The document discusses hydroelectric energy and Kelsey Walker's visions for its future use. It defines hydroelectric energy as using flowing water to create electricity, typically through dams. It notes hydroelectricity's advantages as a renewable and inexpensive source but also its disadvantages, such as disrupting river ecosystems. Kelsey envisions hydroelectric becoming the primary US energy source due to its renewability and existing infrastructure but decreasing globally as demand has recently dropped in places like Europe and China.