BOARD01464

• Board Meeting Date: 11/23/1998
• Description: ISF Section - Informational Items - Staff's Report on "Evaluation of the Standards and Methods Used for Quantifying Instream Flows in Colorado"
• Board Meetings - Doc Type: Memo

adjustments to the R2CROSS rnethodology 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 detennining 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 instrearn 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 popular instrearn 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.

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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 frorn 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 infonnation on various life stages of selected aquatic species
to evaluate habitat impacts relative to incremental changes in streamflow. Output frorn incrernental
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 result in specific, target instream flow values unless habitat
protection standards can be defined.

In <1986, the Water Developrnent and Streamflow Committee of the Western Division ofthe
American fisheries Society (WDAFS) conducted a survey to detennine the status of instream flow
legislation in North America (Reiser et. al 1989). 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 I).

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