The interface zones between large rivers and tributary streams may play an important role in structuring fish assemblages.  By studying these systems from a landscape perspective (e.g., riverscapes), the properties of stream networks that influence fish assemblage structure may be revealed.  The objective of this study was to test the association of fish community structure and habitat in adventitious streams.  We predicted an abrupt change in community structure between adventitious streams and the Kansas River that coincides with changes in physical habitat.  Our analysis was conducted at two scales, a basin wide stream survey and more intensive collections in three adventitious streams in the Kansas River basin.  The fish community of the Kansas River was distinct from its tributaries.  However, many large-river species used these tributary streams in various ways, and size structure and species composition varied with distance from a mainstem river.  In general, riverine species occurred in lower gradient reaches of adventitious streams that flow through the historic flood plain of the main river.  Continuing research will quantify the seasonal changes in the distribution and abundance of fishes within these riverscapes.