Understanding the relative influence of deterministic and stochastic processes in structuring species distributions is a central focus of community ecology, as these assembly mechanisms drive spatiotemporal diversity patterns. The importance of deterministic (e.g., environmental filtering) and stochastic (e.g., dispersal limitation; source-sink dynamics) forces can vary among species according to their traits, and have primarily been linked to species presences. However, mechanisms driving species absences may also vary among species. Disentangling the prominence of various assembly processes in structuring the occurrences of rare and oftentimes imperiled species can be used to generate conservation recommendations, highlighting the significance of this pursuit. We sought to quantify the relative importance of environmental filtering, dispersal limitation, and source-sink dynamics in explaining the presences and absences of two species each of common and rare fishes in the Kansas River basin in Kansas, USA. Species occurrence and environmental data were collected during 1994-2019 across 314 sites and were analyzed using non-metric multidimensional scaling. As predicted, results revealed determinism via environmental filtering was the dominant force underpinning the distributions of all species, explaining 75-100% of presences and 66-75% of absences. We expected greater stochasticity operating through dispersal limitation for rare compared to common species, yet found a similar percentage of absences were explained by this process for rare (mean = 27%) and common (mean = 30%) species alike. Regardless, our finding that over a quarter of rare species absences were attributable to dispersal limitation suggested repatriation at environmentally suitable sites could be used as a viable strategy to meaningfully increase the prevalence of these species across the riverscape. Our study thus highlights how understanding community assembly mechanisms can be used to guide conservation actions, and is among the first to relate assembly theory to repatriation planning.