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1979 THE ENDANGERED SPECIES: A SYMPOSIUM 43 of all time to fishes and aquatic habitats in the western United States. This period was followed closely by a very active period of dam building, with concomitant increases in irrigated agriculture, especially since about 1930, when large reclamation projects began providing water to irrigate what is now some 10 million acres of land in the West. The de- cline in abundance of the native fishes of the mainstream Colorado River is associated closely with construction of these mainstream dams (Minckley and Deacon 1968, Holden and Stalnaker 1975 a, b, Seethaler 1978). De- clines in fishes of tributary streams are also Occurring and are similarly associated with water manipulations of various kinds that re- sult in dewatering portions of fish habitats. Recently, McNatt (1978) has described the process along the San Pedro River of Ari- zona. I present some documentation here for similar problems along the Virgin River of Utah, Arizona, and Nevada. Santa Clara River Boulder Dam Wash Mesquite Diversions Lake Mead <{ Q ~ W 2 The Virgin River drains southwestern Utah and flows through the northwestern comer of Arizona before joining the Colorado River in Lake Mead, Nevada. A salt spring, LaVerkin Springs, enters the river 180 km upstream from its confluence with Lake Mead, forming the upstream limit of distribution for both the Virgin River roundtail chub, Gila robusta seminuda, and the woundfin, Plagopterus aT- gentissimus. Both are here listed as endan- gered and both are presently restricted to the mainstream of the Virgin River below LaVer- kin Spring. In addition, the Virgin spinedace, a threatened species, OCcurs both below and above the springs. Irrigation diversions have been established along the river since the 1860s. Since at least the early 1900s, the Hurricane Diversion, Washington Diversion, and Mesquite Diver- sions (Fig. 1) have been capable of diverting essentially the total summer flow of the river at each of these three diversion points. La- La Verkin Creek Hurricane Diversion Washington Diversion UTAH ARIZONA Fig. 1. Mainstream Virgin River below Hurricane diversion showing total remaining potential habitat for the en- dangered woundfin and roundtail chub, and significant modifications currently restricting their range.