Stream and river hydrology in Kansas has undergone dramatic shifts over the last 100 years owing to reservoir construction, land use changes and groundwater abstraction. Recent work has shown that species introductions contribute more to biotic homogenization than species losses. In a comparison of two river basins, we hypothesized greater homogenization of fish communities within the Kansas basin because of a greater prevalence of larger reservoirs. Bootstrap resampling of historical fish data followed by iterative logit regression quantified species changes while accounting for sampling effort. Different communities occurred in historical (before 1885) versus modern records (Kansas J=0.39, Arkansas J=0.40; J=Jaccard similarity). Four species in the Kansas versus one species in the Arkansas are in decline since 1978 (p<0.05). In the Arkansas, nine native species have been lost (8 before 1936), and 26 added, compared to 17 lost (12 before 1911) and 19 added in the Kansas. Most (72%) additions are reservoir species. Communities have differed from historical since 1948 in the Kansas basin (J=0.36-0.42) and since 1963 in the Arkansas basin (J=0.35-0.38). These changes predate groundwater abstraction intensification (1960’s to 1970’s) and indicate that species introductions, facilitated by reservoir construction (1948-1967), may have driven biotic homogenization in these systems.