The causes of widespread mussel declines remain poorly understood. Most previous studies focused on specific streams, and none have critically evaluated potential causes of mussel declines across a large geographical area. In 2020, we initiated a study across up to 13 states in >75 streams. Study streams represent a continuum of those having highly degraded to high-quality mussel assemblages. For each stream, we are compiling data on or directly measuring a wide variety of potential causes of mussel declines (independent variables), including water quality, land use, stream connectivity, mussel food availability and quality, sediment characteristics, invasive bivalve abundance, and broad-spectrum pathogen diagnostics. Our response variables are mussel assemblage health and individual mussel health and performance. We developed an index of mussel assemblage health that scores assemblages on a continuous scale from 0–10 and is applicable across the study area. We are assessing individual mussel health based on survival, growth, physiological condition, and histological examination of juvenile mussels exposed in silos to ambient stream conditions for three months. Our overarching goal is to examine whether any of the potential causal factors are consistently associated with mussel assemblage health or individual mussel health. We describe the study goals and approaches and discuss preliminary findings. The project depends on a broad, collaborative network of partners from over 34 institutions, agencies, and organizations, including dozens of personnel and substantial in-kind contributions.