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High flows <br />Peale flow and the average monthly flow for the high flow months were all reduced considerably <br />in comparisons of the pre to post dam periods. The average instantaneous peals flow at Dolores <br />River at Bedrock for the period 1918-1983 (non continuous record) was 5,023 cfs and the post <br />dam (1984-2004) annual average was 2,595 cfs (-48%). The maximum flow of record (9,280 cfs <br />in 1973) during the pre dam period was nearly double the average peak. The lowest <br />instantaneous peak flow during the pre dam record at the Bedrock gage was 1,290 cfs (in 1981), <br />whereas the lowest peak during the post dam period was an order of magnitude lower; 164 cfs. <br />Average instantaneous peak for the Dolores River near Cisco, UT gage for the pre dam period <br />1951-83 was 6,900 cfs and the post dam period (1984-2004) was 5,486 cfs. The maximum flow <br />of record --17,400 cfs recorded in 1958-- during the pre dam period was 2.5 times greater than <br />the pre dam average peak. The lowest instantaneous peak flow during the pre dam record at the <br />Cisco, UT gage was 2,110 cfs (in 1981), whereas the lowest peak during the post dam period was <br />1,260 cfs (in 1996). <br />The snowmelt peak along the Dolores River for the period of record typically occurs before the <br />end of June and monsoon peaks may occur from July through September. The average date of <br />the snowmelt peak at Bedrock is 12 May. Average flow during the month of May decreased by <br />40% at Bedrock but stayed about the same (+1.5%) near Cisco, UT. Average June flow in the <br />Dolores decreased by 54% and 18% at Bedrock, CO and near Cisco, UT, respectively. The 10 <br />year recurrence interval flood has decreased from 9,040 cfs to 5,500 cfs (-39%) at Bedrock, CO <br />and from 13,500 cfs to 10,000 cfs (-26%) near Cisco, UT since McPhee reservoir operations <br />began. <br />The 48% and 20% reductions in instantaneous peak flow in the Dolores River at Bedrock, CO <br />and Cisco, UT have likely resulted in shifts in plant community composition and encroachment <br />of vegetation along a significant segment of the Dolores River. As described in the "River <br />processes, riparian vegetation and flow components" section above, the reduction in the <br />magnitude of peak flow reduces the frequency and extent of inundation of the floodplain <br />resulting in decreased nucrobial activity and potentially to increased concentrations of salt in <br />floodplain soils. Reduction in the frequency of the high flows that mobilize and transport <br />material, rework channel deposits, scour and remove vegetation from channel margins, and <br />transport and deliver seeds and propagules of plants across the floodplain all contribute to the <br />changes in plant community stn~cture that have likely occurred along the Dolores River. <br />The annual floods have not been of sufficient magntude to maintain channel margins free of <br />perennial plants (as noted both in the "DRD geomorphology analysis" section of this report and <br />in Kriegshauser and Sommers 2004). These smaller annual floods have likely resulted in a <br />reduction in the turnover rate of marginal bars, point bars and medial bars and islands, resulting <br />in stable channel features relative to pre dam conditions. As a result of this relative stability, <br />chamlel margins, point bars, and medial bars likely have higher cover of vegetation and probably <br />a higher proportion of perennial species than would have occurred during the pre dam period. It <br />is likely that disturbance adapted species, such as narrowleaf cottonwood, have decreased in <br />frequency along the alluvial reaches of the Dolores River as a result of the factors associated <br />with reductions in the magntude of high flows. <br />54 <br />