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r, :.1 I- ""..1 :-... -..J Colonel John N Reese 23 DRAFT Table 2 includes the monthly depletions calculated as a percentage reduction in flow of the San Juan River at Shiprock, New Mexico. The table shows that the range of depletions resulting from operation of an enlarged Red Mesa Reservoir includes no depletion at all in a dry year (1945/1990) to a 2.2 percent depletion in November of a wet year (1948/1986). The highest annual depletion for the years analyzed was 0.29 percent in an average year (1944/1978). The analysis shows that most of the depletion will occur in the months of January through April, although in some wet and average years depletions will occur in November, December, and Mayas well. For the years analyzed, the highest monthly diversion from the La Plata River was 2,314 acre-feet in April of the average year 1978, which constitutes a 1.7 percent depletion to the San Juan River at Shiprock in the same month of the similar year 1944. The highest annual diversion was 3,538 acre-feet in the wet year 1986, which constitutes a 0.23 percent depletion to the San Juan River at Shiprock in the similar year 1948. As previously stated, while water from the La Plata River would be diverted to Red Mesa Reservoir primarily during the winter and early spring, the Project could ultimately affect flows throughout the year. Modifications to Navajo Dam operations are required by the reasonable and prudent alternative for the Animas-La Plata Project to achieve a more natural flow regime in the San Juan River. Reducing the unnaturally high winter flows would allow more water to be stored for release during the runoff. However, the proposed reduction in winter releases from Navajo Dam may not be entirely achieved if water is needed downstream to replace that depleted by Red Mesa Reservoir. If less winter water is stored in Navajo Reservoir, less water might be available for release during the runoff and other times of the year. The frequency at which Red Mesa Reservoir diversions would affect operations of Navajo Dam depends on the frequency at which large Red Mesa Reservoir diversions coincide with relatively low San Juan River flows. The Project's effect on Navajo Dam operations also depends on the outcome of ongoing research and the final recommendations for winter releases from Navajo Reservoir. In the worst case, the Project's effect on (percent reduction of) runoff flows in the San Juan River would be considerably less than the calculated percentage depleti~ns shown in Table 2 because the Project diversions would constitute a substantially lower portion of the San Juan River runoff flows than of the San Juan River winter flows. For example, the largest monthly diversion for the years analyzed is 2,314 acre-feet in April 1978 which is 1.7 percent of the San Juan River flow at Shiprock in April of the corresponding average year 1944. This same April 1978 diversion is only 0.76 percent of the San Juan River flow in June (the peak of the runoff) 1944. The entire quantity of water diverted in 1978, 3,062 acre-feet, is 1.00 percent of the San Juan River flow in June 1944. A possible result of the Project, at least in some years, is to reduce winter flows without affecting Navajo Dam operations. In this case, the reduction in winter flows could contribute to the return to a more natural flow regime in the San Juan River during the winter. The results of ongoing research could help to identify and quantify the effects of reducing winter flows on endangered fishes.