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• -3- INTRODUCTION • As we near the end of the 1970's, we find that most major rivers of the world have been exploited by man to satisfy the agricultural and urban needs of the human populations in adjoining regions. Experience has shown that the process of development produces profound changes in the river environment; changes that can lead to unexpected social, economic, and environmental problems. The Colorado River of the American Southwest is a river with such a history. Although modern man began to utilize the waters of the Colorado only slightly more than a century ago, the intensity with which it has been and continues to be exploited has made it one of the most developed rivers of the world. Nearly 100y of its average annual flow is appropriated for agricultural, domestic, and industrial consumption. The purpose of this paper is to review the history of development of the lower Colorado River, describe the effects of ,development on the natural environment, particularly in relation to the native fishes,. and to explain how government agencies including the California Department of Fish and Game have dealt with the problems of fishery resource management in this ever- chaaging habitat. THE COLORADO RIVER The Colorado River is the fifth longest river is the United States. It originates in the Alpine zone of the Rocky Mountains at about 40° N latitude., and empties into the Gulf of California at 31° 40' N latitude (Tabl'e 1). Its 2,250 km length drains a watershed of b.27 x 105 km2 or approximately 7.7x of the surface area of the continental United States. By world standards, however, it is not among the largest. For example, .the Nile, the largest, is three times as long at 6,690 km and discharges over five times as much water annually into the Mediterranean Sea, or at least it did prior to construction of Aswan Dam. 'The average annual discharge of the Colorado in the years prior to its development was 17,500 hm3. Under present conditions, water no longer flows into the Gulf of California; the last time it did so was in 1961. All of the runoff of the Colorado is now either intercepted and consumed in headwater tributary areas, or is exported and consumed outside the basin. The Nile and the Colorado are similar in that both flow through dry, hot deserts in their lower reaches. However, the Nile originates in the tropics, while the Colorado arises in the north temperate zone. Except for the upper- most headwaters that arise mostly from the Rocky Mountains, the summer climate of the Colorado can be characterized as hot and dry. In the winter the climate of the-upper river area is cold, with snowfall a common occurrence, while in the lower Colorado area the winter climate is warm and mild. For purposes of this report, the lower Colorado River is the 584 km portion from Hoover Dam (Lake Mead) to the Gulf of California (Figure 1). All but the uppermost 130 km and lowermost 64 km of this section constitute the border between the states of California and Arizona.