2001-06-04_REVISION - M1977493 (2)

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the significance of the delivered water to the Eagle River system. When the dilution of a new or increased dischazge is greater than or equal to 100 (receiving stream) to 1 (discharge) during low-flow conditions, the new or increased dischazge is considered not to be degrading the quality of the receiving stream. The location where this determination must be made is that portion of the receiving stream from the discharge point to the first major tributary (i.e., the first segment of the stream affected by the dischazge). For the EPR delivery analysis, the relevant receiving water is Segment 3 of the Eagle River Basin, between the confluence of the EPR Outfall and the South Fork of the Eagle River (the Eagle River, Segment 2, begins at the confluence of the East and South Forks). Flow data from the USGS gauging station on the East Fork are sparse, and only one data point falls during the period when the EPR was delivering water (Table 3). However, even from this sparse data set, it can be seen that flows of 3 cfs or less occur in the East Fork during much of the year (average flow excluding one high spring runoff event). Because daily EPR Outfall flows range from zero to more than 6 cfs during the low-flow months (Table 4), dilution ratios as low as 1 to 2 can be inferred, faz smaller than the 100 to 1 dilution considered to represent insignificant degradation. A significance determination based on volume likely would derive a similaz conclusion. Therefore, using either the flow rate or volume significance determination approach, antidegradation review cannot be avoided based exclusively on the flows in the outfall and the East Fork. The EPR Outfall flow can also be compazed to USGS flow data for the continuously operated Red Cliff monitoring station on the Eagle River, approximately 12 miles downstream from the junction of the EPR Outfall and the East Fork, where flow data aze plentiful. These data can be used to represent the significance of flows from EPR by comparing them to the Eagle River during periods of water delivery. Table 4 provides the average daily flows for each month of 1999 and 2000 (after the initiation of EPR outflow) at the Red Cliff monitoring station, along with average flow at the EPR Outfall. As can be seen from this table, the dilution potential in the Eagle River at Red Cliff is approximately 4 to 1 during the period when the EPR is delivering water, again smaller than the 100 to 1 dilution called for in the flow-and-volume significance determination approach. Baseline Available Increment Approach The baseline available increment significance determination approach considers the effect that the new or increased dischazge will have on the receiving stream's water quality. In this analysis, the new or increased discharge will be considered not to result in significant degradation if the dischazge consumes less than 15% of the available water quality increment in the receiving stream. The available increment is the difference between the low-flow pollutant concentration in the receiving stream and the relevant standards. This determination is to be made for that portion of the receiving stream extending from the discharge point to the first major tributary (i.e., the first segment of the stream affected by the discharge). For the EPR, the analysis should be made on Segment 3, the East Fork, from below its confluence with Outfall waters to its confluence with the Eagle River. Water quality data are available for the East Fork above the discharge point (Station ER_Al), below the discharge point (Stations ER_B1 and 8601631.005 0101 0501 0380 \~bouldert Watabrgects\t9d7_cllmakept repon_2 dx