Hydroelectric power is one of older, more established types of renewable energy. Dams and other hydroelectric facilities account for between six and seven percent of all electricity generated in the U.S. Most of these were constructed decades ago, however, and there are relatively few opportunities to add large-scale hydro facilities. Concerns about the effect of dams on wildlife also limit the expansion of large-scale hydro power.
More promising for future expansion is microhydro power generation systems. Rather than producing a large amount of electricity in one place, microhydro is more compatible with a distributed generation model. Typically, such installations do not feature dams, so some of the environmental impacts of traditional hydro power are averted. There is some potential for microhydro in the Shenandoah Valley, though the economics of microhydro systems are not as well established as with most other renewable energy sources.