Laserfiche WebLink
Historic. Baseline. and With Project Sediment Balances <br />Tables 13 through 15 show the running balance for the historic, baseline, and <br />project conditions, respectively. The right hand columns of each table show <br />the number of days within a year that the balance was set to zero. The general <br />trend during a water year is the gradual accumulation of sediment until the <br />peak flow months when sediment is moved out by high flows. The running balance <br />may be interpreted as showing the accumulation of sediment deposition in pools, <br />backwater areas, beaches and other localized areas. Years for which the <br />balance is not set to zero are generally lower or average annual flow years, <br />preceded by low or average flow years. The with project balance is never set <br />to zero, and has a continual upward trend which is reduced somewhat by the high <br />flow years of 1983-1986. <br />Sediment Balances of Reduced and Redistributed Peaks <br />As shown earlier, the running balance of historic and baseline conditions are <br />very similar. More confidence can be placed in the 'historic' Deerlodge Park <br />daily flow record than with the daily flow records constructed from the ratios <br />of mean monthly flows, as described previously. For these reasons, the <br />majority of program runs with reduced and/or depleted daily flows of April <br />through June were made using the 'historic' daily flow at Deerlodge Park, since <br />the intent was to determine 'minimum hydrographs' that maintain channel <br />morphology. <br />When the peak flow months of April through June were reduced by various <br />percentages and redistributed to power generation months of July-September and <br />December-March, similar patterns were observed. Tables 16 through 20 show the <br />running balances for retaining 900, 800, 75%, 740, and 73% of the daily flow <br />values, respectively. As more and more of the peaks are reduced, only the high <br />flow years of 1983-1986 are able to reduce the balance towards zero. The mean <br />annual discharge remains essentially the same, only the daily distribution of <br />flow changes. Mean annual flows are not exactly equal because the reduced <br />peaks from one water year are redistributed into the remainder of that year <br />(July-September) and December-March of the next water year. The program 'goes <br />back' and reduces the April-June daily flows from water year 1940, so as not to <br />skew the flows in water year 1941. Table 21 summarizes the budgets shown in <br />Tables 16 through 20.