Recent studies of stream fish assemblages have begun to incorporate landscape properties in explaining the dynamics seen at individual sites in riverine systems. In this approach, stream and river landscapes (riverscapes) are viewed over greater spatial and temporal scales to understand the dynamics seen at individuals sites. Adventitious streams are feeder tributaries of mainstreams at least three orders greater in magnitude. Interface sites between adventitious streams and their mainstreams are often more diverse and variable than either the feeder stream or mainstream alone. We analyzed fish assemblage data from 21 sites over three periods (1967, 1995 and 2001) from an adventitious stream drainage (Piasa Creek: Jersey, Madison and Macoupin CO., IL) to test the hypothesis that assemblage variability and diversity would be greater at downstream interface sites than those further upstream. There was a correlation between distance from the mouth of the stream and assemblage variability over time, as well as a negative correlation between distance from the mouth of the stream and mean diveristy over time. We hypothesize that the observed patters in community variability and the distribution of some species within the drainage are best explained by the interactions between the big river fauna and the stream fauna.