Relationships between fish assemblage composition and substrate variation is poorly understood in large rivers. Information on fishes occurrence and behavior and substrate variation were examined in field observations for the Middle Wabash River and fine scale artificial streams experiments. The results from field observations suggested strong concordance for variation in abundance of fishes with habitat variation among sites, resulting in a longitudinal river gradient as dominant in the Middle Wabash River. In addition, shifts in fish behavior within artificial stream experiments demonstrated that species-specific habitat selection behaviors were influenced by interactions within a fish assemblage. The combination of artificial stream experiments and field observations can identify fine scale trends that bioassessment surveys cannot tease apart, and highlighting the need to examine species-habitat relationships at more than one scale. / Department of Biology