Low plains drifters

A signpost on the road leading to Kompong Luong advises that the village is "maximum 7km, minimum 2km" away. Kompong Luong, a collection of bamboo huts bobbing on Tonle Sap Lake in central Cambodia, moves around as the water level rises and falls. Residents tow behind them the necessities of life: shops, a schoolhouse, clinic and pagoda.

A signpost on the road leading to Kompong Luong advises that the village is "maximum 7km, minimum 2km" away. Kompong Luong, a collection of bamboo huts bobbing on Tonle Sap Lake in central Cambodia, moves around as the water level rises and falls. Residents tow behind them the necessities of life: shops, a schoolhouse, clinic and pagoda.

The village is one of several settlements around Tonle Sap whose inhabitants lead an entirely aquatic existence. In a lifestyle that has barely changed for decades, the men fish and sell their catch while the women dry the fish and mend nets. Children paddle to class in canoes and the whole community floats, including the hairdresser's, grocery store, petrol pumps and karaoke bar.

Tonle Sap, the largest inland waterway in South-east Asia, is Cambodia's heart and lungs. Three million people - more than a quarter of the population - live on and around the Great Lake, as it is called. Much of the economy depends upon it; many families rely on it for their livelihood and its 850 species of fish - including giant catfish weighing up to 135kg - are a key source of protein for the nation. Rice is grown on surrounding plains that are flooded annually when the waters rise.

The lake - upstream of Tonle Sap river, which meets the Mekong river in Phnom Penh - is also notable for a unique phenomenon. During the monsoon season, the lower Mekong becomes so bloated that it forces the Tonle Sap to reverse direction and flow away from the sea, flooding the lake with vast quantities of fresh water. When the dry season arrives, Tonle Sap reverts to its usual course, draining the lake basin into the Mekong.

This singular event is celebrated in a water festival, Bon Om Touk, an exuberant spectacle in which longboats compete for three days, finishing in front of the Royal Palace in Phnom Penh. A few years back, the race was staged on the moats that encircle the temple complex of Angkor Wat.

Bon Om Touk, an important part of Khmer culture, takes place in late October or early November, but the wonders of Tonle Sap can be experienced at any time of year. Ferries ply the length of the lake, connecting the capital with Siem Reap, the sleepy colonial town that services Angkor Wat. The six-hour boat trip is preferable to travelling by road, as the nation's highways are atrocious. Besides, to fly means missing out on magnificent scenery.

It was a bedraggled collection of passengers that boarded the cigar-shaped ferry in Phnom Penh following a pre-dawn tropical downpour one autumn morning. Families arrived at the jetty shrouded in plastic raincoats, three or four people to one motorbike, with just the driver's head peeking out. By the time we chugged out of port, the rain had subsided to a gentle drizzle and, as the river widened into the brown expanse of lake, the sun appeared, sending soft rays slanting across the water.

As the ferry navigated between tufts of drowned vegetation, the first of the many communities that cling to the rim of the lake came into view. The water level was so high that it lapped at the front doors of wooden stilt houses clustered on the shore. The dome of a pagoda, built a little inland, was visible behind waterlogged trees. Boys glided past in longboats, fishing for carp, while women punted down little muddy tributaries that snaked off in all directions.

The lake, which covers 2,500 sq km during the November-to-June dry season, can swell to up to 12,500sq km during the monsoon, gaining 70km in length. When it shrinks, the villages move too, seeking deeper waters further out into the lake. Submerged tree-tops reappear and fish are found stranded in the reeds - a sight that prompted an early 20th-century French author, Pierre Loti, to describe Cambodia as the land where fish grow on trees.

Tonle Sap's role over the centuries as a major food source for theKhmer people is depicted in bas-relief carvings at the wonderful Bayon temple at Angkor Wat. The lake, part of which is a World Heritage site, is also a birdwatcher's paradise. Three endangered species - the spot-billed pelican, greater adjutant stork and white-winged duck - can be seen at Prek Toal, a bird sanctuary easily reached from Siem Reap.

From Siem Reap, you can get a taste of life on Tonle Sap by visiting Phnom Krom, a nearby floating community. During the rains, Phnom Krom is situated near the mouth of the Siem Reap river; when the water level rises, it moves a couple of miles into the lake. Rowing boats moored by the narrow road that winds cross rice paddies to the ferry port take tourists out to the village. Phnom Krom is predominantly Vietnamese, although it is also home to substantial numbers of Khmers; locals joke that it is one of the few places where the two races - long-time sworn enemies - live in harmony side by side.

Some residents operate fish farms; you can see thrashing catfish being removed from massive tanks and small silvery fish shovelled into bamboo baskets for processing into prahok, a fermented fish condiment central to Khmer cuisine. A forest of aerials disfigures the skyline; television is one of the few entertainments for waterborne communities.

Friends had warned that the ferry trip can be rough, and they were right. Midway into the lake, the sky turned dark grey and high winds picked up, bringing a slather of rain. The passengers - a mixture of Phnom Penh teenagers sitting cross-legged on the deck and peasant families overloaded with produce and children - huddled beneath blankets and tried to prevent their belongings from blowing away. We reached Siem Reap in the same drenched state in which we had boarded the boat that morning.

GIVE ME THE FACTS

How to get there

There are no direct flights between the UK and Cambodia, but you can go via Bangkok with Thai Airways (0870 606 0911; www.thaiairways.co.uk), Singapore with Singapore Airlines (0870 608 8886; www.singapore air.co.uk) or Kuala Lumpur with Malaysia Airlines (0870 607 9090; www.malaysia airlines.com). Expect to pay from £650 return from London Heathrow to Phnom Penh. Trailfinders (020-7938 3939; www.trailfinders.co.uk) offers return flights from Heathrow to Phnom Penh via Doha in Qatar with Qatar Airways from £600 return.

Express boats run daily between Phnom Penh and Siem Reap, leaving at about 7am from the new passenger terminal near the main post office. The one-way trip costs about $25 (£13). Alternatively return to Phnom Penh by air. Siem Reap Airways (00 855 63 380 331; www.siemreap airways.com) flies several times a day from about $100 (£54) one way.

Where to stay

The Renakse Hotel (00 855 23 722 457) in Phnom Penh offers rooms for around $40 (£22) a night. In Siem Reap, the Grand Hotel d'Angkor (00 855 63 963 888; www.raffles.com) offers rooms from about $200 (£110) a night.

Further information

In Siem Reap, several companies offer trips to Prek Toal and outlying areas of Tonle Sap. Highly recommended is Osmose Nature Tours (00 855 12 832812). Cambodia does not have a tourist office in the UK, but for further information visit www.tourismcambodia.com. The Cambodian Ministry of Tourism also has a website, www.mot.gov.kh.