areas and promote salt marsh development on finer substrates. Warmer water 

 temperatures would stimulate growth of some benthic plants and possibly 

 accelerate grazing by littorina snails and urchins in intertidal and subtidal 

 areas respectively. Increased sedimentation in impoundments would decrease 

 the rocky substrata available for macroalgae and increase the soft, elevated 

 substrata suitable for marsh grasses. Organisms would be displaced from 

 previously occupied heights to new levels. Benthic invertebrates would be 

 severely affected as portions of many populations would not survive the new 

 tidal regimes. As habitats were recolonized, more gradual changes in benthic 

 populations might result through sedimentation. Organisms up the food chain 

 potentially would be affected by changes in the benthic invertebrate 

 populations. Shorebirds, in particular, might be impacted greatly in feeding 

 areas (see chapter 14, "Waterbirds") • 



Tidal power projects also could affect fish and wildlife populations directly. 

 Changes in riverine and estuarine conditions could result in decreased homing 

 of anadromous fishes. Fishways might be needed to allow for upstream passage. 

 Depending on the level of operation and equipment used, turbine mortality 

 might be a serious problem to fish and other aquatic life. Ice might form 

 more readily in an impoundment, resulting in decreased use by wintering 

 waterfowl populations. 



Reduced tidal flushing behind a tidal dam could degrade water quality if large 

 amounts of untreated sewage or other pollutants accumulated in the 

 impoundment. This would be of special concern in relation to aquaculture 



sites . 



Effects of dams on freshwater habitats . Impoundments behind the 

 hydropower dams can affect the physical, chemical, and biological 

 characteristics of rivers severely. In Maine, where hundreds of small dams 

 have been built on rivers and their tributaries, impoundments are of 

 particular concern. Dams and their associated storage reservoirs frequently 

 alter flow and temperature regimes above and below the dam. They also may 

 create barriers to upstream and downstream movements of migratory fishes. 

 Dams and barriers on Maine streams are the major cause of the historic decline 

 of anadromous fish resources in Maine. 



The magnitude of the impacts of an impoundment depend upon the size of the 

 dam, operating mode (run-of-river or store-and-release) , the flow rate through 

 the impounded area, turbine size and speed, and the retention time of water 

 within the impoundment. Generally, larger dams with relatively low average 

 flow rates, wide ranges in operating flows, and long retention times, will 

 have the most severe impacts on the stream and its biological community. 

 Since most Maine dams are relatively small, their impacts, while significant, 

 are probably not as severe as the impacts of larger dams might be. 



Regardless of its size, any dam can be expected to have some impacts. After a 

 dam is built the area immediately upstream typically is inundated, converting 

 it from a running-water habitat to a standing-water habitat, with concurrent 

 changes in its biological community. Bottom fauna in a newly impounded area 

 are usually very unstable and frequently are dominated by chironomid midge 

 larvae (Hynes 1970). Gradually, biological communities stabilize, sometimes 

 resembling communities typical of lakes in the same area. In other instances, 

 floral and faunal assemblages never may be as stable as lacustrine areas, 



3-46 



