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~-, (~ "';i ('~ ~:, .-, U U &. '1.hd . adjustments to the R2CROSS methodology are likely to become more commonplace. Water is an extremely valuable resource in the semi-arid, western United States. The decision to appropriate an instream flow water right generally involves determining the level of flow that is required to protect the instream value of interest and the costlbenefit tradeoff of preserving that instream value versus having the water available for traditional, out-of-stream water uses (McKinney and Taylor 1988). There are a number of instream flow quantification techniques that are currently used in the western United States to protect instream values. This paper begins with an overview of these techniques. It also briefly summarizes the R2CROSS methodology, as well as several of the most popu1ar instream flow quantification methodologies that are utilized in other semi-arid, western states, and suggests ways that these methods may be used in Colorado. The paper concludes with a staff recommendation and suggestions for additional study and future research on the subject of quantifying the biologic instream flow requirements of Colorado streams. Overview of Current Instream Flow Quantification Methodologies . Instream flow quantification methodologies can be classified into two general categories (McKinney and Taylor 1988). "Standard setting methodologies" are techniques which identify the minimum flow standards that are required to protect certain instream flow values of interest. Standard setting methodologies can be further divided into non-field and habitat retention types. Non-field methods are those which derive instream flow recommendations from historic streamflow records rather than on-site, field data. Examples include the Tennant or Montana methods. Habitat retention methods, like the R2CROSS and wetted perimeter methods, utilize hydraulic field data to examine relationships between stream discharge and indices of fish habitat. Standard setting methodologies are useful for developing target instream flow values that preserve a specified amount of aquatic habitat. In contrast, "Incremental Methodologies", like IFIM/PHABSIM, couple very extensive sets of hydraulic field data with biologic information on various life stages of selected aquatic species to evaluate habitat impacts relative to incremental changes in streamflow. Output from incremental methodologies are often used to evaluate the relative impacts of water resource development scenarios (eg., dam releases, hydropower projects, etc.) on downstream aquatic habitats. However, incremental methods do not generally resu1t in specific, target instream flow values unless habitat protection standards can be defined. In 1986, the Water Development and Streamflow Committee of the Western Division of the American Fisheries Society (WDAFS) conducted a survey to determine the status of instream flow legislation in North America (Reiser et. al1989). Of the 46 states that returned the questionnaires, 15 states reported legislative recognition of instream flows for fish and aquatic resource protection. In its survey, the WDAFS found that the IFIM (Bovee and Milhous 1978, Bovee 1982, Milhous et al. 1984) is the most widely used instream flow quantification technique (Figure 1). . 3