Southeast Asia’s largest freshwater lake, the natural resources of the Tonle Sap, the Kingdom of Cambodia’s Great Lake, are under increasing pressure from uncontrolled development. One of the world’s richest freshwater fisheries, about one quarter of the lake area’s 1.2M population lives in floating villages on the lake itself. Both the problems and the opportunities of the natural resources in the Tonle Sap-Great Lake area are under scrutiny as commercial pressure builds from buoyant markets in tourism, agriculture and fisheries and from associated development.

Shoreline rice paddy production around the Great Lake, some 300,000t annually, accounts for 12% of national paddy production in Cambodia. The Tonle Sap is also a significant element in the international hydrology of the mighty Mekong river, particularly of its lushly productive delta in neighbouring Vietnam. Tributaries with a total catchment area of some 71,000km2 drain directly into Tonle Sap. Around fifteen promising irrigation and power projects (including multipurpose ones) have been identified within Tonle Sap’s watershed.

Although Tonle Sap-related hydro power is not in Cambodia’s immediate energy development plan, a team of international consultants has estimated that potential water resources development around the lake could realise some 1522GWh/year, from an installed capacity of 306.5MW, and irrigate a further 318,212ha (dry season) to 358,900ha (wet season). The report of a comprehensive study, completed earlier this year, takes a significant step toward providing an environmentally sound strategy and co-ordinated planning for the Tonle Sap-Great Lake (TS-GL), its watershed area, and the deltaic Mekong’s mainstream.

Under the aegis of the Royal Government of Cambodia (RGC), the Cambodian National Mekong Committee (CNMC) and the mekong-river-commission (MRC), supported by the United Nations Development Programme (UNDP), have collaborated in the TS-GL study and production of a comprehensive report, Natural resources-based development strategy for the Tonle Sap area, Cambodia. Both investigation and report were implemented by NEDECO of the Netherlands, in association with Thailand’s MIDAS Agronomics. External funding of US$0.9M was provided by UNDP.

Tonle Sap is an integral part of the Lower Mekong Basin, particularly of its huge delta. The delta is defined as starting at Kratie, some 180km by river upstream of Phnom Penh. At Kratie, the drainage area of the Mekong is 646,000km2, and its average annual flow registers 14,000m3/sec. About 20% of this flow originates in the upper basin (Tibet, China and Myanmar), which covers 30% of the Mekong’s total catchment. This proportion increases to 40% in the March-April low flow period, augmented by snow melting from the Tibetan plateau.

Since 1957, Laos, Thailand, Cambodia and Vietnam have co-operated informally in development of the lower Mekong basin. In April 1995, however, the four riparian countries entered into the Agreement on the Co-operation for Sustainable Development of the Mekong River Basin (the Mekong Agreement). While Cambodia struggles toward equable, secure government and administration, the external forces of foreign tourism and overseas trade are increasingly affecting Tonle Sap’s sensitive social, economic and environmental interdependence.

For the purpose of both the NEDECO study and the report to the CNMC, the general area of Tonle Sap-Great Lake itself is defined as bound by two roughly parallel highways, RN5 and RN6, between Phnom Penh and the border with Thailand. During high flood levels, however, the Tonle Sap area becomes one large lake, with an area of approximately 1.300,000ha, at an average annual maximum flood level of 9.25m asl. The dry season, in great contrast, shrinks the lake area to only about 250,000ha, at a level of only 1m asl. This great annual fluctuation of its waters, in terms of agriculture and fisheries, endows the people of the Tonle Sap with a unique way of life.


About one-quarter of the some 1.2M (1996) total population of the Tonle Sap area live in floating villages. Thus water is, and is likely to remain, the ruling factor for communications, commerce and sociology. While freshwater fisheries and paddy production, including floating rice, worth an annual US$70M each, dominate Tonle Sap’s economy today, foreign tourism is expected to play an increasingly important role in future. Carefully planned and managed water-based development is seen as the best way to conserve Tonle Sap’s unique natural beauty and traditional way of life.

The lake yields 120-150Mt of fresh-water fish annually, much of which is ex-ported, yet the Tonle Sap today is devoid of defined harbours. Rough piers and land-ing places proliferate wherever a usable road can be accessed. Modern ports, it is estimated, could handle an immediate 27Mt of fish annually, increasing to some 39Mt in five years time.

Over-fishing is a looming problem, although recorded fish production remains stable. Reduction in the sizes of important species, plus the disappearance of certain species and depletion of catch volumes, seem to indicate over-fishing. The open-access nature of the Tonle Sap’s fisheries, with a short-term leasing system, is believed to promote illegal (thus unsustainable) fishing methods. Reports have been received of a growing but unrecorded export trade, mainly to China, for Tonle Sap fish trucked overland via Thailand and Laos.

Although wetland fishery and agriculture predominate, Tonle Sap’s potential role in freight and passenger traffic is highly significant to Cambodia’s national development plan. The waterway’s importance as a transport medium cannot be overstressed, although it is flanked by two national highways, RN5 and RN6, plus a railway roughly parallel to its southern shore. But Cambodia’s highways and railway system are all in very bad condition, unable to support viable traffic. Cambodia’s general infrastructure suffers from neglect caused by several decades of bloody strife. Due to guerrilla activity, for the time being surface travel is advisable only within certain very strict security parameters.

A major tributary of the Mekong river, the Tonle Sap river joins the braided Mekong at the four-channel Quatre Bras confluence in Phnom Penh, Cambodia’s capital city. An inland port, Phnom Penh is reached by seagoing vessels via the Mekong river estuary in southern Vietnam. Waterborne passenger and freight traffic already exists between Phnom Penh and Siem Reap, the Tonle Sap’s busiest landing place. The Tonle Sap’s waterborne business is set to grow with the area’s projected commercial, agricultural and tourism development.

The Tonle Sap’s catchment is estimated to be 84,000km2 in area, including the 13,760km2 area of the Great Lake itself. A small part of this catchment is located within Thailand. At Prek dam, on the Tonle Sap river about 30km above Quatre Bras, net average annual outflow toward the Mekong mainstream is approxi-mately 25,000M m3/year. Prek dam records reveal an annual Monsoon-driven inflow averaging about 51,500M m3/year, from mid-April to October, against 76,500M m3 average outflow/year for the October to mid-April period.

Maximum inflows from the Mekong to the Tonle Sap usually occur in August, sometimes reaching a rate of 10,000m3/sec. Maximum outflow, usually in November, can reach a rate of 12,500m3/sec. The dry season low water level average is about 1.1m asl, although once in twenty years it may fall as low as 0.7m asl, or remain as high as 1.7m asl. In both cases water levels are influenced both by sea level and by dry season discharge into the deltaic Mekong below Phnom Penh. Dry season tidal variations of up to 0.3m are observed at Prek dam.

Thus some two-thirds of the water in the Tonle-Sap floodplain is supplied by the mainstream Mekong, the remaining one-third coming from watercourses flowing directly into the lake. The dry season outflow from the Tonle Sap, which doubles the flows available downstream of Phnom Penh in January, continues to increase them by nearly 40% even as late as March. This emphasises the crucial importance of the Tonle Sap as a reservoir for dry season cropping in the Mekong delta, Vietnam’s rice bowl.

Due to the unique flow-constricting nature of the narrow channels downstream of Quatre Bras, the Tonle Sap serves as an accumulator for the main river system. In the wet season, water is pushed into the lake by the upstream spate; in the dry, water flows out, as the mainstream flow dwindles, and the lake area diminishes considerably. The revers-ing flow characteristics are caused by downstream channel constrictions, thus the Tonle Sap’s navigational channels upstream of Quatre Bras could be deep-ened by dredging without radically alter-ing the unique seasonal hydrology of the lake and of the mainstream Mekong.

In terms of flow, the Mekong river is one of the ten largest rivers in the world, discharging 475,000M litres annually into the South China Sea, from a catchment area of almost 800,000km2. Rising in Tibet’s snow-covered mountains, the Mekong flows through China, Myanmar (Burma), Thailand, Laos and Cambodia to its delta in Vietnam. The Lao Peoples Democratic Republic and the Kingdom of Cambodia are almost entirely located within the Mekong watershed.

In the dry season, however, the water levels at Phnom Penh are influenced by sea level, which prevents the level of the Tonle Sap falling below about 0.6m asl. Water flow in the lower Mekong basin reflects the annual cycle of Southeast Asia’s Monsoon climate. With the onset of the Monsoon in May-June, the river level rises, rapidly at first, to reach maximum levels in September-October. The highest annual maximum flood discharge recorded at Kratie was 67,000m3/s (1939), while the lowest was 40,000m3/s (1965). Nearly 90% of the Mekong’s annual flow at Kratie occurs between June and November.

Average rainfall, 80% of which falls in the Monsoon season, is in the range 1300-1400mm on the Tonle Sap’s southern shore (Battambang, Pursat), slightly higher on the northern (Siem Reap, Kompong Thom). Temperatures are in the 30-35°C range throughout the year, causing high evaporation, in the range 125-150mm/month. Long dry spells, sometimes even in the Monsoon, can cause percolation losses of 30mm or more a month.

Upstream flow in the Tonle Sap channel begins between April and June, followed by overbank flow to the lake as the river rises. Although the Tonle Sap is the Mekong’s largest natural flood reservoir, it is important to note that it is not the only one. During the Monsoon season large quantities of Mekong water are stored in flood plains along the mainstream and its tributaries in Laos and Thailand. Even larger quantities are stored in Vietnam’s verdant delta, as well as other areas of Cambodia, when the Mekong overtops its banks every year.

According to a study made for the Mekong Secretariat (forerunner of the Mekong River Commission) in 1992, sediment discharge at Kratie may be about 200Mt/yr (some 125Mm3/year), which is equivalent to 0.22mm/year erosion products from the catchment. Sediment, it is calculated, is deposited throughout the Mekong delta at an average rate of 1.5mm/year, mainly in the high-flow August-October period.

Small coastal tankers and other ocean-going vessels already reach Siem Reap in the wet season. Year-round navigation for large craft is not yet possible, due to dry season shallows. A hydrographic survey is being completed by Finnmap. Deepening the channel to 2.1m at low water level would open the Tonle Sap to year-round navigation by vessels of up to 1.5m draft.

Feasibility studies are being implemented on projects to build proper port facilities at Chong Kneas, Kompong Luang and Snoc Trou. Besides fish, modern facilities at the three proposed ports could handle an annual 7800 tons of long distance cargo and 165,000 long distance passengers, as well as some 20,000 tourists.

Irrigation systems throughout Cambodia were inventoried in 1993-4. Some 63 systems were identified in the general Tonle Sap area bounded by RN5 and RN6, most of them between the shoreline and the highways. Another 69 schemes were identified outside the highways, but within the catchment area of the Tonle Sap’s tributaries. The total existing irrigation area is some 55,000ha in the wet season, 19,000ha in the dry. Many storage reservoirs built during the Khmer Rouge administration, remain in the Siem Reap area.

Barrage proposed

A barrage to control flows into and out of the Tonle Sap was first investigated about thirty years ago. A site across the Tonle Sap channel near Kompong Chhnang was proposed. A barrage, it was believed at the time, would reduce flooding in an area of 100,000ha around the lake, and augment flows in the Mekong downstream of Phnom Penh by 2,500m3/sec. The cost of the barrage was estimated to be some US$330M in 1984 (equivalent to US$600M in 1998). Although fish locks were included in the concept, it was then believed that stabilisation of water levels would increase fish production.

Controversy arose, however, on the question of exactly what the effect would be on the Tonle Sap area’s traditional agriculture and fisheries, in terms of cost against benefit. A master plan for the Mekong Delta in Vietnam (NEDECO, 1993) showed that agricultural production could be increased by 50% without the need for additional fresh water. Moreover both Vietnam and Cambodia are experiencing benefit from increasing flows in the dry season, due to reservoir construction in the upstream Mekong basin.

The CNMC/MRC Report reasons that medium-sized hydro power projects at Stung Battambang and Stung Pursat could be significant elements of the national power development plan by 2010. An opportunity also exists to develop a mini hydro power plant at Phnom Kulen and Siem Reap.

Water storage in the Tonle Sap’s tributaries, particularly Stung Battambang and Stung Pursat, are specified in the Report as priority projects. Without such development, there is likely to be a dry season water-deficit in the Tonle Sap area within the next ten years.