The Island Falls power dam spanned the main river channel at Big Eddy Falls, where there was a drop of . In addition, a number of earthen dams were built along the margin of the head pond (fore bay) to prevent overflow. About one mile (1.6 km) south, a 900-foot (275 m) concrete spillway dam known as "A-dam" was built across a low area on dry land later to be flooded. The fore bay was filled during July 1930, completely submerging the three low falls from which Island Falls gets its name. In August 1930, A-dam was opened and the massive flood of escaping water flushed out a second channel, thus creating an island. The stretch of river affected by the Island Falls development extends from Big Eddy Falls, where the power plant is located, to Mussena Rapids, a distance of approximately thirteen miles (20 km).