Sullivan, county, southwestern New Hampshire, U.S., bounded to the west by Vermont; the Connecticut River constitutes the border.
The river constitutes the border between Minas Gerais and Goiás states and briefly separates Minas Gerais from Mato Grosso do Sul state.
It's a complicated answer, beginning with what constitutes the "border".
(the Niagara River constituting the border), and to the south by Tonawanda Creek, which is incorporated into the Erie Canal (itself part of the New York State Canal System).
Delaware, county, southeastern Pennsylvania, U.S., located southwest of Philadelphia and bounded to the east by Cobbs Creek and to the south by New Jersey and Delaware, the Delaware River constituting the border.
Addison, county, western Vermont, U.S. It is bounded by Lake Champlain (constituting the border with New York state) to the west and the Green Mountains to the east.
Burlington, county, central New Jersey, U.S., bounded by Pennsylvania to the west (the Delaware River constituting the border) and the mouth of Great Bay in the Atlantic Ocean and the Mullica River to the southeast and south.
Essex, county, northeastern New York state, U.S. It comprises a mountainous region bounded by the Ausable River to the northeast, Vermont to the east (Lake Champlain constituting the border), Lake George to the southeast, and the Hudson River to the southwest.
Salem, county, southwestern New Jersey, U.S. It comprises a coastal lowland bounded by Delaware to the west (the Delaware River constituting the border), Oldmans Creek to the north, the Maurice River to the southeast, and Stow Creek to the southwest.
Montgomery, county, central Maryland, U.S. It consists of a piedmont region bounded by the Patuxent River to the northeast, Washington, D.C., to the south, and Virginia to the south and west (the Potomac River constituting the border).
Prince George's, county, south-central Maryland, U.S. It consists of a piedmont and plains region bounded by the Patuxent River to the northeast and east and the Potomac River (constituting the border with Virginia) and Washington, D.C., to the west.
