We examined species composition and structure of riparian forest (woody plant stems ≥2.5 cm dbh) and herbaceous layer (vascular plants ≤2 m in height) of two upper Allegheny River islands (Hemlock and King Islands, Forest County) in the nonglaciated Allegheny Plateau of northwestern Pennsylvania. Twelve species were recorded from the large tree stratum (stems ≥10.0 cm dbh) at Hemlock and King Islands with Acer saccharinum and Platanus occidentalis as leading dominants. Sixteen species were recorded from the small tree stratum (stems < 10.0 cm dbh but ≥2.5 cm dbh) at both islands; Fraxinus americana, Carya cordiformis, and Salix nigra were dominant. Fifty-nine vascular plant species were recorded from the herbaceous layer of both islands with Phalaris arundinacea, Viola sororia, Geum canadense, Pilea pumila, Lysimachia nummularia, L. ciliata, Alliaria petiolata, and Polygonum scandens as dominant species. Seventeen percent of the herbaceous layer flora of both islands was non-native including three species (Alliaria petiolata, Polygonum cuspidatum, Rosa multiflora) that are invasive pests. Tree density (stems ≥2.5 cm dbh) ranged from 78 to 138 stems/ha, indicating a relatively open, savanna-like forest. Unimodal diameter distributions for Acer saccharinum and Platanus occidentalis revealed an underrepresentation of smaller size classes suggesting that either these species recruit episodically or that altered hydrologic regimens have led to reproductive stagnation.