East River Electric Power Cooperative, Inc. was organized
in October 1949, in Sioux Falls, South Dakota.

The cooperative established its headquarters in Madison in November 1950, and Virgil Hanlon was named the first general manager. The headquarters was officially moved to Madison in 1951. Later that year, East River was awarded its first transmission construction loan to build the first 69 kV system in eastern South Dakota.

In 1954, the first hydropower from Fort Randall Dam flowed to East River.