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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 Safe Harbor, Holtwood, and Conowingo dams.
Geography and History
Before the twentieth century, the Conejohela Valley was a marshy floodplain, with smaller annual floods and more catastrophic 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 natives and for colonists.
The varied terrain created many nurturing biological habitats, but rendered passage across the valley nearly impossible. This stifled trade across the lower Susquehanna in colonial Pennsylvania and Maryland, spurring the 1730 opening of the historic Wright's Ferry and the first Columbia-Wrightsville Bridges, once believed to be the longest covered bridges in the world.
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 SEPTA Regional Rail) and to control the annual flooding, as well as 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. However, the Safe Harbor Dam had the largest impact on the river ecosystem. When it first closed its gates on September 29, 1931, it flooded over ten miles of the upper Conejohela Flats, creating the artificial Lake Clarke. Most of the valley was flooded; the few islands that remain in Lake Clarke are a gathering of low, marshy flats about five miles north of Safe Harbor Dam.
The remaining Conejohela Flats provide habitats for a number of species. The varying depths of inundated islands on the bottom of Lake Clarke support numerous fresh water feeder fish, pan fish, and large predatory game fish species. The Safe Harbor Dam has become popular for local fishing.