Tributary and mainstem corridors represent important fish-connectivity avenues in large riverscapes. We evaluated the connectivity of 40 Monongahela River tributaries in southwestern Pennsylvania and their respective mainstem junctions using a variety of gears. Twelve tributaries were so fragmented by physical and water-quality impediments, comparisons could not be made. Among the 28 remaining tributaries, classified as adventitious (l^st–3^rd order) or ordered (4^th–5^th order), we evaluated fish communities using the Jaccard coefficient of similarity, a cluster analysis, and a Venn diagram. Adventitious tributaries shared 82% of their total faunal complement with ordered tributaries and 29% with the mainstem, while 70% of the ordered ichthyofauna was common to the mainstem. The ichthyofauna of the adventitious tributary network was more distinct and isolated from the mainstem than that of ordered tributaries. In fragmented riverscapes such as this, islands (tributaries) of biodiversity may warrant special protection.