President Donald Trump signed bills Monday extending the amount of time before construction must start on five hydroelectric dams across five states.

The bills cover dams located in Delaware, Maryland, North Carolina, Virginia and West Virginia. Each piece of legislation gives the Federal Energy Regulation Commission (FERC) the authority to extend the deadline for construction to begin up to six years and reinstate each project’s construction license if it expires.

Hydropower is considered clean energy and does not produce any carbon emissions. Environmentalists often oppose such projects because of the dams’ effects on river ecosystems. The Hydropower Reform Coalition, an association of 150 different groups, wants significant reforms on hydropower to limit the impact on rivers. (RELATED: Environmentalists Are Protesting A Clean Power Project)

Hydropower is a significant source of renewable energy in the U.S. While only 13 percent of energy in the U.S. in 2015 was produced through renewable sources, 46 percent of that was produced through hydroelectric dams, according to the Institute for Energy Research.

U.S. hydroelectric facilities are some of the oldest energy generators in the country. The average age of a dam in the U.S. is around 64 years old. The 50 oldest generators in the U.S. are hydroelectric dams built before 1908, according to the U.S. Energy Information Administration (EIA).

Hydropower accounts for roughly 6 to 7 percent of all electricity generated in the U.S. every year, the EIA reports.

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