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ENDANGERED AND THREATENED FISHES OF THE UPPER COLORADO RIVER BASIN INTRODUCTION The Colorado River basin forms its headwaters high in the Rocky Mountains of northcentral Colorado (headwaters of Colorado River) and southwestern Wyoming (headwaters of Green River). Its journey from the source of the Green River to the Gulf of California extends for more than 1700 miles, and the drop in elevation exceeds 2 miles. The erosive energy of the ancient river carved tremendous canyons -- including the Grand Canyon. The official demarcation point for water use that separates the upper Colorado River basin from the lower basin is at Lee's Ferry, Arizona, about 15 miles below Glen Canyon Dam, which forms Lake Powell. This bulletin contains information on the endangered and threatened fishes of the upper Colorado River basin. The demands for water in the lower basin, however, have greatly influenced the environmental changes in the upper basin -- namely, the creation of large dams and reservoirs. Except for the mountainous areas, most of the Colorado River basin consists of arid and semiarid land, and much of it is true desert. Flows fluctuate wildly during a year and between wet and dry years. Historical flows at Yuma, Arizona, have ranged from lows of a few hundred cubic feet per second (cfs) to almost 400,000 cfs. Erosion is high in the basin, and enormous sediment loads are transported in most of the major tributaries to the mainstream of the Colorado. It has been estimated that before major dams tamed this wild river and settled out most of the sediment, more than 100,000 acre feet of sediment was deposited in the Gulf of California each year. Thus, it can be surmised that fishes living, adapting, and evolving in this harsh environment, characterized by great extremes in flows, turbidity, velocities, and temperatures, would form a unique group of species. The Colorado River has had no broad connections with surrounding river basins such as the Missouri and Columbia for millions of years. This great time of isolation promoted the development of unique, often bizarre fishes specifically adapted to harsh environments. Most of the native fishes of the basin have long been isolated from their closest rela- tives and have undergone sufficient evolutionary change to be recognized as species endemic to the Colorado River basin -- that is, species that are native only to the Colorado basin and found nowhere else in the world. The Colorado River basin, as a whole, has the highest percentage of endemic species of any river basin in North America. Among the unusual mainstream fishes specialized for living in the Colorado and Green River and their major tributaries are the squawfish, the bonytail and humpback chubs, and the razorback sucker. The squaw- fish is a predatory, pike-shaped minnow, reputedly reaching lengths of 5 to 6 feet and weights of 60 to 80 pounds. The bonytail chub and humpback chub, with their oddly streamlined shapes, are designed to cope with turbulent flows. The razorback or humpback sucker, one of the largest species in the sucker fam- ily, is characterized by a pronounced body hump with a sharp edge. It was recognized long ago that much of the arid land in the basin could be converted to agriculture if irrigated. With the start of construction of Hoover Dam in 1930, a series of large dams and reser- voirs were constructed during the next 30 years to insure a reliable supply of water for irrigation and for power generation and flood control. These dams and reservoirs extend along the mainstream from Im- perial Dam, just north of Yuma, Arizona, to Fontenelle Dam, which backs up the Green River to near its source in the Wind River Mountain Range of Wyoming. The man- made reservoirs such as Lake Mohave, Lake Havasu, Lake Mead, Lake Powell, and Flaming Gorge Reservoir are completely new aquatic environments unlike any environment that the native fishes have evolved in or are adapted to. These reservoirs provide enormous recreational use and sustain attractive sport fishing for non-native species, introduced by man. Native fishes are essentially gone from the impoundments and from the cold, clear tailwaters below the dams. The introduction of non-native fishes began almost 100 years ago, when it was recognized that the popular food and sport fishes of the sunfish family (such as the largemouth bass and crappie), the perch family, and the catfish family were com- pletely absent from the Colorado River basin. Also, carp, several species of minnows and suckers, and rainbow, brown, and brook trout have been widely introduced. 7