San Juan Dolores Draft Instream Flow Recommendation

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bluehead sucker needs, there would also be sufficient water to maintain pool habitats relied upon by roundtail chub. When developing recommendations for flow rates to support flannelmouth sucker and bluehead sucker, BLM and CPW personnel examined tables and graphs produced by the PHABSIM model that show amounts of"weighted usable area" (suitable habitat) available at various flow rates for each of the two species. The BLM and CPW personnel then identified the most "efficient" flow rate for providing habitat protection. In this case, the most "efficient" flow rate is defined as the minimum flow rate that protects at least 90 percent of the habitat that is potentially available within the stream 'channel for both species. For example, if a PHABSIM modeling run showed that an equal amount of weighted usable area was available at either of two different flow rates, then lower flow rate was identified for protection because it is more efficient. In addition, when identifying minimum flow rates, BLM and CPW personnel also considered the amount of time weighted usable area is available, specifically the number of days within a calendar year. CPW and BLM considered this factor because much of the potential habitat in the Dolores River is available during only a very short time period during the peak of snowmelt runoff. Application of R2Cross Criteria The primary objective of most cross section methodologies, including R2CROSS, is to maintain quality riffles. Riffles are the most vulnerable habitat to dewatering and riffles are important for invertebrate productivity. As noted previously, riffles comprise only a very small portion of the total habitat area in this stream reach, so the importance of maintaining riffle habitats is magnified even further. BLM and CPW performed a reconnaissance to identify the various types of riffles within this stream reach, based upon width, substrate, and average water velocity. The two agencies then selected four representative riffles for further R2Cross analysis. BLM and CPW personnel applied the following R2Cross criteria in evaluating the R2Cross modeling runs: • Maintain 70%of wetted perimeter, given that the channel width typically exceeds 60 feet. • Maintain 1.3 feet per second average velocity and maintain 1.0 average depth, which creates at least marginally suitable habitat for flannelmouth sucker and bluehead sucker. The R2Cross model provides reliable predictive results for flows that are up to 250% of the flow measured during the data collection effort. It also provides reliable predictive results for flows down to 40%of the flow measured during the data collection effort. This range, from 40%to 250% of flows measured during the data collection, is referred to as the "confidence interval" for R2Cross modeling. When the flow rate that meets the instream flow criteria fell outside of this confidence interval, data from that cross section were not used to develop instream flow recommendations.. The results from cross sections with usable results (inside the confidence interval)were averaged to develop the recommended flow rates.