Dam removal is a common conservation tool that has many potential benefits for freshwater mussels. We conducted qualitative and quantitative mussel surveys in the Mill River system, Massachusetts, where four dams have been removed or modified to benefit aquatic organisms. These data represent a baseline for future monitoring of the effects of dam removal or modification. Mussel assemblages were composed of six species and were dominated by Elliptio complanata; Lampsilis radiata was the second most abundant species. Two species of Special Concern in Massachusetts, Ligumia nasuta and Leptodea ochracea, were rare, as were Pyganodon cataracta and Utterbackiana implicata. We conducted catch-per-unit-effort (CPUE) surveys at 77 sites; mussels occurred throughout much of the watershed except for the lower portion of the Mill River. The highest CPUE values were found immediately downstream of the two lakes in the system. We conducted quadrat-based surveys at nine sites, including one site in each of the lakes. Precision of estimates of total mussel density was ≥80% at most sites, which will allow detection of moderate to large changes over time. Monitoring of changes for rarer species may require a watershed-based approach based on CPUE because quantitative estimates had wide confidence intervals.