Small, adventitious tributaries (<3 orders of magnitude smaller than the stream it flows into) are a conspicuous feature of many river–floodplain systems, but their value as fish reproduction and nursery habitat is not well understood compared to oxbow lakes and the main river channel (MRC). Moreover, connectivity of tributaries to the MRC is often less impacted by anthropogenic modifications (e.g., dams and levees) compared to oxbow lakes. From April to July 2012, larval and juvenile fish were collected in the Fourche LaFave River (Arkansas, USA) system to better understand fish nursery habitat function of tributaries relative to oxbow lakes and the MRC. Nonmetric multidimensional scaling ordination of juvenile and larval fish genera revealed distinct fish assemblages in MRC and floodplain habitats. Ordination of juvenile fish at the species level resulted in distinct fish assemblages in tributary versus oxbow lake habitats. Tributaries had more unique species and higher abundance of shared species than oxbow lakes and MRC. Additionally, of the 46 species identified, all but six were collected in lower tributary reaches. Connectivity was strongly associated with both ordinations and was important in describing patterns of fish variation among habitats and between tributaries. Of the tributaries sampled, the least fragmented stream had the most similar fish assemblages between upper and lower sections. Findings of this study revealed tributaries are an important, yet overlooked, feature in the river–floodplain model. Especially in years of drought, channel–floodplain connectivity can be limited, but tributaries can be used by fishes for reproduction and nursery habitat.

Floods play an important role in regulating ecological patterns and processes in river–floodplain ecosystems. Yet, the hydrologic connectivity between many river–floodplain ecosystems has been severed by anthropogenic activities that have markedly altered these ecological processes. Superimposed over anthropogenic reductions in river–floodplain connectivity, climate change is also shifting flood characteristics to produce more intense, unpredictable floods. To more efficiently manage river–floodplain ecosystems, additional research characterising the ecological consequences of unpredictable floods on fish movements is required. In this study, we used a modified fyke net to characterise the patterns and identify the important drivers of lateral movements of fish emigrating from the floodplain during contraction of a high‐magnitude, late‐summer flood in the Fourche LaFave River, Arkansas, in 2007. We captured approximately 43,200 fish emigrating from the floodplain over an 8‐day period, representing 38 distinct species from 12 families. We related fish‐emigration patterns to different stages of flood contraction and linked species‐specific emigration to water‐level fluctuations and associated changes in floodplain habitat availability and quality. We also show that some species exhibit body‐size emigration patterns, where larger individuals emigrated from the floodplain earlier than smaller individuals. To meet conservation goals for maintaining healthy riverine ecosystems and the biodiversity they support, it will be necessary to continue characterising and identifying the ecological consequences of shifting flood characteristics within river–floodplain ecosystems.