From A Wiki of Ice and Fire
The River Gate is one of the seven huge gates that surround the huge walls of King's Landing. It is known as the Mud Gate by the population of the city. It is near the center of the southeast wall and connects Fishmonger's Square and the wharfs on the Blackwater Rush. Opposite the River Gate on the southern shore of the Blackwater is the kingsroad, which leads to Storm's End.
A Clash of Kings
The commander of the River Gate, Ser Jacelyn Bywater, is made the new commander of the City Watch of King's Landing. The River Gate is assaulted with a battering ram by the army of Stannis Baratheon during the Battle of the Blackwater.