because of large water-level fluctuations (drawdowns) caused by dam 

 operations, especially power generation. Shoreline habitat is particularly 

 endangered by water-level changes. 



Reduction of peak flows downstream of a dam can reduce scouring and cause 

 organic detritus to accumulate (Ridley and Steel 1975). This in turn can 

 favor rooted aquatic vegetation and disrupt invertebrate and fish populations. 

 Reduction of peak flows also can interfere with environmental cues that 

 trigger fish migrations. 



Impounded water usually warms slower in spring and remains warm longer in fall 

 than freely flowing water, resulting in a more even temperature regime 

 downstream (Ridley and Steel 1975). Hence, releases from impoundments can 

 change temperatures downstream and interfere with the normal timing of the 

 life cycle of some stream animals (e.g., smallmouth bass, Atlantic salmon, and 

 brook trout) . Similar changes also can occur in the impoundment itself and in 

 upstream areas. 



Surface water released from an impoundment often contains more algae and less 

 silt than freely flowing water (Ridley and Steel 1975). This can change 

 biomass and sediment transport and buildup downstream, affecting vegetation, 

 macroinvertebrates , and fish. Increased transport of planktonic algae is 

 particularly important, because it can favor populations of filter feeders, 

 such as larval blackflies, which can then produce a significant nuisance 

 effect as adults emerge. 



The most obvious and frequent impact of a dam is its interference with the 

 migration of fish. Studies in Maine and Canada show that at least 5% to 10% 

 of adult Atlantic salmon are unable to migrate past a dam, even where fishways 

 exist (DeRoche 1967). Of course, dams without operating fishways impede far 

 more fish. 



Some fish are more likely to be impeded by dams than others. For instance, 

 salmon, which can clear obstacles 11 feet (3.4 m) high under favorable 

 conditions (Jones 1959) are much more likely to pass barriers than Atlantic or 

 shortnose sturgeon, which are sluggish swimmers, or American shad or striped 

 bass, which are not prone to leaping. In Maine, the only major anadromous 

 fishes that pass barriers and fishways easily are alewives and salmon. The 

 American eel, a catadromous fish, also passes barriers well (see chapter 11, 

 "Fishes"). 



Dams also can block downstream migrations of young fish. Most downstream 

 migrants swim along the bottom, so that many young fish fail to take advantage 

 of dam spillage and fishways. In the Androscoggin River, a 10% loss of river- 

 produced Atlantic salmon smolts would occur at each power dam encountered 

 during downstream migration and a 5% loss of tributary-produced smolts for 

 each dam encountered (DeRoche 1967). Higher mortalities, about 11% per dam, 

 could be experienced during unusually poor water years, when dam spillage is 

 nearly nil and water is passing mostly through turbines (Schoeneman et al. 

 1961). 



Dams and stream pollution have been implicated as the major causes of the 

 reduction of anadromous fish runs in Maine to far below historical levels. 

 Striped bass virtually have been eliminated as a spawning population. 



3-47 



10-80 



