Cambodia’s principal lake, the Tônlé Sap (Great Lake), is the largest in Southeast Asia. From the northwest, the Tônlé Sap drains into the Mekong via the Tônlé Sab River, entering the Mekong at Phnom Penh. Each year during the monsoon season (approximately May to October), the waters of the Mekong increase and reverse the flow of the Tônlé Sab, which begins to drain into the lake. The lake then expands dramatically, flooding the provinces along its banks. When dry weather returns, the river reverses its course again and flows back into the Mekong, draining the northwestern provinces. At the height of the flooding, the Tônlé Sap reaches more than 10,000 sq km (4,000 sq mi), or about four times its size in the dry season. The lake is one of the richest sources of freshwater fish in the world.