The Environmental Protection Agency defines only "low-impact small hydropower" as a renewable energy source. Some states also count only small hydropower facilities toward renewable energy targets. This is because large dams have significant ecological impacts, such as disrupting fisheries or silting rivers. Some states limit new hydropower construction.
The "fuel" for hydropower is the energy in fast-flowing water, which continues to flow through the turbines that convert the energy into electricity. Scientifically, hydropower is renewable, even if government agencies don't define it as such for the purposes of incentivizing new, low-impact renewable investments.
In 2019, hydropower accounted for 6.6% of overall U.S. electricity generation.