Power obtained from the natural movement of masses of water. Hydroelectric power plants convert the energy contained in flowing water, like rivers and streams, into electricity. Low impact hydro plants also called 'run of river' plants, are considered renewable sources of electricity. Larger hydro projects, known as storage hydro or high impact, cause concern because dams can change natural river flows, degrade water quality and block fish migration. Hydropower currently provides about 10 per cent of the electricity generated in the United States - a percentage unlikely to increase dramatically, both because few new sites remain for the construction of large dams and because of general opposition to building large new facilities on environmental grounds. Hydropower currently provides almost 60 per cent of electricity generated in Canada.