Summary: The majority of today’s large rivers have been simplified and fragmented, constraining their ecological structure and function. Evidence underscoring the roles of biodiversity and complexity in stabilizing ecosystem processes in river networks has grown, necessitating thinking beyond mainstem rivers to consider how tributaries contribute to large river fish populations. When tributaries provide 1) habitat diversity, 2) connectivity, 3) ecological asynchrony, and 4) density-dependent processes, they help facilitate completion of life stages for fishes inhabiting greatly modified large river systems. This talk describes two case histories from two greatly altered and distinct large-river tributary systems – the Lower Missouri River and Lower Colorado River – to highlight what has been learned about how and why large river fishes use tributaries. Further, we describe future research directions to advance our understanding of tributary roles and inform conservation actions. These examples show how habitat diversity, connectivity, ecological asynchrony, and density-dependence processes can help guide restoration activities across a diverse range of mainstem rivers and their tributaries. Using these assets as a guide, we suggest these can be transferable to other large river-tributary systems.