The Ellsworth Power House and Dam, the latter also known as the Union River Dam, is a hydroelectric power generation facility on the Union River in Ellsworth, Maine. The dam, located just north of downtown Ellsworth, impounds the river to create Leonard Lake, named for project's engineer, James Leonard. The powerhouse is a Renaissance Revival building located at the western end of the dam. The power plant, built in 1907, was one of the first peaking power plants built in the state, and the hollow concrete dam is one of the highest Ambursen-type buttress dams ever built. The facility was listed on the National Register of Historic Places in 1985.