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<br /> <br />The historical diversions and impoundments on <br />the Dolores River, discussed in later sections, have <br />resulted in changes to both the seasonal flow regime <br />and peak flow rates within the study area. The loss <br />of water from the river system is more evident at low <br />flows. This can be seen in Figure 7 where, from June <br />through November, the mean monthly flows at <br />Dolores were greater than the flows at Bedrock, 125 <br />miles downstream. The data in Figures 8 and 9 <br />specifically define these low flow impacts. For <br />example, referring to Figure 8, the Bedrock 10-year, <br />7-day low flow was 0.1 ft3fs, while at Dolores the <br />same recurrence interval flow was over 22 ft3fs. The <br />flow duration curves in Figure 9 indicate that the low <br />flows at Dolores exceeded the low flows at Bedrock <br />more than 75 percent of the time. <br />The impacts to peak flow rates from water <br />diversions are more significant at the lower Recur- <br />rence Interval (RI) events. As can be seen in Figure <br />10, the 1.5-year RI peak flow is essentially the same <br />for both Bedrock and Dolores. <br />However, as the RI increases, so does the <br />difference between the peak flows at these two <br />stations. This possibly results from the water <br /> <br />40 <br /> <br />30 <br /> <br />10 <br />"'-. <br />- <br />u. <br />~ 20 <br />o <br />u:: <br /> <br />10 <br /> <br />diversions becoming less significant with higher <br />flows, and the higher flows being more a function of <br />drainage basin features such as runoff characteristics, <br />climate, and basin size. <br />The natural occurrence of high flows in a river <br />system provides the energy needed to maintain the <br />river's fluvial features. The bank-full discharge has <br />been found to be the most dominant or effective flow <br />for maintaining these features. The bank-full <br />discharge is defined as the stage of a river channel <br />above which the banks are overflowed onto the <br />floodplain. This discharge closely correlates to a <br />1.5-year RI flood event (calculated from an annual <br />flood series, or a 0.90 RI if calculated using a partial <br />series). On the average bank-full flow is equalled or <br />exceeded 4-7 days annually (Leopold and Dunne, <br />1978). Table 3 compares the 1.5-year RI peak flow <br />for gauging stations at Dolores, near McPhee, and at <br />Bedrock, against the l.S-year 7-day high flow. <br />Without the historic water diversions below the <br />Dolores station, it would be expected that the peak <br />and 7-day high flows for both McPhee and Bedrock <br />would be somewhat higher. In addition the period of <br />record used for the flow calculations at the McPhee <br /> <br />At Dolores <br />(191 7-22; 1971-81) <br /> <br /> <br />At Bedrock <br />(1895-1903; 1910-12; 1921-81) <br /> <br />10 <br /> <br />100 <br /> <br />Recurrence Interval Years <br /> <br />Figure 8. Dolores River 7-day low flow statistics. <br /> <br />15 <br />