While freshwater mussels are often negatively impacted by large reservoirs, the influence of smaller low-head dams on resident mussel fauna is variable. A 2017 planned dewatering of Robinson Lake, a small water-supply reservoir located in the Llano River, Texas, presented an opportunity to quantify the native unionid community. We also compared unionid communities between Robinson Lake and a riverine portion of the mainstem Llano River to assess how impoundments may influence assemblage structure, and we evaluated the conservation implications for two Endangered Species Act (ESA) candidate species. In total, we salvaged and relocated 1,012 live unionids representing five species from Robinson Lake, including ESA-candidate species Lampsilis bracteata and Cyclonaias petrina. Lentic specialists were observed exclusively in Robinson Lake, while lotic specialists and habitat generalists occurred in the Llano River. Though community composition differed, we did observe overlap among sites, suggesting that Robinson Lake contains a subset of the unionid community within nearby riverine reaches, and it supports more lentic-adapted species. Contrary to previous habitat assessments, observations of L. bracteata reproduction in Robinson Lake suggests that this species is able to adapt to lacustrine environments and establish populations within small impoundments, though catch rates suggest higher densities in lotic habitats. As increased utilization of water resources and changing climactic patterns continue to impact spring-fed river systems of the Edwards Plateau region, such impoundments may become important conservation units for L. bracteata during major drought conditions.