Khartoum provides nearly 70 [%] of the flow of the river. The Abay, which originated from the 1800 m (6000 m) altitude near Lake Tana in northwest Ethiopia, flows out of Lake Tana through a series of rapid beaches, first to the southeast through the deep valley, and then to the west around the Choke Mountains. Fold northwestward into Sudan, where Khartoum meets the White Nile. It is about 1600 km long (1000 miles) and the main tributaries are the Dinder and Rahad rivers.