From A Wiki of Ice and Fire - A Song of Ice and Fire & Game of Thrones
The Blackwater Rush, commonly called the Blackwater, is a river in the southern riverlands and the crownlands with some headwaters in the westerlands. It is one of the major rivers of the Seven Kingdoms of Westeros.
With headwaters near Stoney Sept, the Blackwater flows southeast and is joined by two tributaries, one east of Stoney Sept and one from the Gods Eye. King's Landing, the seat of the Iron Throne, is situated at the mouth of the Blackwater Rush at Blackwater Bay. The Goldroad crosses and follows some of the river. Tumbler's Falls is also located on the Blackwater Rush.
In the days of the Hundred Kingdoms, the mouth of the Blackwater was claimed by the Darklyns of Duskendale, the Masseys of Stonedance, and the river kings of Houses Mudd, Fisher, Bracken, Blackwood, and Hook.
Prior to the War of Conquest, the southern shore of the river was claimed by Argilac Durrandon, the Storm King, and the northern shore by Harren Hoare, the King of the Isles and the Rivers. Aegon the Conqueror founded first the Aegonfort and later King's Landing at the mouth of the Blackwater. A cog on the Blackwater was used as a temporary sept while a permanent one was under construction at Visenya's Hill.
After being stopped at the Red Fork in the Battle of the Fords, Lord Tywin Lannister joins Lord Mathis Rowan and Lord Randyll Tarly near the headwaters of the Blackwater. At Tumbler's Falls they meet Lord Mace Tyrell, who has brought barges and a large host with him. From there they sail down the Blackwater and land a half day's ride outside of King's Landing.
During the Battle of the Blackwater, many of the ships of Joffrey Baratheon and his uncle Stannis Baratheon are set ablaze by wildfire after a great chain divides the river from Blackwater Bay. The Lannisters and Tyrells then attack Stannis's army in the rear and force him to retreat.
Before the port of King's Landing can be reopened, the Blackwater must be dredged of sunken ships. Three out of every four quays are damaged.