Hydroelectric generation is the predominant form of electric generation in Canada. A major part of the economic history of Canada consisted of the development of sites with large hydroelectric potential such as Niagara and Shawinigan. Hydro developments have had a substantial effect on rivers and lakes in Canada. There have been relatively few large hydro sites developed since the 1970s as the environmental and human effects to be avoided or mitigated in such large projects make them increasingly difficult and costly to plan and build.