From A Wiki of Ice and Fire
The Wendwater is a river located in the southeastern Crownlands and northeastern Stormlands. It is the largest river in the kingswood. Beginning at a narrow lake in the southern kingswood, it flows northeast to Blackwater Bay. North and west of the Wendwater is the forest of the kingswood, while to the east is Massey's Hook. According to semi-canon sources the seat of House Wendwater is located along the river in the kingswood.
The dragon Sheepstealer of Dragonstone was known for grabbing sheep from flocks in Blackwater Bay from Driftmark to as far as the Wendwater.
A Clash of Kings
En route to King's Landing, Stannis Baratheon's fleet anchors at the mouth of the Wendwater. Ser Imry Florent, Lord High Captain of Stannis's fleet, calls for a meeting of captains to instruct them in the battle plan once they reach the Blackwater Rush. Three days later they attack King's Landing in the Battle of the Blackwater.
References and Notes