Connectivity describes the degree to which matter and organisms can move among spatially defined units in a natural system. River connectivity is typically described in longitudinal, lateral, and vertical dimensions within the river corridor and the watershed and can be conceptualized as a continuum from fully connected to disconnected over diverse temporal and spatial scales. Explicit characterization of connectivity helps understanding of disparities among short-term, local rates of flux and inferred fluxes over larger scales. Connectivity also strongly influences the response of rivers to natural and human disturbances. Investigations of connectivity facilitate the recognition of processes that cross traditional disciplinary boundaries, as well as understanding of nonlinear behavior and retention within rivers. Enhancing the ability to identify, quantify, and predict the processes that create and maintain connectivity is integral to the effective management of rivers.