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e . I. .~ ".:. .~-:~. ~-:~:~ "\( San Juan River The San Juan River originates in southwestern Colorado and heads westward through the states of New Mexico and utah until it empties into Lake Powell. At the Bluff, Utah, gauge the river has a drainage area of about 23,000 square miles and has varied from a maximum of about 70,000 cfs in 1927 to various periods of no flow during drought years. The following table lists the average annual discharge at three important sites on the San Juan River, Farmington, Shiprock, and Bluff for the 1935 - 1962 period prior to the construction of Navajo Dam and the 1963 - 1989 period following its closure: Average Annual Discharge (acre-feet) San Juan River San Juan River San Juan River at Farmington at Shiprock at Bluff 1935 - 1962 1,622,000 1,675,000 1,746,000 1963 - 1989 1,466,000 1,460,000 1,643,000 Since 1963 Navajo Dam has significantly altered the flow of the San Juan River by storing the spring peak flows and releasing water during the summer, fall, and winter months. As shown in Figure 10, this has resulted in an approximate doubling of average winter baseflows and a 45 percent decrease in the,average spring peak flow at the Bluff gauge. Similar comparisons can be made at the Farmington and Shiprock gauges, shown in Figures 11 and 12. Consumptive use storage and deliveries and the maintenance of the trout sport fishery are the primary factors for dam releases in normal and dry years while the control of the spring snowmeit runoff is the driving force for the releases in wet years. For the purpose of analyzing the effect of the Animas-La Plata project, future dam operations are expected to be consistent with historic operations, 1963 through 1989. Frequency analyses were performed on the historic gauge records of these three San Juan River gauges to understand the probability of extreme high and low flows as well as average conditions. The historic gauge records are listed in Tables 2 through 4 in acre- feet and in Tables 6 through 8 in cfs. These were again segregated into pre-Navajo and post-Navajo periods, 1935 through 1962 and 1963 through 1989. The results of these analyses have been plotted as Figures 13 through 18. with the project in place, the city of Bloomfield would divert water from the San Juan River at a point upstream from the confluence of the Animas River and the water depleted would be replaced by project water from the Animas River. As a result, the project would decrease the flow of the San Juan River from the diversion point to the confluence of the Animas River throughout the year. The previous discussion of the effects of the project on the Animas River at Farmington would also apply to the San Juan 7