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Lost, A Desert River and Its Native Fishes: A Historical Perspective of the Lower Colorado River GordonA. Mueller By us. Geological Survey P.O Box 25007, D-8220 Denver, CO 80225-0007 Paul C. Marsh and Arizona State University Department of Biology Box 871501 Tempe, AZ 85287-1501 Abstract. The Colorado River had one of the most unique fish communities in the world. Seventy-five percent of those species were found no where else in the world. Settlement of the lower basin brought dramatic changes to both the river and its native fish. Those changes began more than 120 years ago as settlers began stocking nonnative fishes. By 1930, nonnative fish had spread throughout the lower basin and replaced native communities. All resemblance of historic river conditions faded with the construction of Hoover Dam in 1935 and other large water development projects. Today, few rememlJerwhanhe Colorado River was really like. Seven of the nine mainstream fishes are now federally protected as endangered. Federal and state agencies are attempting to recover these fish; however, progress has been frustrated due to the severity of human impact. This report presents testimony, old descriptions, and photographs describing the changes that have taken place in hopes that it will provide managers, biologists, and the interested public a better appreciation of the environment that shaped these unique fish. Key Words: Colorado River, history, impacts, lower basin, native fish. Introduction The waters ofthe Colorado River have been used by humans for more than 11,000 years. Water was more pre- cious than gold in the desert, and the Colorado River proved critical in the settlement, growth and economic development of the American Southwest. Today, it pro- vides irrigation water for more than 3.7 million acres of farmland and delivers water to 30 million people in the United States and Mexico. Reservoirs blanket 675 mi2 of the floodplain and can store five-and-a-half times the river's annual flow. Remaining portions ofthe lower river resemble canals and deliver reservoir water to farms and cities in Phoenix, Tucson, Los Angeles, Yuma, and Mexico. Native fish have paid a high price for humans' un- quenchable thirst and tinkering in the Colorado River Basin. Today, the river is totally diverted and only reaches the sea during major flood events. Long reaches of the Lower Salt, Gila, and Colorado Rivers and 92% of the wetlands historically found in the Colorado River Delta have dried up due to upstream water diversion and use. Along with the obvious physical impacts of water abstraction, more than 70 nonnative fish species have been introduced. Half of these species have established