Which bay separates the Delmarva Peninsula from the mainland of the United States?

The bay separating the Delmarva Peninsula from the mainland United States is Chesapeake Bay.

Chesapeake Bay is the largest estuary in the United States. It lies between the Delmarva Peninsula on the east and the mainland of Maryland and Virginia on the west, opening to the Atlantic Ocean between Cape Henry and Cape Charles. The northern part is in Maryland, while the southern portion extends into Virginia.

The bay is approximately 200 miles long and receives water from more than 150 rivers and streams. Its watershed covers parts of six states—New York, Pennsylvania, Delaware, Maryland, Virginia, and West Virginia—as well as Washington, D.C. The Susquehanna River is its largest tributary and enters at the bay's northern end.

Chesapeake Bay is a drowned river valley, formed when rising sea levels flooded the lower Susquehanna valley after the last Ice Age. It is not the Delaware Bay: Delaware Bay lies on the eastern side of the Delmarva Peninsula, while Chesapeake Bay forms the peninsula's western boundary. The region is especially known for blue crabs, oysters, striped bass, wetlands, and a long maritime history.