A quantitative study based on unionoid bivalve presence-absence data, species abundance, shell morphology, and sedimentological data provides a more detailed paleoecological interpretation of the Upper Cretaceous Hell Creek Formation. The Proparreysia and the Pleurobema faunal assemblages can be separated on the basis of taxonomic diversity and species composition. While the first consists of 19 species, including trigonal and elongate shell morphologies, the latter consists of only 3 species characterized by elongate shells. A riverine environment is interpreted for most species from the quantitative analysis of shell morphology. Only one species (cf. Pleiodon sp.) is from a lacustrine habitat. The main river channels of the Hell Creek paleodrainage system are the habitat of bivalves of the Proparreysia assemblage through a combined interpretation of the morphology and diversity data. In contrast, the bivalves of the Pleurobema assemblage are from secondary channels and tributaries of the main river channels. The mode of deposition of the two assemblages is very similar, as they are preserved predominantly in crevasse-splay deposits. The long-term ecosystem stability, high habitat heterogeneity, and a variable degree in floodplain connectivity of the habitats in the midreach of the Hell Creek paleodrainage system resulted in the high taxonomic diversity of the unionoid bivalves and the separation of the unionoid faunal assemblages.