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UJL~3{16 . Discussion of Existing Methodologies and their use in Colorado R2CROSS Nehring (1976) concluded that the R2CROSS methodology is appropriate as "a first level of evaluation". He also recommends that it be used on "headwater streams at high elevations" and on streams "where encroachment by diversion, pollution, and development is not anticipated as a serious problem." The CWCB staff agrees that the single-transect R2CROSS methodology is the best available methodology for developing instream flow recommendations on the majority of Colorado's coldwater streams. It provides a site-specific, cost-effective, quantification of the streamflow required to "preserve the natural environment to a reasonable degree." R2CROSS-based instream flow recommendations on coldwater streams should be verified against recommendations resulting from other methodologies whenever possible. For example, on gaged stream reaches, R2CROSS-based instream flow recommendations can easily be compared with recommendations that result from the Tennant Method using stream gage records from the USGS, the Colorado State Engineers Office, or other accepted sources. In addition, output from the R2CROSS staging table can be used to generate a plot of wetted perimeter versus discharge. This plot can then be used to compare an R2CROSS instream flow recommendation against one derived from the Wetted Perimeter method. The verification ofR2CROSS recommendations against the Tennant and Wetted Perimeter methods could be accomplished with very little additional effort or expense. When the resulting recommendations are similar, this verification process would add additional confidence to the R2CROSS-based instream flow recommendation. . The CWCB and CDOW staff should also work to improve the hydraulic component of the R2CROSS model. Nehring (1979) suggested a "tendency of the R2CROSS model to overestimate the average velocity by an average of 45% from measured field values" and that "some action should be taken to balance measured water stage levels and field velocity measurements." The tendency for R2CROSS to overestimate instream velocities at modeled flows less than the field-measured discharge is most likely due to the assumption of constant roughness in the current version of R2CROSS. Relative roughness, or resistance to flow, within a stream tends to increase as flow decreases. Therefore, it is expected that the assumption of constant roughness in the R2CROSS model would result in inflated estimates of predicted velocities as modeled flows decrease. The opposite would be anticipated at modeled flows greater than the field-measured discharge as the assumption of constant does not allow flow resistance to decrease at higher flows, as it would in the stream. The staffs have generally addressed this modeling constraint by collecting field data during times of low flow. Collecting data at flows near the resulting recommendation minimizes the need for excessive extrapolation of modeled results. Staff is also in the process of investigating various techniques for enhancing the R2CROSS model to include a variable roughness factor. The CWCB and CDOW staffs should also cooperate to verify and document the biologic links to the R2CROSS hydraulic model. As stated previously, CDOW began developing "seasonal 12 .