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. . . Pl... ...~ -... .'.}. r. ,~, liU~~lJJ Evaluation of the Standards and Methods Used for Quantifying Instream Flows in Colorado Colorado's Instream Flow/Natural Lake Level Program ("ISF Program") was initiated in 1973 with the passage of Senate Bill 97 (SB 97) which vested the Colorado Water Conservation Board (CWCB) with statewide authority to appropriate instream flow and natural lake level water rights. SB 97 was subsequently amended by SB 414 in 1981, SB 91 in 1986, SB 212 in 1987, SB 181 in 1989, and SB 64 in 1996. Colorado's instream flow legislation is contained within 9 37-92- 102(3) of the Colorado Revised Statutes (1997). Colorado's ISF Program is administered under the guidance of the "Rules and Regulations Concerning the Colorado Instream Flow and Natural Lake Level Program" which were adopted by the CWCB on November 10, 1993, and amended on September 13, 1994 (Colorado Water Conservation Board (1994). The "Rules" were amended again in 1998 to reflect the legislative changes that resulted from the passage ofSB 64. Instream flow rights are held by the CWCB on behalf of the people of the State of Colorado to "preserve the natural environment to a reasonable degree." The CWCB currently holds more than 1,800 water rights covering approximately 8000 miles of Colorado streams and almost 500 of the state's natural lakes. In Colorado, instream flow appropnatIOns are generally based on biological recommendations provided to the CWCB by various state and federal agencies. To date, the majority of the Board's appropriations have been on coldwater stream reaches or high elevation lakes. Biological flow recommendations have generally been based on the premise that the amount of water needed to preserve an aquatic indicator species, like a trout, is the same amount of water necessary to preserve the entire natural environment. The most frequently used methodology for developing biological flow recommendations for Colorado's coldwater streams has been R2CROSS (Nehring 1979; Espegren 1996). The R2CROSS methodology has proven to be advantageous because (1) it requires data from only a single field visit, (2) the procedures are very easy to learn and implement, and (3) data interpretation is simple. In addition, recommendations from R2CROSS output are generally more representative of site-specific stream conditions than non-field methods. R2CROSS recommendations have also been found to be quite similar to, and more cost effective than, recommendations developed from more data intensive techniques (Nehring 1979). Finally, the hydraulic criteria that are used to develop R2CROSS-based instream flow recommendations are specifically designed to protect the biologic flow needs of coldwater fish species within riffle habitats (Nehring 1979). Consequently, the R2CROSS methodology has been, and will continue to be, an important quantification technique for developing instream flow recommendations on coldwater stream segments throughout the state of Colorado. 1