As Southeast Asia’s largest fresh body of water, Tonle Sap Lake acts as the central lifeline for over 1.2 million Cambodians living along its banks. In addition to hosting the most productive inland fishing on the planet, the similarly named Tonle Sap River has the distinction of being the world’s sole waterway which runs in both directions.
That is to say, its flow of water reverses every six months out of the year, causing the lake to swell to an astonishing five times its size between the wet and dry seasons. And like a living, breathing organism, its residents have skillfully adapted to the Tonle Sap’s rhythmic, annual inhalation of water, flora and fauna.
Annual Cycle of the Tonle Sap
During the dry season, which typically lasts from November to May, the Tonle Sap flows towards the sea, much like any other stream, in a south-easterly direction. Roughly 100 km (62 mi) downstream, the Tonle merges with the mighty Mekong River, which then branches off into several waterways throughout Vietnam’s thriving Mekong Delta region and into the South China Sea.
Over the wet season, however, the Mekong River becomes so inundated with torrential rains throughout Southeast Asia in addition to the melting snows from Tibet (4,500 km/2,800 mi upstream), that its excess waters take an alternate, north-western route up the Tonle Sap River.
In turn, Tonle Sap Lake expands from an area of 2,700 to 16,000 sqkm (1,050 to 6,200 sqmi, see map above-left) at the peak of the wet season, while its widest point grows from 35 to 105 km (22-65 mi). And because the depth of the Tonle Sap rises so drastically, from roughly 1-metre to 9-metres, lakeside structures are built to adapt in one of two ways: houses that either float or stand on stilts up to 10-metres (33 ft) high.
The Life-Giving Tonle Sap
Remarkably, Cambodia is home to one of history’s largest pre-industrial settlements. During the Khmer Empire’s zenith, circa 1200 AD, Angkor Wat was home to an estimated 900,000 inhabitants. This feat is widely attributed to its prime location along the northern banks of the Tonle Sap, as evidenced by Angkor’s countless temple bas reliefs depicting scenes of everyday life related to fishing and irrigation.
Nearly a millennium later, little has changed in terms of reliance on the Tonle Sap for sustenance. Much like their ancient Angkorian ancestors, fish and rice are two staples in nearly every Cambodian’s diet, with freshwater fish accounting for two-thirds of their daily protein intake.
Though dry season means the shoreline recedes from underneath the floorboards to tens of kilometres away, the muddy Mekong waters leave behind rich soil deposits ideal for rice cultivation. Thus, fisherman all across the wetlands hang up their nets in lieu of farming equipment for half the year.
Not only does the Tonle Sap provide for those in its immediate vicinity, but it also acts as both a safety valve and reservoir for those living further downstream in Vietnam’s Mekong Delta. Unlike the Red River Delta in northern Vietnam, which is quite prone to natural disaster, the Tonle Sap’s ability to absorb excess water prevents flooding in the southern Mekong Delta during the heavy rains. Conversely, drainage from the Tonle Sap contributes half of the Mekong’s water flow during dry months.
Discover the Tonle Sap with Heritage Line
As a unique, ever-changing ecosystem harbouring over 200 species each of birds and plants as well as 150 types of fish, the Tonle Sap gained formal recognition as a UNESCO Biosphere Reserve in 1997.
Two of Heritage Line’s luxury river vessels, Jayavarman and The Jahan, explore the Tonle Sap and Mekong Rivers’ gorgeous green landscapes, ancient temples and charming village life. Lower Mekong River cruises range from 3-, 4- and 7-nights between Siem Reap and Saigon.
When water levels on the Tonle Sap are high enough for ship navigation (typically from late August to mid-November), passengers are fortunate enough to embark on a direct sailing across the vast Tonle Sap Lake to and from Siem Reap, home of Angkor Wat.