We reviewed the results of seven extensive and two reach-specific fish surveys conducted on the mainstem Willamette River between 1944 and 2006 to document changes in the summer distribution and species richness of non-native fishes through time and the relative abundances of these species today. The results revealed: (1) the addition of four non-native species since 1944; (2) significant summer range expansions by three recent introductions; (3) three apparent failed introductions; and (4) smallmouth bass, largemouth bass and common carp as the most abundant non-native species making up 4.2%, 1.2% and 1.1% of the total electrofishing catch in 2006. We discuss the positive effects of fishing multiple gears within varied habitats and the importance of longitudinal sampling distance on the catch species richness of large-river fish surveys. Regular standardized monitoring of large-river fish assemblages could improve the management of invasive species.