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10 including the identification and inventory of the water resource base of the area (Wassenberg et al. 1979). In 1974, the Second National Water Assessment was undertaken by the U.S. Water Resources Council and represented "the first nationwide examination of instream flow conditions and the implications of accelerated offstream uses" (Bayha 1978). In his Water Resources Policy Reform Message of June 6, 1978, President Carter identified water conservation and instream flows as national goals. On July 12, 1978, he issued a directive to federal agencies to improve, where possible and in cooperation with the states (which have principal responsibility for protection of instream flows), the operation and management of existing water resources projects to protect instream uses. Although protection of the instream use of water is now recognized as a national concern, the most difficult aspect of this issue, the assessment of instream flow needs, remains a challenge. 1.3 Effects of Hydroelectric Generation on Instream Flow The issue of instream flow maintenance in hydropower development is essentially a problem of evaluating the effects of planned modifications in hydrologic patterns. Both large and small-scale hydropower projects can alter natural flow regimes, often with adverse effects on downstream water users and the aquatic ecosystem. The effects of dams on downstream (tailwater) biotic communities were reviewed previously (e.g., Fraser 1972b; Ward and Stanford 1979b; Hildebrand 1980; Loar and Hildebrand, in press) and need not be reiterated. However, it should be noted that while the effects may be relatively well documented, the underlying mechanisms or causal factors responsible for these changes are, with few exceptions, poorly understood. Alterations in flow regimes below hydroelectric dams can include both spatial and temporal changes in the amount of water moving through a natural stream channel. Large-scale spatial changes occur I r