<i>Abstract.</i>â€â€Fish assemblages in the Willamette River basin (Oregon) were once substantially degraded by water pollution, channelization, dams, nonnative fish, and conversion of natural forest and savanna to agriculture and urbanization. Restoration actions have included basinwide waste treatment, physical habitat rehabilitation, recovery of the Oregon Chub <i>Oregonichthys crameri</i> to stable status, and stocking reductions of nonnative fish to protect native fish. State and federal sewage treatment regulations and funding, federal endangered species regulations and funding, and reduced funding and support for stocking nonnative trout led to those rehabilitated fish assemblages. Periodic fish and habitat monitoring has documented the following improvements in fish assemblages: (1) decreased occurrences of pollution-tolerant species and increased occurrences of pollution-sensitive species and native main-stem species, (2) increased number of abundant Oregon Chub populations, and (3) persistence of resident native Rainbow Trout <i>Oncorhynchus mykiss</i>. Notably, no known extinctions of native fish species have occurred in the Willamette River, water quality index scores in the lower river have improved from poor to fair, and water quality in the upper river remains good to excellent. In conclusion, enactment of laws and regulations for environmental protection and the collective actions of state and federal agencies, tribes, municipal governments, universities, land trusts and conservation groups, watershed councils, and private landowners have led to a substantially rehabilitated river. However, population and economic growth, climate change, nonnative fish, winter steelhead (anadromous Rainbow Trout) and spring Chinook Salmon <i>O. tshawytscha</i> listings, a superfund site, channel alterations, toxic substances, poor fish passage at dams, and altered flow regimes remain challenges. Four lessons learned are that (1) pollution control and improved water quality and flows are essential to the recovery and persistence of native fish populations, (2) recovery of endangered species is achievable but requires knowledge of their life history needs, (3) the greater ecological fitness of native stocks facilitates their persistence, and (4) research and monitoring, combined with public communication and collaboration, are essential for habitat and native fish assemblage rehabilitation.
