Freshwater Molluscan Fauna 
115 
almost totally eliminated by drawdowns of a lake in Maine. Libois and 
Hallet-Libois (1987) found that thin-shelled unionids such as Anodonta 
suffered high mortality during a 3-week drawdown of the River Meuse 
in Belgium. From those studies, and the observations gathered here, it 
appears clear that as long as drawdowns are used as a management 
measure in Greefield Lake, the unionid population cannot recover. 
Not all of the effects of drawdowns are obvious and direct. Lake 
drawdowns drastically reduce the amount of nearshore habitat available 
for the fish community and were observed to result in substantial 
mortality of fish through strandings, increased predation by wading 
birds and gulls, and cold shock. This loss to the fishery is directly 
related to the health of the unionid population, because the Unionidae 
rely on a fish host for the glochidial stage of their life cycle. A reduction 
in the species diversity or abundance of fishes will, therefore, reduce the 
number of glochidial hosts available. 
Drawdowns also reduce the cross-sectional area of the impounded 
portions of tributaries. This reduction in cross-section, with tributary 
inflow remaining the same, causes an increase in water velocities in 
normally lacustrine areas, and that results in a massive shifting and 
redistribution of the bottom sediments. Several Anodonta cataracta 
were observed with trails in the sand behind them, apparently attempting 
to reestablish themselves. While shifting substrates can be a normal 
occurrence in lotic environments, the lacustrine organisms of the lake 
probably have difficulty coping with such changes. 
During reimpoundment, which takes from 2 to 3 weeks, virtually 
no fresh water is released from the dam. Therefore, undiluted waters 
from the Cape Fear River reach the base of the dam with each high tide, 
and all fish and benthic organisms in the outlet stream are exposed to 
abnormally high salinities. Uniomerus tetralasmus and Villosa vibex 
were recorded below the spillway by Johnson (1970), and Dawley (1965) 
reported Gillia altilis from that area. Habitat for those species has been 
altered by increased salinity levels. 
As discussed previously, all of the tributaries of Greenfiled Lake 
have been channelized to enhance drainage. These creek channels are 
periodically maintained by dragline, which results in almost total removal 
of all benthic organisms and aquatic vegetation. The consequences of 
such actions for molluscan populations are undoubtedly profound; 
Greenfield Lake and its tributaries will be repopulated only from 
undisturbed waters upstream or downstream of the maintenance area or 
from outside sources. Because channels run from the lake all the way to 
the headwaters, recolonization is presumed to be very slow. 
