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.. ECOLOGY OF RIVERINE FISHES IN REGULATED STREAM SYSTEMS 59 , considerable change. Impounded waters preclude obligate riverine fishes, since they require flowing water for all their ecological requirements (Fraser, 1972). There is some evidence that white sturgeon prefer free-flowing areas and would not maintain viable populations in reservoirs (Haynes et al., 1978). Such is also the case for blue suckers (Pflieger, 1975; Smith, 1979) and for the widely publicized snail darter (Peraina tanasi) (Williams and Finnley, 1977). Other species that require only rivers for spawning may be found in the reservoir for a few years following impoundment but will disappear due to a lack of recruitment if appropriate spawning areas are inundated. For species such as the paddlefish in the Missouri River, the adults often live in reservoirs but migrate upstream to spawn in tributary rivers (Pflieger, 1975). These pop- ulations will be adversely affected if reservoirs cover their spawning areas. Lewis (1974) noted a decrease in fishes of the family Mormyridae, which are primarily riverine species, when the River Niger was dammed to form Lake Kainji and attributed the change primarily to food availability. Bottom feeding insectivores no longer had a plentiful food supply in the reservoir. Blake (1977) indicated that the loss of riverine species in the lake was prima=ily due to a reduction in reproductive success. Balon (1978) and Petr (1978) review similar situations in other tropical reservoirs. Therefore, it appears that reservoirs impact adult riverine fishes primarily by not providing basic living conditions, such as food supply, and by not providing adequate habitat for successful reproduction. Other factors that may be important include changes in water chemistry and interactions with other species that become abundant in the reservoir. Reservoirs also create adverse situations for downstream migrating juvenile fish. Passage of young Pacific and Atlantic sal- mon through large reservoirs is only one-third as fast as that through free-flowing sections (Raymond, 1969; Dominy, 1973; Raymond, 1976). This delay causes increased mortality due to increased exposure to disease and predation (Collins, 1976). The thermal stratification of large reservoirs can also cause passage problems to young salmon (Collins, 1976). The upper layers may be too warm for the fish, and the lower layers too low in oxygen. Furthermore, reservoirs can accumulate pollutants to levels that become toxic to the juvenile fishes (Dominy, 1973). . Dams result in habitat alteration in tailwater reaches, which may have major impacts on riverine fishes. Downstream changes are not nearly as obvious as the change seen in a reservoir but are often just as effective in eliminating riverine fishes. A major effect of dams on downstream habitat is a change in temperature. The temperature below a dam usually depends on the level of with- drawal from the reservoir. A dam with a high intake will generally