Temples aside, Siem Reap has another unique attraction in the fascinating Tonle Sap, the massive freshwater lake that dominates the map of Cambodia. The lake 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 depend on it 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 over 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 which have spawned amid the flooded trees. February sees a bumper fish catch, much of it going to satisfy the insatiable Cambodian appetite for prohok.

Fishing is big business on the Tonle Sap, and the government has awarded large concessions to wealthy businessmen at the expense of local fishermen, who have to either practise their trade illegally or rent a share from a concessionaire. The majority of these fishermen are part of the lake’s huge itinerant population, mostly stateless ethnic Vietnamese, living in mobile floating villages on the lakeshore. The houses – which are utterly basic, with (unscreened) holes in the floor as toilets – are built on bamboo rafts and lashed together to keep them from drifting apart. Vietnamese villagers having been here for decades, they have not assimilated into Khmer society and are generally loathed by the Khmer – though they are not averse to exploiting their potential as tourist attractions.

The lake was designated a UNESCO Biosphere Reserve in 1997 – a status which reconciles sustainable use with conservation. One core area of the reserve, Prek Toal, is a sanctuary for a wide range of water birds, including three endangered species – spot-billed pelicans, greater adjutant storks, and white-winged ducks. Prek Toal lies on the northwest edge of the lake in the dry season and is easily reached from Siem Reap, though you’ll have to take an organized tour.

This lake may not always be here though; its fragile eco system is under threat, as upstream on the Mekong the Chinese continue with the controversial building of dams.