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Evolution of the Kalu Ganga - Bolgoda Ganga Flood Plain System, Sri Lanka

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Evolution of the Kalu Ganga - Bolgoda Ganga Flood Plain System, Sri Lanka Journal of the Geological Society of Sri Lanka, Vol. 14, 41-53 Journal of the Geological Society of Sri Lanka Vol. 14 (2011), 41-53. C.B. Dissanayake Felicitation Volume Evolution of the Kalu Ganga - Bolgoda Ganga Flood Plain System, Sri Lanka P. Wickramagamage Department of Geography, University of Peradeniya, Peradeniya, Sri Lanka. (email: [email protected]) ABSTRACT The Bolgoda Ganga catchment shows all the signs of a relict hydrogeomorphological feature. It has many hydrogeomorphological anomalies, such as disproportionately large estuary and floodplain, presence of a valley extending South to North, presence of an off- shore canyon off the mouth of the Bolgoda Ganga. Furthermore, the Bolgoda Ganga and Kalu Ganga catchments were inter-connected even before the construction of the Kepu Ela. This study attempts to elucidate the evolution of this drainage system and the geo- morphology of the area. The geomorphology of the Southwestern coastal plain of Sri Lanka has evolved during a period of rapidly occurring environmental changes, such as warming of the climate since the last glacial maximum and fluctuation of sea levels during the Holocene. The drainage system has developed in response to these changes. Examination of the geomorphology and stratigraphy of the Bolgoda Ganga flood plain indicates that the most probable palaeo- river that flowed into the Bolgoda estuary was the Kalu Ganga. The strongest evidence in this regard is the presence of gem gravel similar to those of the Ratnapura area (Kalu Gan- ga Basin) in the Bolgoda sedimentary terraces and the floodplain. The changing characte- ristics of fluvial sediment in the floodplain indicate that the climate also changed during the recent past. This change is from a wetter climatic phase, possibly represented by the Ratnapura pluvial phase, up to the present climate. It is responsible for the upward fining sequence of fluvial sediment in the upper terraces and changes in the lower course of the Kalu Ganga. INTRODUCTION The evolution of the Southwestern coastal ward. Several late Holocene high sea level plain of Sri Lanka has taken place against a stands (up to 3.7m) have been reported from background of a rapidly rising sea levels dur- many parts of the world at 5700, 4900, 3700, ing much of the post-glacial period since the 2400, 2200, and 1000 BP (Fairbridge, 1961). last glacial maximum around 18,000 years BP. Even though some doubt that there ever were By then the sea level was about 100 m below higher sea levels during the late Holocene that of the present time (Dyre, 1997, p.1) ex- (Kidson, 1986), evidence to the contrary has posing much of the continental shelf around been reported from tectonically stable lands Sri Lanka. The worldwide sea level rose during such as Sri Lanka (Katupotha, 1995; Weerak- the last 20,000 years rapidly with the melting kody, 1988, 1992). Katupotha (1995) suggests of polar ice sheets as the climate began to that there were at least three episodes of warm inundating the lower parts of the coast- higher sea level stands in Sri Lanka during the al plain. However, the sea level change during late Holocene around 5500 BP, 4000 BP and the last 20,000 years was not uniformly up- 2500 BP. Fairbridge (1961) claims that sea lev- 41 Wickramagamage, Evolution of the Kalu Ganga - Bolgoda Ganga Flood Plain System el was about 3-5m above the present level There is a man-made canal called Kepu Ele between 6000-4600 BP. The Sri Lankan evi- connecting the Kalu Ganga Basin with the Bol- dence also shows higher sea levels up to goda Ganga. about 5m above the present level (Weerakko- Another important anomaly associated dy, 1988). with the Bolgoda Ganga is the existence of an The rising sea level would have brought offshore canyon opposite the outfall of the about changes such as the drowning of the Panadure Ganga. Such canyons exist off the lower courses of rivers, changing the base lev- mouths of large rivers like Niger. They are be- el of erosion, reducing the competence of the lieved to be products of subaerial erosion by a rivers to carry sediment, etc. The fluctuation powerful river or erosion by turbidity currents in the sea level would have led to alternation associated with a large river flowing into the between freshwater and brackish water con- sea (Swan, 1983). The continental shelf of Sri ditions in the basins of sedimentation in the Lanka has ample evidence to show that the low-lying areas of the coastal plain. It has also stream valleys continue into the continental been noted that the floodplains of many rivers shelf of the West coast (Deraniyagala, 1958). in Sri Lanka are covered with thick layers of But at present, Bolgoda Ganga does not have alluvium, extending to a depth well below the a sufficiently high discharge capable of pro- present sea level (Cooray, 1984; Erb, 1984). A ducing such a large canyon. growing body of evidence suggests that the The Kalu Ganga in the South is separated glacial periods are associated with aridity and from the Bolgoda basin only by a low ridge interglacial periods with humid conditions which has openings in many places. When the (Premathilake and Nilsson, 2001). These water level of the Kalu Ganga is high, water changes in the climate and associated sea lev- could have entered the Bolgoda Ganga el fluctuations would also have had a pro- through these natural openings even before found effect on the drainage pattern of the the construction of the Kepu Ele. coastal plain of the island. The present drai- All these anomalous characteristics indi- nage pattern of the Southwestern coastal cate the possibility that the Bolgoda basin was plain reflects hydrological feedbacks to these a part of a much larger ancient river system. changing environmental conditions. This is The potential candidate for this palaeo-river remarkably evident from the Bolgoda and Ka- system is the Kalu Ganga which may have car- lu Ganga Basins. The Bolgoda basin sand- ried more water during a past pluvial climatic wiched between the Kalu Ganga Basin to the regime (possibly during the Rathnapura Phase South and the Kelani Basin to the North is described by Deraniyagala, 1958). The objec- somewhat of an anomaly due to the follow- tive of this paper is to evaluate the available ing: evidence to trace the palaeo-hydrogeo- The Bolgoda River and the estuary (two morphology of this region. lakes, the Bolgoda Ganga and Panadura Gan- ga), are disproportionately large compared to The Study Area the total area of the Bolgoda basin and its The area covered by this study includes present discharge. It is more like the lower the floodplains of the Kalu Ganga and the Bol- reach of a large river. The combined area of goda Ganga, which are two adjacent river ba- the lakes and channels is about 18 km2 in a sins and an inter-basin area situated on the basin of only 395 km2. The two lakes situated southwestern coastal belt of Sri Lanka. The in the lower basin appear to be remnants of a two river basins are inter-connected naturally large estuary. The marshes around them are through gaps in the ridge separating them and the silted up parts of this palaeoestuary. The also by a canal constructed by the Dutch for present drainage pattern is a total mismatch navigational purposes through a pre-existing for such a large estuary. gap of the ridge. During floods, waters of the The second anomaly is that the main Kalu Ganga enter the Bolgoda Ganga catch- channel that flows from South to North con- ment through these openings. necting the two lakes is connected on the The Kalu Ganga, which flows in a North- South by a silted up broad valley. The present westerly direction up to the Kepu Ele, turns channel of the Kalu Ganga is separated from Southwest making a sharp bend and flows this valley only by a slightly elevated land. into the sea at Kalutara, about 48 km South of 42 Journal of the Geological Society of Sri Lanka, Vol. 14, 41-53 Figure 1: Kalu Ganga and Bolgoda Ganga Flood Plain Area Colombo (Fig. 1). The position of this outfall highest in the country. This is because the up- varies annually and in some years even seaso- per catchment of the Kalu Ganga is situated in nally over a distance of several kilometers the highest rainfall region of the country. The along the coastline (Swan, 1983). This move- headwaters of the Kalu Ganga originate at an ment is a direct result of the gradual growth elevation of about 2000 m and flow along nar- of a spit blocking the river mouth during the row strike valleys occasionally cutting through low discharge in the dry season. Due to low strike ridges forming a typical rectangular tidal range, river mouths in the island have a drainage pattern (Fig. 2). tendency to be constricted or completely The Bolgoda Ganga by contrast has a very blocked by the growth of sand bars across the low annual discharge which is mostly coming mouth obstructing the passage of water. The from the two small tributaries, the Panape Ele river mouth sand bars are found on both and Maha Oya. They directly discharge into banks indicating the frequent reversals of the the North and South Bolgoda Lakes respec- direction of the long-shore drift. The long- tively. These are small streams of local origin. shore drift also changes seasonally, where the The outer edge of the continental shelf off the Northerly drift is more prominent during the Bolgoda Ganga outfall is marked by a canyon.
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