Which river marks the boundary between Georgia and South Carolina?

The Savannah River is formed by the confluence of the Tugaloo and Seneca rivers at Hartwell Dam, Georgia, U.S. It constitutes the boundary between Georgia and South Carolina as it flows southeastward past Augusta and Savannah, Georgia, into the Atlantic Ocean after a course of 314 miles. Early Spanish explorers called the river Río Dulce (“Sweet River”), and its Indian name was Isondega (“Blue Water”); the name Savannah was probably derived from the Spanish sabana (“flat country”) or from the Sawana Indians.

Source: Britannica