The Blackwater Rush, frequently just called the Blackwater, is a significant river in southern Westeros, where the borders of the Westerlands, Riverlands, Crownlands and the Reach meet. It runs out of the western hills and mountains eastwards to the sea, passing Stony Sept and the capital at King's Landing along the way.
The river is wide at its mouth on Blackwater Bay and is navigable even for large galleys some distance upriver. For this reason the river is a major trade and transport artery for those bound into the interior of the continent. The Blackwater's river mouth at King's Landing is now defended by winch towers and a huge chain, built on the order of Tyrion Lannister shortly before the Battle of the Blackwater to prevent ships from crossing upriver to land flanking forces west of the city. Veterans of this battle style themselves 'of the Blackwater' to denote their participation during this engagement.
The Roseroad crosses the river to the capital by means of a ferry. The Goldroad crosses the river further upstream, but it is unclear if this is by ferry, ford or bridge.