The Conejohela Flats are a combination of low-lying islands and mud flats on the Susquehanna River in Pennsylvania about three miles south of Columbia and Wrightsville, located just offshore from the small settlement of Washington Boro in Lancaster County.
The Conejohela Flats are alluvial islands that develop from the forces of erosion and deposition and the building up of large amounts of sediment (soil, silt, sand, and coal silt derived from upstream coal mining waste), which alters their shape and size. Major floods from severe weather events can dramatically change islands. Ice during the winter devastatingly carves out large chunks of land and soil altering the landscape. The dredging of river channels, construction of dams, pollution and erosion of soil caused by poor agricultural practices, intensive logging of the watershed, and acid drainage from coal mining in the past and still today has harmed natural areas such as the Conejohela Flats and on a greater scale the health of the Susquehanna River and the Chesapeake Bay. An effort has been
made in recent years to restore the health of the river and Bay, but much work remains to be done.
The Safe Harbor Dam (located on the Susquehanna River about 6 miles south of the Conejohela Flats) was built in 1931 to generate electricity. With construction of the dam, the river behind it backed up, flooding many of the low areas of land and islands creating the 11.5 square-mile reservoir known today as Lake Clarke. The dam also became a huge trap for upriver sediment that flowed into the shallows of the river, expanding and creating new islands such as the Conejohela Flats.
The operators of Safe Harbor Dam are required by their Federal Energy Regulatory Commission (FERC) license agreement to not raise the water level of Lake Clarke above a certain set amount so that a percentage of mud flats are exposed from April 15-October 15 during the shorebird migration period. Safe Harbor is permitted to increase the water level to generate more hydroelectric power during the non-migration season.