Twin Branch Dam is the tallest dam with a hydroelectric power station on the St. Joseph River in Indiana. When it was first commissioned, it was called the Hen Island Dam.^[2] The name Twin Branch Dam comes from the Twin Branch Creek which empties into the St. Joseph River just above the dam.^[3] The prior name for the dam, "Hen Island Dam," came from an island that used to be just below the dam where a riverboat resident used the island to raise geese and chickens.^[3] There were two islands, one named "Hen Island" and the other "Goose Island". Both islands were taken by the river by the 1950s.^[3]

In 1835 was the first attempt to build a dam on the St. Joseph River near the Twin Branch Dam it was commissioned by the St. Joseph Iron Works.^[3] The attempt was abandoned in 1837.^[3]

The dam was built by the Sanderson & Porter, company of New York City.^[1] The dam was commissioned on February 26, 1900.^[1] The company that commissioned the dam was the St. Joseph and Elkhart Power Company. It was incorporated on February 28, 1900, with a first issue of capital stock for $50,000.^[4] The company took over the Home Electric Light and Power Company.^[5]

They acquired the rights to over a thousand acres of land that would be flooded.^[1] Because of the size of the dam and the view that it would impact the planned project upstream by the Indiana Power Company, litigation between the two companies was filed.^[1] The case was Indiana Power Company vs. St. Joseph and Elkhart Power Company St. Joseph C.C.^[6] The planned project for the Hen Island Dam was successful in the Circuit Court, the Indiana Supreme Court, and the United States Supreme Court refused to hear the case.^[1] The legal battle took over two years and was settled between the two companies.^[1]

The St. Joseph and Elkhart Power company was also sued by the St. Joseph Navigation Company in Federal Court, seeking an injunction to stop the dam.^[7] The case was filed because the dam would block navigation up the river.^[7]

The original plan was for the plant to have six alternating three-phase generators of 800 kilowatts each, with thirty large turbine wheels connected in tandem.^[8]

There were 30 arc lights that had been installed to allow construction to take place at night on the project.^[9] Over 2.5 million feet of lumber was used to construct the dam.^[1] The lumber was used to build cribbing and drive piles 20 feet into the river.^[1] There were over 2,000 piles driven into the river bed.^[1] They were filled with gravel and stone. The dam when it was built housed four horizontal turbine wheels that measured 51-inch.^[9]

In 1922 the dam was in need of repair Indiana & Michigan constructed a concert cover over a steel frame.^[1][3] In the 1922 upgrades the power plant finally got the two new generators to bring the dam's generators to the planned six.^[3] In 1957, the dam only generated 7,200 kilowatts of power.^[3]

When the Hen Island Dam was in the process of being built it was billed as "The Great Dam Now Under Construction at Hen Island is a Stupendous Feat of Engineering. Interesting Legal Battle."^[1] The power plant was connected to the Elkhart Power Company and power was first produced on December 16, 1903, while the dam was still under the control of Sanderson & Porter.^[10]

In 1904, Charles A Chapin, purchased the interest that South Bend Electric Co had in the dam for over a $1,000,000.^[11] St. Joseph and Elkhart Power company merged with the Indiana & Michigan company in 1907.^[3]

A $3.4 million upgrade was made to the power plant in 1989,^[12] with two generators replaced as part of the upgrade.^[12] The generators were replaced after a period of nine-years being out of service, one was out of service because of shaft failure, the other was broken down in 1979, performing only at 60% operation.^[12]

This hydro-electric dam is capable of producing 2,600 kilowatts. The dam has been owned and operated by American Electric Power/Indiana Michigan Power since 1922.^[13]

• List of dams and reservoirs in Indiana