Conejohela Valley

The Conejohela Flats are a group of islands in the flooded Conejohela Valley, a large floodplain along the southernmost 30 miles (50 km) of the Susquehanna River in Pennsylvania and Maryland in the United States. The valley was flooded primarily during the early 1900s by the construction of the Holtwood, Conowingo, and Safe Harbor dams from 1910 to 1931.

Before the twentieth century, the Conejohela Valley was a marshy floodplain, with extensive wetlands. The Susquehanna River flooded annually in the spring, and there were more damaging floods approximately once per decade. Thick forests surrounded a mixture of small waterfalls, rapids, and marshes. A wide, flat valley formed; the frequently wide river was a substantial barrier to crossing, both for Native Americans and for colonists.

The varied terrain created many nurturing biological habitats, but human passage across the valley and river was extremely difficult. The Susquehanna is very wide in this area, as it nears its mouth on the Chesapeake Bay. This stifled trade across the lower Susquehanna in colonial Pennsylvania and Maryland. In 1730, the Wright's Ferry was established to cross the river. Later the first Columbia-Wrightsville Bridge was built, once believed to be the longest covered bridges in the world. It was replaced by bridges built of structural steel, large enough to carry a range of vehicles. The modern bridge opened in 1930; it was dedicated in 1980 as the Veterans Memorial Bridge.

Three dams were built across the Conejohela Valley during the first four decades of the 20th century to provide hydroelectric power for southern Pennsylvania (including electrical power for Amtrak and SEPTA Regional Rail) and to control the annual flooding. It was intended to keep sediment from the flats out of the Chesapeake Bay. The first dam across the lower Susquehanna, the Holtwood Dam, was completed in 1910 as McCalls Ferry Dam. The Conowingo Dam followed in 1928.

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