Hydroelectric is the quiet workhorse of electricity generation. The nine biggest power plants in the world are hydroelectric. Three Gorges in China is the biggest by far, with a capacity of 22.5 GW. (One gigawatt can roughly power 725,000 homes in the US, probably more in China.)
The Kashiwazaki-Kariwa nuclear power plant in Japan has been shut down since Fukushima, thus the top nine are all hydroelectric. Big coal and nuclear plants come close to matching the 6.5 GW output from bottom-ranked Sayano-Shushenskaya Dam but do not surpass it.
Big hydro is indeed renewable energy, however not without environmental issues. Huge dams displace large number of people. The weight of all the water can trigger earthquakes. Eventually the lakes do silt up. And there are other issues too.
Dammed rivers have also impacted processes in the broader biosphere. Most reservoirs, especially those in the tropics, are significant contributors to greenhouse gas emissions (a recent study pegged global greenhouse gas emissions from reservoirs on par with that of the aviation industry, about 4% of human-caused GHG emissions). Recent studies on the Congo River have demonstrated that the sediment and nutrient flow from the Congo drives biological processes far into the Atlantic Ocean, including serving as a carbon sink for atmospheric greenhouse gases.
Large dams have led to the extinction of many fish and other aquatic species, the disappearance of birds in floodplains, huge losses of forest, wetland and farmland, erosion of coastal deltas, and many other unmitigable impacts.