Description: This data was created as part of the Missouri Aquatic Gap Project. Our three aquatic Subregions; Central Plains (CP), Mississippi Alluvial Basin (MAB), and Ozark, largely correspond with Pflieger's (1971; 1989) aquatic faunal regions, with slight modifications to some boundaries and names. Subsequent quantitative and qualitative investigations into the zoogeographic patterns of other aquatic taxa, including mussels, crayfish, and other macroinvertebrates have consistently corroborated the ecological distinctiveness of the CP, MAB, and Ozarks (Oesch 1995; Pflieger 1996; Rabeni et al. 1997; Rabeni and Doisy 2000). Our three Subregions also closely correspond to Bailey's (1995) Ecological Provinces and Omernik's (1987) Level II ecoregions. This same correspondence has been demonstrated in several other studies from a variety of physiographic settings (Hughes et al. 1987; Whittier et al. 1988; Jackson and Harvey; 1989; Harding et al. 1997; Angermeier et al. 2000) and reveals the strong influence that climate, geology, soils, vegetation, and landform have on the geographic distribution of riverine biota at this spatial scale. This correspondence between abiotic landscape and climatic features and biological assemblages is also what makes this level of the hierarchy somewhat distinct from the preceding levels. The upper three levels are largely zoogeographic strata while Subregions are largely ecoregional strata of regions. There are certainly physiographic (principally climatic) differences among upper levels of the hierarchy (e.g., Arctic Region vs. Mississippi Region). However, distinctions among these upper level units (Zones, Subzones and Regions) are largely biological and relate specifically to the family and species-level taxonomic differences among the different units, which have largely resulted from the distinct evolutionary or phylogenetic histories of each unit. Differences among our Subregions, on the other hand, are related more to the physiographic factors used to delineate ecoregions, which result in distinct hydrologic and physicochemical conditions and thus distinct environmental conditions within each unit. Not surprisingly, differences in the biological assemblages among our three Subregions are not "merely" taxonomic, but relate principally to differences in the ecological attributes (e.g., physiological tolerances, morphology, behavior, and reproduction and foraging strategies) of the species that are characteristic of the distinct riverine environments found in each Subregion.