Reservoirs are common anthropogenically created ecosystems that provide novel fish habitat alongside societal benefits including drinking water, flood control, and recreation. Reservoirs and their inflowing rivers consist of longitudinal gradients of connected habitats that vary biologically, physically, and chemically. Components of river-reservoir ecosystems (RRE) encompass characteristics that resemble elements of natural riverscapes, such as productive floodplains and riverine deltas. Previous research in RRE has demonstrated the importance of aquatic ecotones at the lotic-lentic transition for supporting diverse and abundant assemblages. Most studies have quantified changes in fish assemblage structure across gradients in RRE in terms of abundance or biomass, often only describing patterns in one system. Whether these patterns are generalizable across multiple RRE, remains to be tested. To gain a more comprehensive understanding of relative benefits for fishes in RRE, we quantified relationships between fish assemblage structure, abundance, larval fish growth rates, a metric of performance, and a suite of potential abiotic and biotic correlates across 8 RRE. We collected fishes monthly in Ohio during summer 2024 (n = 96 sites), with growth determined using hard structures (otoliths). During the study period, we captured 6,101 larval fishes (<= 20mm) and 30 unique taxa across sites. We observed significant increases in larval fish abundance in upstream reaches of RRE, correlated with increases in available littoral and floodplain habitat (p < 0.001). Fish assemblage structure was similar across habitats from river inflows to the dam, and more variable across time as a function of increasing temperature and differences in spawning periodicity. Preliminary analysis of samples suggests fish growth to be highest within the aquatic ecotone at the lotic-lentic transition (p < 0.01). Results from this study will provide information on the relative benefit of habitats within novel ecosystems with implications for native fish conservation and management of productive sport fish populations.