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function of their magntude, frequency, and duration. For example, the magnitude of high flows <br />intercts with the existing channel and valley shape to determne the maximum width, depth and <br />velocity of the flow, including the extent of floodplain inundation. High-flow magnitudes also <br />determine the size of bed substrates that can be mobilized and whether bed sediments are <br />transported as bedload (i.e., rolling or bouncing along the channel bed) or as suspended load (i.e., <br />suspended in the water column). The frequency of high flows determines not only how often <br />processes such as sediment transport, bank erosion, and vegetation scouring occur, but also <br />temporal sequence of high-flow events and the amount of intervening time in which channel <br />adjustments associated with low flows, such as encroachment of riparian vegetation or <br />infiltration of fine sediment into the channel bed, can occur. High-flow duration influences the <br />total amount of sediment transport and other geomorphic work performed by the high flows and <br />can, by determining the extent of floodplain inundation, mediate effects on riparian vegetation. <br />These high-flow components, as well as the timing and rate of change of high flows, are also <br />extremely important to aquatic orgaiusms (Poff et al., 1997). <br />Alterations to high flows in the Dolores River <br />Impacts to the flow regime of the Dolores River occurred before the constn~ction of McPhee <br />Dam. Prior to constn~ction of McPhee Dam, the mean aiu7ual flow at Dolores (above McPhee <br />Dam) was 763 cfs and decreased to 465 cfs at Bedrock (94 miles downstream from the dam). <br />The decrease was caused by diversions in place prior to constn~ction of McPhee Dam (BLM, <br />1990). McPhee Dam increased the depletion of the annual flows from 30% to 69% of natural <br />flow (USDI BLM, 1990). Construction of McPhee Dam in 1984 affected the flow regime of the <br />Dolores River by altering the spring peak flows and the magnitude and variability of the base <br />flow. Between 1986 and 2004, the spring peak was essentially eliminated downstream from the <br />dam for six of the 19 years of record (Figure 1 and Figure 2). k7 an average runoff year, both the <br />magnitude and duration of the spring peak flow are decreased (Figure 3). Correlation of the peak <br />flows above and below the dam show a distinct decrease in the peak flows below the dam <br />(Figure 4). <br />23 <br />