The Guayas River system, which flows southward to the Gulf of Guayaquil, constitutes the most important of the drainage systems in the Costa Internal. The Guayas River Basin, including land drained by its tributaries, is 40,000 square kilometers in area. The sixty-kilometer-long Guayas River forms just north of Guayaquil out of the confluence of the Babahoyo and Daule rivers. Briefly constricted at Guayaquil by hills, the Guayas widens south of the city and flows through a deltaic network of small islands and channels. At its mouth, the river forms a broad estuary with two channels around Puná Island, the deeper of which is used for navigation.