This study followed the response of exotic crayfish, Orconectes virilis, to restoration of flow and removal of exotic fish in Fossil Creek, Arizona, components of a large dam decommissioning project.  We predicted that crayfish would increase with restored flow but that increases would be higher where exotic fish were removed due to a release from competition and predation.  Results indicated that crayfish densities rapidly increased within one year, with a two-fold greater increase in areas where exotic fishes were removed relative to where they remain.  A smaller scale, manipulative experiment tested how crayfish, at different densities, affected algae, detritus, and macroinvertebrates to determine if there were detectable densities below which crayfish effects would be minimal.  Effects on algae, leaf litter and detritus were minimal when crayfish were maintained at 1 m^-2, which is slightly higher than current maximum densities (0.76 m^-2). In contrast, at higher densities (3 m^-2) crayfish strongly reduced leaf litter and macroinvertebrates, important food resources for native fish.   Projected crayfish densities, calculated from field studies, indicate that densities could reach the level of the high-density treatments within 0.8 – 2.3 years.