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<br />except September and October, with the most significant differences in late winter and early <br />spring. Figure 4-1 shows the average monthly flow for the various periods, <br /> <br />Flow duration curves were developed to show the time variability of flows between the two <br />periods, A flow duration curve is a plot of flow versus percent exceedance. Percent <br />exceedance is a particular flow that can be expected to be exceeded that percentage of time, <br />For example, a flow at 75 percent exceedance means that 75 percent of the time the flows <br />exceeded this value and 25 percent of the time they were less than this value. The flow <br />duration curves were developed using average daily flow values to prevent averaging out <br />or masking the variations in flow that could be caused by using monthly averages. The <br />flow duration curves for each month are shown in Figures 4-2, 4-3, and 4-4. Observation of <br />the winter months reveals that as a general rule the pre-project flows were relatively greater <br />and more constant. The flow duration curves for the spring months reveal that the post- <br />project flows were less than the pre-project flows for all but a short period in June. <br />Inspection of the flow duration curves for the summer months shows that in most cases the <br />post-project flows mimic the flow duration curve of the pre-developed flows, with the pre- <br />project flows actually having lower flows for percent exceedance values greater than <br />approximately 60 percent. <br /> <br />Tables 4-3, 4-4, and 4-5 show the calculated percent exceedance values for periods from <br />1943-1969,1970-1994, and 1943-1994, respectively. Figures 4-5, 4-6, and 4-7 compare the 25, <br />50, and 75 percent exceedance flows for the different periods. The method used to calculate <br />percent exceedance in this report should also be noted. The method interpolates the <br />position of these percentages and is not an actual value, unless the percentage interval of <br />the total data series can be divided equally into the percentage desired. <br /> <br />Another parameter for showing the time variability of flows between the pre-project and <br />post-project period is plotting positions for the extreme records. Weibull's plotting was <br />applied to the minimum daily flows because it is the most commonly used method. The <br />results from Weibull's plotting are shown in Figures 4-8, 4-9, and 4-10, The results confirm <br />what the flow duration curves showed: the winter and spring months have a greater <br />frequency of lower flows for the post-project period, and the summer months have a greater <br />frequency of lower flows for the pre-project flows. <br /> <br />Methods for statistical distributions are available for estimating return periods of the high <br />flows. The Log-Pearson Type III method was applied to statistically evaluate high flows. <br />The maximum daily values were used from both periods to determine the return period of <br />high flows annually, and also for the months of June, July, and August. The results are <br />shown in Figures 4-11, 4-12, 4-13, and 4-14. <br /> <br />A flow evaluation was performed to determine frequency and magnitude of flows that have <br />been recommended by the USFWS for channel maintenance. The duration of the <br />recommended peak flows is 14 days. The 14-day moving average was calculated. The <br />percent exceedance was then calculated for the maximum 14-day average from each year. <br />Figure 4-15 shows the flow duration curve for both pre-developed and post-developed <br />flows, The 14-day average value should be used with some conservatism because the flow <br />is an average of larger and smaller flows over the 14-day period. The results reveal that <br />during the post-project period the percent exceedance of the 14-day average for 1,800 CFS <br />and 3,000 CFS was approximately 46 percent and 29 percent, respectively, <br /> <br />SLC\ 1341 07\REPTl,DOC <br /> <br />4.2 <br />