The varied terrain created many nurturing biological habitats, but human passage across the valley and river was extremely difficult.
It also had the unintended consequence of preventing sediment from the Flats and points upstream from reaching the Chesapeake Bay.
When it first closed its gates on September 29, 1931, it flooded more than 10 miles of the upper Conejohela Flats, creating the artificial Lake Clarke.
The few islands that remain in Lake Clarke are a gathering of low, marshy flats about five miles north of Safe Harbor Dam.
The low-lying islands and riparian ecosystem also support dozens of species of birds; the flats are popular among birders.