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3 (Sect. 1.3), lack the necessary guidance for assessing the biological effects which might result from such modifications in stream flow. Q This report has been prepared to provide the guidance that does not r currently exist for -assessing instream flow needs for fishery resources below hydroelectric projects. The various methods are reviewed (Sect. 2.0) and evaluated (Sect. 3. 0), and strategies are presented (Sect. 4.0) for selecting the most appropriate method based on (1) project design and operation, and (2) nature of the aquatic resources., Before discussing specific methods, however, additional background information is provided on the instream flow issue from an historical (Sect. 1.1), regulatory (Sect. 1.2), and hydropower (Sect. 1.3) perspective. 1.1 Historical Background on the Instream Flow Issue r 1.1.1 Development in the West Significant conflicts between offstream and instream uses of water first developed in the West where water supplies are limited and offstream uses (e.g., irrigation) are well-defined by law. Western water law is based on the appropriation doctrine which has two fundamental principles: (1) first in time is, first in right and (2) beneficial use of the water is the basis of the right (Gould 1977). Because of the scarcity of water in many regions of the West, water development has been equated with growth, prosperity, and success (Lamb and Bayha 1978). In short, beneficial use was defined in economic terms. In many states, permitting water to remain in the stream (e.g., for the protection of fish and wildlife habitat or recreation and aesthetic values) was not recognized as a beneficial use. Now, however, most western states recognize the need to protect instream values and, in some cases, have been able to acquire protection through legislative changes (e.g., Dewsnup and Jensen 1977; Lamb and Bayha 1978).