Changing a river into a lake is basically what a large dam does. one reason we do this to create reservoirs of fresh water. however, dams change the local ecosystem in a big way. what is one negative impact of building a dam? a) dams are needed to create hydroelectricity. b) dams can create smog that pollutes the lake. c) dams can reduce the intense heat of summer. d) sometimes people who lived along the banks of the river are displaced.