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10 HlGH-FLOW FREQUENCY DATA The flood-control storage needed to limit the release of reservoir out- flow is dependent on the amount and distribution of the inflow. Therefore, knowledge of inflow is needed and may be evaluated in terms of magnitude, frequency and duration of natural high flow. The magnitude and frequency of natural high flow data for durations ranging from instantaneous peaks to 183 days have been developed from the daily streamflow records of 101 gaging stations to define frequency curves such as shown in figure 2. As a know- ledge of the process is not required in order to understand the development of storage requirement data, the methodology is presented in the appendix of thi s report. As described in the appendix the frequency curves at a site are based on the observed data extended to a long-term period of unregulated conditions and modified by the historical data on flood peaks. The ordina te scale of fig- ure 2 is logarithmic and the abscissa scale is normal probability. To the extent that discharges experienced during a long term period in the past are a sample of future expectancy, the abscissa scale may be read as the percent chance of exceedance. Percent chance of exceedance can be converted to recurrence interval by dividing into 100. Thus a 20-percent chance is equal to a 5-yearrecurrence interval or one that may recuron the average of once every 5 years. The nine separate high-flow frequency curves in figure 2 show the max- imum average flow expected to endure for selected periods of time. For ex- ample, the maximum 7 -day me an flow has a 2 -pe rcent chance of being great- er than 21,000 cfs in anyone year. This rate of flow during the 7 -day period is equivalent to a total volume of 292,000 acre-feet of runoff or a me an depth of 5.93 inches of runoff from the drainage basin. High-flow frequency data determined from such curves as those in fig- ure 2 are listed in table 2 for each of 101 streamflow stations for which 10 or more years of unregulated record are available. The information is ex- pressed in cfs per square mile for five values of chance of exceedance be- tween 2 and 50 percent. Each streamflow station is identified by a number that is shown on the map of figure 1 and on othe l' maps of this report. Also given is the location, years of record used for analysis and drainage area. Appendix 1 contains details of the development and limitations of these data. The data in table 2 provide only an intermediate step in the investiga- tion of storage requirements for this report, but they have been presented so that users may study other problems, such as frequencies of flood volume s of ,'unoff, at these sites. The data in table 2 show the considerable range in flood volumes across the State. For example, stations 66 and 4 are located