Biogeographic studies that aim to describe processes of the coevolution of Earth and its inhabitants rely on the accurate descriptions of modern and historical distributions of taxa, habitats, climates, and land masses. In order to understand the response of freshwater fishes to deglaciation, size distribuitons of river basins and species richness were described for lower Michigan. To that end, basins were created from a composite 30 meter digital elevation model and compared to mapped, natural basins. Collection data from University of Michigan Museum of Zoology were then intersected with basins in latitude - longitude space. This produced a dataset in which taxa and basins are both georeferenced and from which species richness by basin and basin area can be extracted. Species richness was treated as a continuous random variable that varies jointly with basin area and the number of specimens collected in a basin. A traditional species-area curve was then plotted as the marginal density function of species richness with basin drainage area. For lower Michigan species richness can be approximated by the inverse of a exponential random variable with respect to basin area. This suggests that lower Michigan fishes have not yet reached a dynamic equilibrium state with respect to immigration and extinction rates within basins created after the retreat of Wisconsinan ice sheets.