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~ o , .' 3 co Colorado River basins. In 1948, the Upper Basin states (wyoming, Colorado, utah, and New Mexico) signed an agreement apportioning the upper basin share between the states. Arizona was also apportioned 50,000 acre-feet in that agreement. Each of the States, in concert with the Bureau of Reclamation under the authority of the Colorado River storage project Act (CRSP), initiated the development of their portion of the waters of the Colorado River basin. The passage of the CRSP Act allowed for the construction of many large mainstem impoundments on the Colorado River and various tributaries including Navajo Dam on the San Juan, Flaming Gorge on the Green River and the Aspinall Unit on the Gunnison River. , ~ .. ~-' While the construction of these impoundments was essential for the development of water storage and flood control and to allow the Upper Basin States to develop their water resources, their construction and operation altered natural river ecosystems and, thereby, the native floral and faunal communities of the Colorado River. At the time of dam construction, little attention was given to the needs of the native species that evolved in the Colorado River system. As a result, natural riverine habitats were altered, migration routes blocked, and selective chemical treatments were applied to eradicate native species in favor of non-native sport fish species. The physical and biological changes to the environment, such as the disruption of the natural flow regime and changes in water temperature and quality, that were brought about by the construction of these large mainstem impoundments, led to the endangerment of four native fish species of the Colorado River. Two of these species (Colorado squawfish and razorback sucker) inhabit the San Juan River, a third (bony tail) probably occupied the river, and the fourth (humpback chub) may have historically occurred in the river. 1.3.2 Other Human Induced Impacts other human-induced impacts to the Colorado River System and specifically to the San Juan River include contamination from oil and gas development and agricultural return flows which may contain heavy metals and other forms of contamination. Urban development in the Basin, with attendant urban runoff, sewage effluent, and watershed alterations have also affected the aquatic environments of the San Juan River drainage. The cumulative effects of these and other possible impacts have resulted in reduced population levels of the native fish species, with two species, the Colorado squawfish and razorback sucker being threatened with extinction. One other species, the bony tail, appears to be extinct in the San Juan River.