Developing and recommending protective flows for stream fishes requires precise information on how fishes use their habitat. Collections to establish fish-habitat relations are typically conducted during daylight hours, but because of diel habitat shifts exhibited by many warmwater stream fishes, we were concerned that determining protective flows only from day collections would result in recommended flows that were not protective. We tested for diel differences in habitat selection by stream fishes and evaluated the effects of these differences on simulated usable habitat area as flows varied. Logistic regression modeling and habitat-selection analyses for five fish species showed substantial differences between day and night habitat relations. A two-dimensional hydrodynamic model that used habitat selection and flow specific habitat-availability data indicated that habitat selection data collected during the day were generally not sufficient to protect adequate habitat throughout the diel period for the majority of species. Key words: hydrodynamic modeling, fish habitat, stream fish, prairie river
