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. Rio Grande located about 4,000 feet downstream of the dam. Flow at the gaging station is regulated by storage in Rio Grande Reservoir. No tributaries enter the river between the dam and the gaging station. Reservoir storage records are available for the period of May 1969 through 1980. All earlier records have been lost. The inflow inflow to the reservoir was calculated on a monthly basis by the following expression: Inflow = Outflow + Change in Storage The inflow calculated by this method should be accurate within the I imitations of the USGS gage. The gage is rated as "good," meaning that about 95 percent of the daily discharges are within ten percent of the measured values. A summary of reservoir inflow is presented in Table 2. Monthly inflow for 1950 through 1968 was estimated through use of the USGS gage records and analysis of the inflow pattern for 1970 through 1979. The monthly percentage of annual streamflow was deter- mined for 1970 through 1979. These records were ranked: three driest years (1971, 1974, 1977); four average years (1970, 1972, 1976, 1978); and three wettest years (1973, 1975, 1979). The average monthly percentages were then determined for the representative dry, average and wet years. The annual discharge records at the USGS gage for the period of 1950 through 1969 were broken down on a monthly basis through use of the dry, average and wet year monthly flow percentages. This method of inflow estimation is subject to errors caused by annual reservoir change in storage of as much as 40,000 acre-feet. The calculated inflow is correct on an overall basis to within the accuracy of the USGS gage. . Reservoir Capacity According to the original filing map prepared in 1903, the storage capacity of Rio Grande Reservoir is 41,587 acre-feet. This value was used in the flood routing calculations in the 1978 dam safety inspec- tion report prepared by Bruns, Inc. A revised filing map was prepared . -15-