The Congo is roughly divided into four topographical regions. The coastal region consists of a low, relatively treeless plain, with occasional high spurs jutting down from the Mayombé Escarpment. The escarpment region is made up of a series of parallel folds of moderate height (600–900 m/2,000–3,000 ft) that are almost completely forested. To the east and north of the escarpment, and forming the watershed between the Niari and Ogooué river systems, lies the plateau region, with savanna covering more than 129,000 sq km (50,000 sq mi) and separating the Zaire and Ogooué basins. The northeastern region of the country is a swampy lowland covering some 155,000 sq km (60,000 sq mi); flooding is seasonal, with different tributaries of the Congo/Zaire overflowing into one another. The country has two river systems: that of the coastal rivers, which flow into the Kouilou River, and that of the Zaire River and its tributaries.