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58 P. B. HOLDEN these species are not game fish but, rather, are minnows, suckers, darters, and other non-game fish. Therefore, most of the informa- tion concerning the ecology of fishes in regulated streams comes from sport or commercial fishes, such as salmon and trout, which require rivers for only pert of their ecological requirements. EFFECTS OF DAMS The impact of damming, or regulation, of rivers on obligate riverine fishes is generally negative. Approximately 60% of the fishes presently listed as endangered or threatened in the U.S. are obligate riverine species. Although not all were decimated by regulation of their habitat, dams are one of the major causes in the decline of obligate riverine species. The impacts of regulation can be classified as Immediate--those impacts that become immediately apparent when a dam becomes opera- tional; and Delayed--those impacts that do not become evident for several years after dam completion. Immediate Impacts of Regulation One of the most obvious immediate impacts of dams is the block- age of upstream and/or downstream migration. This has been identi- fied as a major problem to Pacific salmon (Raymond, 1968; Trefethen, 1972; Collins, 1976; Robinson, 1978), Atlantic salmon (Dominy, 1973), paddlefish (Branson, 1974; Pflieger, 1975), and several Atlantic species of herring (Alosa sp.) and bass (Morone sp.) (Nichols, 1968). These species all need to spawn in flowing water, and dams have blocked their natural routes to upstream spawning areas. Shikhshabekov (1971) indicated that several species of fish in Russia resorbed eggs when their migrations were slowed or stalled due to regulation. Dams present a special problem to downstream migrating juvenile salmon and steelhead trout. Since many dams these fish encounter are for hydroelectric generation, much of the water passing the dam flows through turbines. Turbine-associated mortality is high. Also, passage through a dam often ends in stilling basins where predators, such as northern squawfish (Ptychocheilus oregonensis) and sea gulls, await the disoriented fish (Long and Krcma, 1969). The construction of fishways, to aid upstream migrating adults, and the use of screens or the actual trucking of juveniles around dams have decreased the negative impact of some dams on Pacific salmon and steelhead trout (Long and Krcma, 1969; Park and Farr, 1972; Collins, 1976; Raymond, 1976). The other major immediate effect of dams is habitat alteration. The alteration of a free-flowing river to an impoundment is a ~ ~ \ ~ l'