地学雑誌 Journal of Geography 103 (6) 623-636 1994 Preliminary Study of Sedimentation in Lake Tonle Sap, Cambodia Shinji TSUKAWAKI *, Masafumi OKAWARA **, Kim-Leang LAO * * * and Motohiko TADA ** Abstrac t Lake Tonle Sap, the largest lake in the Indochina Peninsula, lies in central Cambodia. Unique sedimentation is expected in the lake due to drastic changes in its water area between the rainy and dry seasons. As the first step to examine the sedimentary processes of the lake, bottom sedi- ments of the lake and the Tonle Sap River, and surface soils of alluvial deposits were collected and examined in order to reveal the origin of the lake bottom sediments. As the results it becomes clear that clay minerals in the bottom sediments of the northern part of Lake Tonle Sap are derived both from surface soils on the alluvial deposits around the lake, and metamorphic and granitic rock bodies lying in the Mekong River basin. The latter is trans- ported, as suspended sediments by backward current in the rainy season, into the lake from the Tonle Sap River. The presence of marine creatures in bottom sediments of the lake suggests that the lake was closely connected with the sea during the last sea-level high stand. Furthermore, there is a strong possibili ty that annual changes due to alternating rainy and dry seasons will be recorded in the lake bottom sediments over the long geological period. control of seasonal fluctuations in water level I. Introduction due to alternating rainy and dry seasons, char- Lake Tonle Sap, the largest lake in the acteristics of the tropical monsoon region. In Indochina Peninsula, lies in central Cambodia. addition, the lake has been closely associated The lake is known as "the elastic water with the lives and culture of the Cambodian world", because its water area expands drasti- people since the Khmer Dynastic periods. cally in rainy seasons. It is also known as "the Consequently, it is expected that climatic and mud ocean" which refers to deep brown colour environmental changes, and their relationships of the water all year round due to a large with the rise and fall of various civilization amount of muddy suspended sediments con- in the Indochina Peninsula can be reconstructed tained in water. Such settings hold out a over the period based on analysis of the lake promising prospect for investigations of unique bottom sediments. sedimentary processes in the lake under the Bathymetry, water quality and aquatic re- * Department of Geology , College of Liberal Arts, Kanazawa University ** Department of Civil and Environmental Engineering , Faculty of Engineering, Iwate University *** Environmental Center , Japan Quality Assurance Organization 62 Fig. 1 Lake Tonle Sap and topographic features of Cambodia Dotted line around the lake indicates flooded areas in the rainy season. sources of Lake Tonle Sap have been reported of the largest rivers on the Asian continent, (Mitusio et al., 1970 ; Kottelat, 1985 ; Lao, and the surface soils of alluvial deposits in the 1992, 1993 ; Matsui et al., 1993). However, northern part of the Tonle Sap Basin which no geological and sedimentological investiga- seem to be one of the important sources of tions have been carried out in the lake, except bottom sediments of the lake. for brief descriptions of bottom sediments of II. Lake Tonle Sap and Natural the lake and adjacent rivers (Tsukawaki and Environments of Cambodia Moriai, 1993 b ; Tsukawaki and Lao, 1994) . As the Cambodian political unrest has persisted Figure 1 shows topographic features of Cam- since the 1970's, only a limited survey can be bodia. Lake Tonle Sap lies in the central carried out. Therefore, as the first step in a part of the Tonle Sap Basin, a gentle topo- series on pursuing the above-mentioned sub- graphic depression with a NW-SE trend, in jects, the present paper illustrates preliminarily central Cambodia. The Cardomon Chain and the sedimentary processes of the lake based the Elephant Mountains with the highest peak mainly on the sedimentological and biological of 1,744 m above sea level, and the Dangrek properties of bottom sediments in the northern Escarpment, 765 m above sea level, all com- part of the lake and the Tonle Sap River which posed mainly of sedimentary rocks of the Upper connects the lake to the Mekong River, one Indosinias Formation ranging in age from Cre- 624 taceous to Palaeocene (Gubler, 1933 ; Saurin, inundated during high stands of water level as 1935 ; United State Geological Survey, 1971 ; well as paddy fields (Pl.-3, 4). Workman, 1979) form the western, southern There is a network of numerous rivers and and northern boundaries of the basin, streams flowing directly into the lake from its respectively. The overwhelming part of the surroundings, but their flow is adequate only basin is covered mainly with alluvial deposits in the rainy season, except for a certain num- composed of unconsolidated silt, sand, gravel, ber of rivers, such as the Siem Reap River laterite (United State Geological Survey, 1971) originating in the Kulen Mountains which are and a variety of soil types such as acidic litho- a salient topographic high in the northern soil and lateritic clay which are typical soil part of the Tonle Sap Basin. Small rivers and types in a humid tropical region (Bridges, streams around the lake partly flood in the 1978 ; Garami and Kertai, 1993). The thick- rainy season. Many muddy deltas form in the ness of the alluvial deposit reaches more than vicinity of river mouths in the coastal area of a few Eundred metres at the Mekong Delta in the lake in the rainy season (Pl.-5). Viet Nam, whereas up to 15 m at Phnom Penh Again in the rainy season, the flow of water City (Workman, 1979). Several topographic which contains a large amount of muddy sus- highs such as the Kulen Mountains, Phnom pended sediment derived from surface soils Bakheng, Phnom Bok, Phnom Dei and Phnom distributed in the Tonle Sap Basin through Krom are scattered in the northern part of the this group of rivers joins water of the Mekong basin (P1.-1, 2). River, originating in and bringing much melted The climate of Cambodia is controlled main- snow from the northern slopes of the Hima- ly by the tropical monsoon system influenced layas in the western part of China together with by such local topographic highs as the Cardo- the heavy monsoon rain. Therefore, water of mon Chain and the Dangrek Escarpment. the Tonle Sap River flows backwards to the Therefore the dry season (November to May) lake in the rainy season, and water level of is clearly distinguished from the rainy season the river increases more than 8 m in compari- (May to November) with a brief semidry season son with it in the dry season in Phnom Penh in July, year in, year out (Fig. 2). City (Pl.-7, 8). On the other hand, in the Lake Tonle Sap is long, narrow and gourd- dry season, the inflow from the rivers decreases shaped, having a NW-SE longitudinal axis markedly and water flows out from the south- of 120 km and a maximum width of 40 km in eastern part of the lake through the Tonle the dry season. The water area of about 3,000 Sap River to the Mekong River. km2 in the dry season swells more than three- The lake water is reddish brown to yellow- fold (about 10,000 km2) in the rainy season ish brown, and extremely low in transparency (National Astronomical Observatory, 1993), thus (50-100 cm) even in the dry season (Mitusio the lake lacks fixed coastal lines. At low water et al., 1970). The temperature of surface water level the depth is small wholly (3 to 6 m) is extremely high (26-30 •Ž) all year round (Mitusio et al., 1970) and at high water level and the pH is almost neutral (around 7.0) (Mi- the deepest parts are no more than 14 m tusio et al., 1970 ; Lao, 1992, 1993). Two (National Astronomical Observatory, 1993). hundred and fifteen species of fishes which come Forests in the coastal areas of the lake are under 127 genera and 47 families including 625 626 balloonfishes and rays which are generally made into a thin section and examined under marine forms are recognized in the lake, the the microscope. Since the contents of micro- Mekong River and adjacent rivers in Cambodia organic remains in the samples are insufficient (Kottelat, 1985). for statistical analysis, the results is short of a preliminary grasp. III. Sampling Method and Analytical For clay mineralogy, samples were first Procedures disaggregated in distilled water, followed by The ;samples used for the present study were ultrasonic procedures for four hours, then clay collected in August 1992 at the northern part fractions were concentrated by a centrifuge. of Lake Tonle Sap and north of Bayon which Almost pure clay fractions almost free from is the central sanctuary of Angkor Thom, and impurities were dried at room temperature, March 1993 at the Tonle Sap River in Phnom and prepared for X-ray analysis. X-ray anal- Penh City. Sampling devices were a cylindri- ysis was carried out on oriented powder smear- cal aluminium can, 6 cm in diameter and 12 ed on a glass slide. X-ray diffraction patterns cm long, with an 1 kg steel weight at the lake, for each of the samples were recorded with a and a Seki-type grab sampler, 500 ml in vol- Mac Science type MXP-3 AHF X-ray powder ume, at the Tonle Sap River. Surface soil diffractometer equipped with a graphite mono- samples at Bayon were collected by a 6.5 m chromator, using Cu-Kƒ¿ radiation, tube voltage long hand auger.