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IMPACTS TO THE COLORADO RIVER ECOSYSTEM The Colorado River is the largest river in the southwestern United States. Its headwaters are located in the Rocky Mountains of Colorado and it flows westward through the state of Utah, then turns south where it formed the Grand Canyon. As it exits the canyon it continues its 3,200 kilometer journey through the arid desert to the Gulf of California where it empties into the Pacific Ocean. The Colorado River played a critical role in the settlement and development ofthe American southwest. Today, it's the most regulated river in the Western Hemisphere. During the past century, over 40 major dams or water diversion structures have been built within the water shed. Reservoirs, such as lakes Mead and Powell were built to provide; a dependable water supply, flood control, and hydroelectric power. Reservoirs settled the once turbid waters providing additional benefits in the form oftourism and excellent sport fisheries. Major water depletions occur from Lake Havasu where water is diverted to southern California and central Arizona. Further downstream additional water is diverted into hundreds of kilometers of canals to satisfy agricultural needs. The river supplies water to over 25 million people in the United States and Mexico. Water depletions from the river are so great that the flowing river seldom reaches the ocean. The physical and biological changes caused by water development have been dramatic. Channelization and levees have reduced flooding events, but at the cost of riparian and wetland communities. Reservoirs, stream regulation, and stocking of non-native species have all but eliminated native fish populations. Today, it's estimated that less than 5% of the fish found in the Colorado River are natives. The original biological components of the lower river's ecosystem are basically gone. Concern over the loss of these natural communities began to gain momentum nearly 3 decades ago. Since that period, the razorback sucker, bonytail chub, humpback chub, and Colorado squawfish have been listed as endangered species and are now protected by Federal law. Unfortunately, populations of humpback chub and Colorado squawfish remain greatly depressed, although currently populations appear stable. Wild populations of bonytail chub, first to receive Federal protection in 1980, are thought by many to be extinct. Of the four endangered fish, the majority of restoration work has been with the razorback sucker which is now restricted to less than 25% of its former range (Minck ley et at. 1991, Mueller and Marsh 1995). The razorback sucker (Xyrauchen texanus) is a large, uniquely-shaped fish having a 50 year long life span (figure I). Nearly all suckers collected during the past 3 decades have been adults. These fish are nearing extinction in the upper Colorado River basin (Tyus 1987) where remaining populations number as few as 500 individuals Modde et al. 1996). The largest population is found in Lake Mohave, Arizona-Nevada, a regulatory reservoir formed by the closure of Davis Dam in 1954. Aging studies indicate populations are composed of old individuals. While reproduction occurs and fish hatch, very few young survive to reach adulthood. All existing wild populations are declining and extinction will occur within the next decade unless populations are augmented with younger fish (Minck ley et at. 1991, Burdick 1992, Mueller and Marsh 1995). No direct cause and effect relationships have been identified, however, declines have been attributed to habitat degradation, and competition and predation from non-native fish species. Razorback suckers do successfully spawn at several locations throughout the basin. Eggs incubate and hatch and larval razorback suckers are produced. However, young suckers longer than 25 mm total length are rarely found, because the young are eaten by a host of predators not present a few decades ago. Over 40 non-native fish species have been successfully introduced into the Colorado River basin. Fishery managers have succeeded in optimizing the introduced sport fishery during the past century, often at the expense of native fish communities. Reservoirs, tributaries, and the