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. The Corps has formally established criteria in the water control diagram to provide flood control space to minimize flood damages during a major storm. The water control diagram is shown on Plate 3, and the stage-frequency curve which incorporates the prescribed water control plan is shown on Figure 1. The flood control operation involves filling the reservoir later, while holding less water, less often with more space for flood storage. Basically, the reservoir would not reach gross pool.as frequently; thus, the risk of flooding would be reduced to a 1 percent chance exceedence flood event in any given year. According to the diagram, a variable amount of flood control space would be maintained from March 1 to July 20 with a maximum space of 59,396 acre-feet from May 1 to June 25. This variable space is predicated on filling the reservoir (if possible) by the end of the snowmelt season without exceeding downstream objective flows, The bottom of the flood control pool is at the storage of 25,014 acre-feet (elevation 6,794.9 feet). Flood control operation for the 100-year snowmelt flood would reduce the unregulated (natural) one-day maximum flow at Colona of 4,100 cfs to a regulated flow of 2,000 cfs. Since 1988 when the reservoir and dam was constructed, there has been only 1 year when the snowmelt inflow would have exceeded the downstream channel capacity (2,000 cfs). . . . . 3.3 Comparison of Stage-Frequency Curves. The stage-frequency curve shows the relationship between the reservoir elevation and the interval in years that the reservoir will exceed that particular elevation. The differences between the stage frequency curves relate to the amount and timing of water releases for flood control during several months of the year. Figure 1 shows that without the use of the water control diagram as described in the proposed water control manual, the reservoir has less space for flood control during the peak snowmelt season. Thus, water will pass over the spillway more often, which could cause flooding downstream, In relation to flow releases below the dam (as shown in Table I), the flood control operation with the water control diagram would allow water to be stored later in the year and released earlier in the year. The USBR would store water earlier in the year, but release water later in July. Essentially, the differences in average monthly streamflows between the two flood control schemes are small, no more than 50 cfs per month, and the main differences in releases are during May through August (see Figure 2). There is very little difference between the two alternatives and related stage-frequency curves in regard to causing significant environmental effects. This conclusion is reached by examining the relationship of the differences between the two stage-frequency curves to reservoir fluctuations and flow releases below the dam. . . . . Table 1 compares the average monthly flows below the dam with and without flood control. Table 1 also shows that average monthly flows for with- and without . 6 .