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operation, the manual discusses how impacts may have complex interrelation- sh i ps, wh i ch may lead to unexpected consequences. It is hoped that the discussion will help the reader organize his/her thinking about a partic- ular SSH project under review, and stimulate them to question the hydro developer or outside experts as needed to clear up any doubtful aspects of the project being reviewed. To complete the discussion of environmental impacts of SSH projects, mitigation alternatives are listed; specific miti- gation measures are provided in the companion Mitigation Handbook. Environmental impacts affecting fish and wildlife are the primary sub- ject of this manual. An environmental impact, as defined here, is an "observable effect" on fish, wildlife, or their habitats. All the major en- vironmental impacts of SSH projects, both positive and negative, are dis- cussed in this manual. Generally, this means that only primary (direct) impacts of SSH equipment and activities can be considered; however, some of the most important secondary impacts are also discussed. Because the actual magnitude of an environmental impact depends on site-specific condi- tions, no attempt is made in this manual to suggest relative magnitudes of impacts, except when impacts can be generalized safely over a large number of SSH projects. Information pertinent to SSH development in the United States, includ- ing Alaska, is included in this manual. Small-scale hydro is defined as projects with total project power of < 30 MW generating capacity. This amount of power is sufficient for the normal needs of about 15,000 people (J. Hanson, pers. comm.). The definition of SSH used in the Public Utility Regulatory Policies Act (PURPA: PL 95-617, November 1978) is 15 MW generating capacity. Another term, considered by many people to be synonymous with SSH, is "l ow-head II hydro, wh i ch has been defi ned by the Department of Energy as 20 m (66 ft) of fall (U.S. Army Corps of Engi neers 1979). While both the terms sma ll-sca le and low-head generally refer to relatively small hydroelectric projects, the two terms are def- initely not exchangeable in all situations. A low-head plant on a river of high discharge could easily produce more than 30 MW of power, whereas developments of more than 20 m head that produce less than 30 MW, or even 15 MW, are common, particularly in mountainous areas. The present work covers all the common configurations of SSH plants within the contiguous United States and Alaska, except for pumped storage and tidal power systems. 1.2 SOURCES OF INFORMATION The information on SSH technology and its environmental impacts con- tained in this manual was assembled from published and unpublished litera- ture; consultations with engineers, biologists, and government personnel; manufacturers I 1 i terature; and presentat ions at profess i ona 1 conferences. Much of the pub 1 i shed 1 iterature is 1 i sted at the end of each chapter in 2