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Little more than 200 years ago when European settlers first arrived on American shores, more than 725 species offish swam in the springs, lakes and rivers of the continental United States. Today, at least 12 of those species and four subspecies are extinct-lost forever. Many more of our native fishes are in trouble. Over 35 have been listed or proposed for protection under the Endangered Species Act passed by Congress in 1973. Among these are four which once flourished in the Colorado River system of the arid southwestern United States. The Colorado River drainage rises in the high Rocky Mountains of northcentral Colorado and southwestern Wyoming. As it flows 1,700 miles to the Gulf of California, its elevation drops more than two miles to create some of the earth's most turbulent waters. Historically, the river gained water from its many tributaries as it dropped until it emerged on the Colorado plateau with force enough to carve mile-deep canyons in the bedrock. Spring's melting snow fed the river until it was a muddy torrent. By midsummer, however, its flows would be reduced by the intense heat and negligible precipitation of the Southwest. Measured at Yuma, Arizona, its variable flow ranged from a dry•weather low of 3,000 cubic feet per second to a spring-flood high of 380,000 cubic feet per second. Each year the turbulent river carried to its delta silt loads of over 100,000 acre feet. In this extreme environment of fluctuating streamflows, muddy water and high summer water temperatures, fewer than ten fish species evolved-an incredibly small number for the mainstreams of such a large river system. These unique fishes developed bizarre shapes like humped or ridged backs, winglike fins and streamlined bodies to help them navigate the river during high water. Significantly, 70 percent of the Colorado River's native fishes were found nowhere else in the world! Now some of them are falling victim to the most serious threat to any living being's existence-the alteration of its natural habitat. In the last 100 years the Colorado River system has undergone drastic modification-physically, chemically and biologically. The components of an aquatic ecosystem are so tightly intertwined that any change inevitably triggers others. More than 20 major dams have been constructed on the river and its larger tributaries. Each has changed the riverine habitat, upstream and down. Above a dam, the river's naturally turbulent, silty waters are impounded to form a deep, clear reservoir. Depending on the dam's periodic releases, water levels below a dam fluctuate rapidly. The water released is quite clear and cold. A dam•tamed river offers few areas of shallow backwaters, vital to some fishes as safe nursery, resting and feeding sites. Many fish species not found in the natural river system now thrive in the modified Colorado. Sport fish like bass and trout inhabit the reservoirs. Catfish and carp swim in the main channel with numerous minnows which have become established after escaping the bait buckets of fishermen. These species may compete with the native fishes for food and spawning and nursery sites. Today the survival of most of the Colorado River system's native fishes seems to depend on stretches of rivers left relatively unchanged. In the thirsty southwestern United States, impounding the waters of the Colorado River and its tributaries has made water available to meet some of our most vital needs-the irrigation of crops, the development of energy resources, the generation of electricity ....Abundant and diverse fish, wildlife and plants are also important to a quality human environment. As our requirements for water grow, we must plan and act judiciously to maintain the precarious ecological balance upon which man, as well as fish, depends.