Blackwater Rush

The Blackwater Rush, also called the Blackwater or River Blackwater, is a significant river of Westeros. It flows out of the hills and valleys of the Westerlands several hundred miles to the west into the Narrow Sea at Blackwater Bay. A tributary of the Blackwater feeds the lake of Gods Eye to the north.

In the books
In the Song of Ice and Fire novels, the Blackwater forms the boundary between the Crownlands, Westerlands, Reach and Riverlands. King's Landing, the capital city of the Seven Kingdoms, is built on the mouth of the Blackwater where it enters Blackwater Bay.

Aside from King's Landing, the largest settlement on the Blackwater is the town of Stoney Sept, located not far from its headwaters.