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This river, called the Tonlé Sap, which joins the Mekong at Phnom Penh, is unique. It flows downstream in the dry season but in the rainy season when the Mekong swells and is in danger of overflowing its banks, the water flows upstream and fills a large lake in the center of Cambodia.

This reservoir acts as a natural flood control for the whole of the Mekong Delta. After a few days in Phnom Penh we took this boat up the Tonlé Sap and across the lake to the town of Siem Reap - our staging point for visiting Angkor.