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River throu h the tailrace. All Orchard Mesa diversions durin the non-irri ation season were g g g assumed to be used for power development and returned to the river through the tailrace. For the period during which the USGS gage has been in operation at Palisade coincident with the study period for this hydrologic analysis (October 1990 through September 1993), a comparison was made of the monthly flows generated as discussed above and the actual flows measured at the gage. The calculated flows ranged from -8 percent to +9 percent of the actual flows on a monthly basis, and were within 4 percent on an annual basis. Figure 3.4 illustrates the differences between the estimated and actual flows. Given the accuracy of the USGS gaging records and the Colorado Division of Water Resources diversion records, the procedure for estimating flows appear to produce reasonable results. The estimated flows were also compared to the flows developed by the USFWS (1995) and are slightly less than, though reasonably close to, those values. The relationship between streamflow and gage height of the Colorado River at the Palisade gage has been established by the USGS during the course of its flow and channel cross-section measurements. This relationship was used to relate the flows computed for the top of the 15-mile reach to water Levels. The flow-gage height relationship, obtained from the USGS, is shown as a streamflow rating curve on Figure 3.5. The lower portion of the curve (below the 3.20 ft gage height) was extrapolated by CDM. This relationship was used to estimate reductions in flow depth due to future Ute Water diversions (see Section 3.3.4). It can also be used to provide a historical perspective of flow depths prior to Ute Water diversion. Finally, similar to work by the USFWS (1995), changes in flow depth can be related to estimates of the change in types offish habitat for the 15-mile reach (Section 3.3.5). 3.3.3 Section 7 Baseline Flow Adjustments Through the informal consultation process, the USFWS identified the need to incorporate the effects of previously approved Section 7 consultations into the Colorado River baseline flows. This is necessary to account for depletion effects of project withdrawals that have been approved, but not yet implemented. The USFWS position is that under the federal regulations', the impacts of all previously approved projects must be considered as part of the environmental baseline when evaluating the effects of a new project such as this. Therefore, it is a requirement under the law to adjust the historic Colorado River baseline flows within the 15-mile reach to reflect the reductions due to previously approved Section 7 consultations. To properly account for the Section 7 baseline flow adjustments to the Colorado River flow at the top of the 15-mile reach, it was necessary to identify previously completed Section 7 consultations within the Colorado River Basin and distribute these. reductions on a monthly basis. These total monthly reductions were then subtracted from the historic Colorado River flows at the top of the 15- mile reach prior to evaluating the effects of the Plateau Creek Project future flow reductions. J ' Federal Register/Vol. 51, No. 106/Tuesday, June 3, 1986/Rules and Regulations, 19932. CDM Camp Dresser & McKee O:\8047-110\DOC\APPEND-B.DOC B-1 1