Field collections at more than 2900 sites and the examination of many museum collections and literature allowed me to map the historical and current distribution of several freshwater molluscan faunal groups in the Intermountain region of the United States (Great Basin, Colorado River drainage basin, and upper Snake River sub-basin). Historical and current records show that Margaritifera falcata, Anodonta californiensis, and Ferrissia rivularis have drainage-specific distributions, while Valvata utahensis has a specific drainage pattern, and V. californica (new combination) has a dispersed pattern. Shell morphometric data of Valvata and Ferrissia show extensive shell variation between and within populations. Current surveys show that these molluscan populations have been reduced since the colonization by European descendants over the last 150 years. Margaritifera falcata was found to be extirpated from eastern California, Nevada, and Utah and was common in only 1 stream. Anodonta californiensis populations of 10 or more individuals occurred in only 2 of 13 drainages, as well as in 1 isolated spring. Valvata californica was extirpated in 7 of 10 lakes. Ferrissia rivularis was very rare in 6 of 12 drainages. Range declines among these fauna are thought to be related to alterations of habitat caused by grazing, irrigation, and urbanization, as well as the intensive management of sport fish in these waters.