Unlike river main stems in the northern geographic range of steelhead (Oncorhynchus mykiss), the surface flow of main stems in south-central and southern California can be extremely low, intermittent, or entirely lacking during the dry season. This has led to a general belief among biologists that main stems provide only migratory habitat, which has created challenges for fishery managers because some main stems are now perennial as a result of the continuous release of treated municipal wastewater. During 2000–2002, we monitored juvenile abundance and downstream migration of the south-central California coast evolutionarily significant unit of steelhead in the lower main stem (reaches proximate to the ocean) of a south-central California stream.  This stream has a continuous discharge of tertiary-treated municipal wastewater that forms most, if not all, of the dry-season living space for steelhead. Our principal finding indicates that this artificial discharge provides oversummering habitat for juvenile steelhead.