The mouth of the Tonlé Sap river into the Mekong

The Tonlé Sap is unusual for two reasons:
1. its flow changes direction twice a year, and
2. the portion that forms the lake expands and shrinks dramatically with the seasons.
From November to May, Cambodia's dry season, the Tonlé Sap drains into the Mekong River at Phnom Penh. However, when the year's heavy rains begin in June, the Tonlé Sap backs up to form an enormous lake.