Great Lake and Tonle Sap River
Location:
11°40'-13°30'N, 103°05'-105°05'E; in the lowlands of central Kampuchea from the region of
Siem Reap in the northwest to Phnom Penh in the southeast.
Area:
Great Lake 250,000 -300,000 ha; total inundation zone c.1,500,000 ha.
Altitude:
7 - 15m.
Biogeographical Province:
4.10.4.
Wetland type:
11, 13, 14, 15, 17, 18, 19 & 21.
Description of site:
The Great Lake is the largest permanent freshwater lake in Southeast Asia. The entire lake basin extends for some 250 km from northwest to southeast and is up to 100 km wide. At low water level, the lake is about 120 km long and up to 35 km wide. It covers an area of 250,000-300,000 ha and has a mean depth of less than one metre. The lake ~ surrounded by a broad belt of freshwater swamp forest, generally some 20-30 km wide, but extending for 65 km west from the west end of the lake. This forest contains numerous snal rivers and streams, and innumerable lakes and ponds. The belt of swamp forest is in turn surrounded by a broad belt of rice paddies, up to 25 km wide, which borders on extensifli forested areas. Silts and clays comprise much of the soils.
The Great Lake is connected to the Mekong River at Phnom Penh, some 120 km to tM southeast, by the Tonle Sap River. In June or early July, shortly after the onset of the rainy season, the elevation and strength of the Mekong discharge begins to act as a dam on the bale Sap River. Initially, this causes the Tonle Sap to spread laterally, but subsequently its currete reverses and carries the Mekong floodwaters into the basin of the Great Lake. To the south the lake, there are many natural and man-made diversions which spread the floodwaters over wide area. The Mekong reaches its highest levels in August and September, and by the end September or early October, flooding in the Great Lake basin covers 1,100,000-1,300,000 ha, a large proportion of which is swamp forest. At this time, the lake has a mean depth of 8-l0a. The water level falls rapidly between January and March, and the lowest levels occur in AM and May.
Open water temperatures in the lake range from 28-29°C at the surface and 26-28°C at the bottom, while temperatures in the shaded waters of the inundated forest may exceed 29°C. Th water of the Great Lake is always extremely turbid; this is attributed to shallowness coup~ with wind-induced turbulence which constantly churns the bottom sediments. The pH of the lake fluctuates between 6.6-6.9, while that of the tributaries varies between 6.1-7.1 (Pan l986b). Some of the smaller water bodies around the Great Lake contain humic acids which become highly concentrated during the dry season.
The Great Lake was at the heart of the ancient Khmer Empire. The holy Mount Mahendra (Phnom Koulen) lies to the north of the lake, and the famous ruins of Angkor City are located only 10 km from the northwest shore (13°18'N, 103°52'E). The ruins are situated in a region of fertile plains with a number of canals and reservoirs, up to 1,600 ha n extent, built during historic times.
Climatic conditions:
The Great Lake lies within the tropical wet and dry zone. About 80% of the annual rainfall occurs during the southwest monsoon from May to October, and there is a pronounced dry season during winter with at least one month having less than 60 mm of rainfall. The average annual rainfall at Siem Reap, northwest of the lake, is 1,432 mm (maximum 2,056, minimum 466); the mean annual temperature is 26.7°C (maximum 40.3°C, minimum 9.5°C).
Principal vegetation:
No information is available on the aquatic vegetation of the lake. The lake is surrounded by extensive seasonally flooded freshwater swamp forests dominated by Homalium brevidans, Hydrocarpus anthelmintica, Barringtonia acutangula. Terminalia chabula and Amelia asiatica. In 1960, the total area of freshwater swamp forest in Kampuchea was estimated at 681,400 ha, most of this being around the Great Lake. Large areas have since been cleared for rice cultivation, and the total has recently been estimated at 564,000 ha.
Land tenure:
No information.
Conservation measures taken:
The ruins of Angkor City and the surrounding forests, including 2,000 ha of swamp forest, have been protected in the Angkor Wat National Park (10,717 ha) since 1925.
Conservation measures proposed:
McNeely (1975) proposed the establishment of a large reserve along the southwest shore of the lake to protect a significant tract of freshwater swamp forest and its associated fauna.
This reserve would extend from the Pursat River northeast along the south side of the take to Mongkol Borey, and south to the polder dikes built to help irrigate parts of Pursat and Battambang districts.
Land use:
Large-scale production forest (mangrove) for producing cord-wood for export to Taiwan. The area also supports a local fish-smoking industry and small-scale fishing for domestic consumption, and there is some hunting of Cervus unicolor and Sus scrofa. There are several settlements and patches of cultivation in the surrounding areas.
Possible Changes in Land use:
Fishing and agriculture, mainly cultivation of rice. The fishery of this region is perhaps unparalleled in the world for its extreme intensity and for the ingenious variety of gear involved. Much of the fisheries exploitation results from the straining of fishes during the gradual recession of the floodwaters.
Disturbances and threats:
In recent decades, the accelerated clearance of freshwater swamp forest around
the lake for agriculture, firewood and fish ponds has increased siltation, threatening the existence of the lake. McNeely (1975) concluded that at the current rates of sedimentation, the lake would continue to diminish in size and might disappear in a relatively short time. Further loss of freshwater swamp forest could lead to serious reductions in fish populations, thereby reducing piscivorous wildlife and damaging a lucrative commercial fishing industry (Pantulu, 1986a).
Full implementation of the water resources management projects advocated by the Mekong Committee could lead to a loss of as much as 92% of the high-water surface area of the inundation zone, and cause the near or complete elimination of the seasonal inundation zone fishery (Mekong Committee, 1976a).
In recent years, there has been some contamination of the wetlands with toxic materials released in jute production.
Economic and social values:
The Great Lake is the heart of Kampuchea. It serves as a vital, natural regulator of
Mekong floods in the rainy season, thereby protecting delta areas in southern Kampuchea and Vietnam from even deeper flooding.
The lake supports one of the world's most productive freshwater fisheries, which provides a large proportion of the protein requirement of Kampuchean people. The great productivity has been brought about by the inflow of organic matter into the lake from vegetation on the floodplain. Most records indicate a commercial productivity in the lake of 40-50 kg/ha/yr, representing a total annual yield of 36,000 metric tonnes. The inundation zones of the Great Lake and Tonic Sap are of vital importance as spawning and nursery grounds for a wide range of fish species. By far the greatest production support for the entire lowland delta fisheryderives from the aquatic production in this floodwater complex. The fishery of the inundation f zone has, however, been steadily declining, ostensibly due to increased fishing pressure and accelerated siltation in the Great Lake (Mekong Committee, 1976a). Mean annual yields from the entire Great Lake-Tonic Sap inundation zone were estimated at 139,000 tonnes during the period 1939-51 and 101,700 tonnes during the period 1956-61 (Pantulu, 1986b). In the early l970s, the total catch, including the subsistence take, for this zone was thought to be between 50,000 and 80,000 metric tonnes (Mekong Committee, l976a); in 1984, the total production was estimated at 63,000 tonnes.
The ruins of Angkor City, the capital of the Khmer empire from about 800 A.D. to 1432, are widely regarded as the finest ruins in Southeast Asia. The centre-piece is Angor Wat, a temple constructed in 1100-1150 A.D. and dedicated to Vishnu, a Hindu God. Prior to the warfare of the 1970s, Angkor Wat was the most popular tourist destination in the country, attracting 70,000 visitors in 1968. This tourism declined almost to zero during the war years, but a few tourists have been able to visit the site in the last two or three years.
Fauna:
The lake, together with its surrounding seasonal swamp forests, is home to a large number of endemic fish species and provides a refuge for a w de variety of waterbirds.
Approximately 38 commercially important species of fishes have been recorded in the area. Because of the considerable annual variation in water volume of the system, most fishes exhibit migratory behaviour patterns, with major movements between spawning grounds in the inundated forests and dry season refuges in the major river channels. The Great Lake fishes are usually categorized as either "Poissons Blancs" or "Poissons Noirs", depending upon their migration patterns. "Poissons Blancs" annually migrate with the Mekong flood up the Tonic Sap to the lake; they include carp, clupeids, schilbeid catfish of the genus Pangasius, threadfins and drums. The "Poissons Noirs" are permanent residents in the lake, and include the t murrels Channa spp, Anabas testudineus, catfish of the genera Saccobranchus, Clarias, the spiny eel Mastacembelus spp and the sand goby Oxyeleotris marmoratus (Pantulu, 1986b).
Although waterbirds are reported to be very common throughout the Great Lake basin, very little detailed information is available. Several breeding colonies of large waterbirds have been found in recent years, and several endangered species such as the Milky Stork, Giant Ibis, White-shouldered Ibis and Eastern Sarus Crane (Mycteria cinerea, Thaumatibis gigantea, Pseudibis davisoni and Grus antigone sharpil) are believed to breed in the area. Leptoptilos javanicus was reported in the Siem Reap area in the l960s (Luthin, 1984).
Mammals known to occur in the Angkor Wat National Park in the 1970s included Eld's Deer Cervus eldi and Banteng Bos javanicus.
Special floral values:
No information.
Research and facilities:
No research has been carried out since the onset of hostilities in the ~' early l970s, and most of the area remains closed to outsiders because of continuing threats from the Khmer Rouge militia.
References:
FAO (1980); IUCN (in prep); Karpowicz (1985); Luthin (1984); McNeely (1975); Mekong Committee (1970, 1976a, l976b, 1978, 1981 & 1984); Pantulu (1975, l986a & l986b).
Criteria for Inclusion:
123.
Source:
Le Dien Duc, Tran An Phong and references