The Tonle Sap is at once a reservoir, flood-relief system, communications route, home and larder to the people who live on and around it; even Cambodians who live nowhere near it depend on the lake as a rich food source.
At its lowest, in May, just before the rains, the lake covers an area of around 2500 square kilometres. Himalayan meltwater flows down the Mekong just as the monsoon rains arrive, causing the level of the river to rise so quickly that at Phnom Penh the pressure is sufficient to reverse the flow of the Tonle Sap River, which would normally drain the lake. As a result of this inflow, each year the lake inundates an area of more than ten thousand square kilometres, making it the largest freshwater lake in Southeast Asia. The flow of water reverts to its usual direction in late October or early November, the receding waters leaving behind fertile mud for the planting of rice, and nutrients for the fry that have spawned amid the flooded trees. February sees a bumper fish catch, much of it going to satisfy the insatiable Cambodian appetite for prohok.
This lake may not always be here though; its fragile ecosystem is under threat, as upstream on the Mekong the Chinese continue with the controversial building of dams.