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B. Physical Factors and Activities in the Determination of the Without Fish Scenario for <br />the Colorado River Basin <br />Over the last century, the physical characteristics of the Colorado River Basin have been <br />altered significantly. Historically, barriers to the migration of endangered fishes in the Basin <br />were predominately natural rapids and swift, turbulent waters. While these natural barriers <br />may have impeded the migration of fishes, they were not absolute impediments. The 1902 <br />Reclamation Act lead to major dam development in the Lower Basin. These dams and <br />diversions have fragmented the river systems and obstructed the fishes' ability to migrate. <br />Since 1905, 4b dams have been constructed in the Colorado River Basin. As a result, the <br />physical characteristics of Basin habitats have been altered significantly. These dams have <br />diverted water from the Colorado River system to agricultural, municipal, and industrial uses. <br />Flow regimes have been altered, resulting in changes in water temperature and sediment <br />loading.' <br />Coupled with the development of diversions and dams has been the increase in water <br />consumption. Developed, irrigated land in the Basin is now in excess of 2.1 million acres. <br />Table I-3-1 provides estimates of the water consumed for irrigation and other uses. These <br />uses have depleted river•flows thus changing the nature of the Basin. <br />Coupled with the physical changes in the river system, the uses of the river and the impacts <br />of activities occurring along the river have increased signif cantly over the years. The growth <br />of agriculture over the past 50 years has been phenomenal. Not only has this growth affected <br />the river through water depletion, but also has affected the river in other ways. For instance, <br />contaminant releases from agricultural, muticipal, and industrial activities have affected the <br />overall water quality of the river. Furthermore, facilities sited along the river, such as <br />recreation campgrounds, have affected habitat as have industries that produce oil and gas. <br />`A significant amount of detail is provided in the Biological Support Document regarding the physical changes in <br />the river. <br />I-12 <br />