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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 Holtwood, Conowingo, and Safe Harbor dams from 1910 to 1931.