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B. Physical Factors and Activities in the Determination of the Without Fish Scenario <br />for the Colorado River Basin <br />Over the last century, the physical characteristics of the Basin have been altered significantly. <br />Historically, barriers to the migration of endangered fishes in the Basin were predominately <br />natural rapids and swift, turbulent waters. While these natural barriers may have impeded <br />the migration of fishes, they were not absolute obstructions. The 1902 Reclamation Act lead <br />to major dam development in the Lower Basin. These dams and diversions have fragmented <br />the river systems and obstructed the fishes' ability to migrate. Since 1905, 46 dams have <br />been constructed in the Colorado River Basin. As a result, the physical characteristics of <br />Basin habitats have been altered significantly. These dams have diverted water from the <br />Colorado River system to agricultural, municipal, and industrial uses. Flow regimes have <br />been altered, resulting in changes in water temperature and sediment 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 significantly over the years. The <br />growth of agriculture over the past 50 years has been phenomenal. Not only has this growth <br />affected the river through water depletion, but also has affected the river in other ways. For <br />instance, contaminant releases from agricultural, loss of flooded bottom lands, and municipal <br />and industrial activities have affected the overall water quality of the river. Furthermore, <br />facilities sited along the river, such as recreation campgrounds, have affected habitat as have <br />industries that develop oil and gas resources. <br />'A significant amount of detail is provided in the Draft Biological Support Document regarding the physical <br />changes in the river. <br />I -12 <br />