18], stream size [20] and riparian habitat characteristics, such as forest versus grass-dominated riparian zones adjacent to -mussel beds [21] and riparian wetland forest (positive) and urban development (negative) within buffers located upstream of bed locations [10; 11], have also been found to influence mussel abundance and distribution. At the mussel bed scale, streamed composition, such as predominance of sandy [22] and other fine substrates (positive; [23; 24; 25; 26; 27]), as well as low water velocities [27] and absence of shear stress [28; 29] have been found to characterize inhabited microhabitats. Additional characteristics, such as the presence of woody debris [26; 30] and proximity to the bank [20], have also been reported to promote higher mussel densities. Mussels appear to be very sensitive to contaminants [31; 32], with species-specific increases in mortality in response to declining dissolved oxygen (DO) conditions [33; 34]. Given the preceding array of explanatory factors that influence freshwater mussel abundance and distribution, development of conservation and restoration strategies faces significant challenges. Moreover, studies have identified substantial uncertainties regarding the influences of substrate composition and stream hydraulics on mussel ecology [35; 36]. Nevertheless, the majority of studies indicate freshwater mussel abundance and diversity are negatively associated with excess sedimentation and increased suspended solids [35; 37], increased water temperature [38; 39], and depleted dissolved oxygen [33; 40], factors often linked with agricultural practices and the loss of riparian woodlands [41; 42]. Conversely, stable substrates, fine sediments (usually), stable flows and depths [24; 43], groundwater inputs [25], and accumulations of woody debris [27; 30] appear to favor maintenance of diverse mussel assemblages. Central North America, including the northern coast of the Gulf of Mexico, is a region with very high diversity of unionid mussels [44]. However, relatively few studies (e.g., [33; 36; 45; 46; 47]) have investigated the ecology of freshwater mussels in streams within the Gulf Coastal Plains ecoregion. These streams are unique relative to those in more temperate regions of the U.S. that have been the focus of most freshwater mussel studies, with Gulf coast streams typically characterized by minimal gradients, fine sediments, hypoxia [40], and extreme periodic increases in discharge from tropical storms and hurricanes (e.g., [48]).