The country's most prominent topographical features are the Mekong River, which descends from Laos and continues down into southern Việt Nam, and the Tonle Sap (Great Lake), which drains into the Mekong at Phnom Penh. The central lowlands are surrounded by transitional plains covered by savannah grasses. The heavily forested southern coastline is isolated from the rest of the country by the mountainous expanse of the Cardamom and Elephant Ranges, whilst the northern border abuts the Dangkret Mountains and the Eastern Highlands.