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18 <br />• Dimensionality used in mapping the instream habitat, <br />and <br />• Ability to provide seasonal or speci,es-specific <br />instream flow recommendations. <br />These characteristics provide a focus for the information presented in <br />this section and are discussed in detail in the comparative analysis <br />presented in Section 3.0. <br />2.1 Fixed Percentage <br />p <br /> <br />The best known of the fixed percentage methods is the Montana <br />Method developed by Tennant (1975, 1976). Because of its relative <br />simplicity and minimal data requirements, the Montana Method is one of <br />the most frequently used methods for determining instream flow needs. <br />The procedure involves calculation of the mean annual flow rate (MAF) <br />at a proposed development site and expression of the instream flow <br />needs in terms of a fixed percentage of the mean annual flow (Tennant <br />1976). Various levels of flow needs, or what Tennant called a <br />"recommended base flow regimens," were identified, including flushing <br />flows (200% MAF), optimum flows for all instream water uses (100 to <br />60% MAF), and a gradation of lesser conditions ranging from excellent <br />(60 to 40%) to severe degradation (less than 10% MAF). The minimum <br />instantaneous flow recommended to sustain short-term survival habitat <br />for most aquatic biota is 10% MAF. At this flow, channel width, <br />velocity, and depth are significantly reduced and the aquatic habitat <br />is degraded. <br />The basis for the Montana Method consists of an extensive set of <br />observations on streams in the states north of the Mason-Dixon Line <br />between the Atlantic Ocean and the Rocky Mountains (Tennant 1976). <br />The percentages established for flow needs reflect what has been <br />interpreted to be a consistent relationship between watershed <br />hydrology and physical habitat conditions within the stream channel.