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�Mti"T°FTy United States Department of the Interims <br /> -��+ dry <br /> US ARMY CORPS OF ENGINEERS <br /> ° p ,.3,, RECEIVED <br /> r' ,, _�o U. S. GEOLOGICAL SURVEY <br /> ;;: / - Colorado Water Science Center AUG 0$ 0 <br /> y a9 Western Colorado Office <br /> g4'eM '∎ 764 Horizon Drive <br /> Grand Junction,CO 81506 DURANGO REGULATORY OFFICE <br /> IN REPLY REFER TO <br /> August 5, 2005 <br /> Kara Hellige, Project Manager <br /> US Army Corps of Engineers, Sacramento District <br /> Durango Regulatory Office <br /> 278 Sawyer Drive, Suite#1 <br /> Durango, Colorado 81303 <br /> Dear Ms. Hellige: <br /> The U.S. Geological Survey, Colorado Water Science Center is providing comments regarding Public <br /> Notice No. 200575106 dated July 15,2005. Our primary concerns are related to the proposed <br /> reconstruction of existing fish habitat structures, addition of large boulders, and installation of boulder <br /> edge and riparian terraces in the San Juan River channel upstream of the Hot Springs Boulevard Bridge. <br /> Our concern is heightened as a result of channel work completed in this reach of the San Juan River by <br /> the Town of Pagosa Springs in March of 2005. This channel work destroyed the in-stream measuring <br /> equipment at the USGS streamflow-gaging station, San Juan River at Pagosa Springs (09342500). This <br /> streamflow-gaging station had been in existence since 1910 providing near real-time stage and <br /> streamflow data to numerous federal, state, and local governments as well as recreational water users. <br /> That channel work in the reach resulted in the relocation of the gaging station 200 feet upstream of the <br /> Hot Springs Boulevard Bridge. <br /> Although the plans included with the public notice state that all drop is below the existing channel <br /> invert, analysis of the high water line from the peak flow on May 23, 2005 at the old gage location had a <br /> stage value of 7.04 feet. The preliminary peak flow for Water Year 2005 determined for the San Juan at <br /> Pagosa Springs is 4,890 cubic feet per second. Using the stage-discharge rating that was in use at the <br /> old gage location,the corresponding stage value for this flow would have been 6.47 feet. This means <br /> that the channel has been raised about 0.60 feet possibly decreasing the channels capacity by about <br /> 1,000 cubic feet per second during spring runoff and high streamflow during the monsoon season. <br /> Since relocation of the gaging station,the USGS has made several discharge measurements needed to <br /> develop a new stage-discharge relation at the new gaging station. The accuracy of a stage-discharge <br /> relation depends on channel stability through the gagged reach. The new gaging station location is <br /> located 30 feet upstream of an existing "W"weir that was placed in the stream during channel work <br /> completed in 1993. This weir structure has remained relatively unchanged since it was installed and is <br /> considered to be a stable channel feature. Any proposed work upstream of the Hot Springs Boulevard <br /> Bridge could potentially affect the operation of the new gaging station and could disrupt the ability of <br /> the USGS to provide real-time streamflow data at this site. During the proposed construction phase the <br /> temporary coffer dams will affect streamflow computations at low flows and after the work is <br /> completed,the stage-discharge relation during high flow is expected to be affected as well. <br />