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24 repeated for all months except those in which high flows occur (i.e., the spring months). During high-flow months, the instream flow a, recommendation was set at the median flow of record (50 percentile). The flow duration approach was developed for use on midwestern - streams to satisfy assessment requirements in which extensive field data collection was not possible. However, it can be applied only where a relatively long historical flow record is available. The state of Iowa currently uses an instream flow policy based on the 84 percentile, annual low-flow statistic (Dougal 1979). Hoppe and Finnell (1970) also used a flow duration analysis on the Frying Pan River in Colorado, but they included equations to extrapolate flows to sites above or below USGS gaging stations based on watershed area. 2.4 U.S. Forest Service Habitat Evaluation Several offices of the U.S. Forest Service (Regions 1, 2, and 4) developed assessment methods which examine the relationship between stream flow and physical parameters of the aquatic environment { (Bartschi 1976, Cooper 1976, Isaacson 1976). All are used primarily in the mountainous, western United States on small, wadable streams (<50 m wide) with relatively low gradients. Site-specific field data (i.e., depth, velocity, and substrate type) are collected at one or more flows along transects across a stream channel and are used to describe the aquatic habitat. Transects are selected to be representative of specific types of stream habitat (e.g., riffles, pools, runs, etc.) which might be affected by alterations in flow. The eventual recommendations for instream flows are based on the actual habitat conditions at a site rather than on stream flow statistics, which were the basis of the three methods discussed previously. Among the various regional approaches used by the U.S. Forest Service, there are differences in the way in which physical data are obtained and used to describe aquatic habitat. Transect data can be