An Approach to the Origin of Boga Lake and Pukurpara Lake in the Hill Districts of Bangladesh
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Oriental Geographer Vol. 58, No. 2, 2014 (Published in December 2017) AN APPROACH TO THE ORIGIN OF BOGA LAKE AND PUKURPARA LAKE IN THE HILL DISTRICTS OF BANGLADESH Md. Shahidul Islam* Premanondo Debnath** Jean Marie Garnier*** Abstract: Boga lake and Pukurpara lake are naturally created hilly lakes located on hill tops in the hill districts of Bangladesh. The former is oval in shape and has a maximum water depth of 35 meter and the later is semicircular nut-shaped and has a maximum water depth of 28 meter. They preserve stratified sedimentary sequences, which are suitable for palaeo-environment and palaeo- climatic reconstruction. Using hand-operated piston corer, operating from a floating platform we collected 7 boreholes from Boga lake and 6 boreholes from Pukurpara lake upto 150 cm long. The bathymetry of the lake floors and terrine study shows that both lakes occupy the valleys of the Tertiary hills. Microfossil evidences (diatom) and C14 results suggest the varying rates and direction of monsoon intensity and its cycles of intense torrential rainfalls events during the late Holocene period. It has been argued that there were 13 phases of alternative weak and strong monsoon events during the last 1500 years. The intensified monsoon sequences have had tremendous impact on the terrain morphology and fluvial dynamics in the hilly regions of Bangladesh. The lakes on top of Tertiary hills are not volcanic in origin. Rather intensive monsoon events triggered by mega-scale earthquakes were the driving forces to massive landslide and mass- movement to closure the valley mouths, which initiated the formation of hilly lake basins in the Tertiary landscape and has later been top-up by torrential rain- fed water. Many of such lakes were small in size and have later been filled-up by excessive surface erosion and have turned into wetland ecosystem and marshy plain lands. However, Boga lake and Pukurpara lake, since their origin, till remain their original shape and form, and bear the characteristics of typical hilly lake environment. These two lakes are the examples to reveal how the origin of lakes in the hill districts of Bangladesh are associated with intensified palaeo-monsoon driven heavy rainfall in association with palaeo-seismic activities. C14 dates suggest that Boga lake is much younger (less than 500 yrs BP) than Pukurpara lake (estimated 2000 yrs BP). Keywords: Boga Lake, Pukurpara Lake, Palaeo-monsoon, Palaeo-earthquake, Palaeo- landslides * Md. Shahidul Islam, PhD, Professor, Department of Geography and Environment, University of Dhaka, Dhaka 1000, Bangladesh **Premanondo Debnath, Research Associate, Center for Environmental and Geographic Information Services (CEGIS), Dhaka 1212, Bangladesh ***Jean Marie Garnier, PhD, Professor, Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE), UMR CNRS 7730, AMU (Aix-Marseille Université). BP 80, 13545 Aix en Provence, France 32 ORIENTAL GEOGRAPHER INTRODUCTION A natural lake is localized basin surrounded by land area and is filled-up by water. Natural lakes are generally located in the hilly/mountainous regions or along the course of a mature river (Kuusisto and Hyvarinen, 2000). Lakes are varied in terms of their origin, size, shape, depth and water quality. Some lakes are only a few acres in size whereas other can be thousands of square kilometers. Their depth may vary from a few meters to more than thousand meters. Natural lakes can be formed by various earth system processes. However, some major natural causes are due to tectonic process, glacial activity, volcanic activity and landslides (Allen and Collison, 1986; Garcia-Castellanas, 2006). In the high latitude or tropical high altitude lakes are normally formed due to glacial blockage, and in tropical humid region due to massive landslides. Landslide damlake can be formed due to massive movement of rock, debris and mudflows in an area of narrow valley bordered by steep and rugged hills (Li and Zhu, 2001). Such lakes are commonly found in Hindu Kush Himalayas, where rainfall is high, earthquakes are common and slopes are steep. Bangladesh is primarily a low laying deltaic country formed by sediments carried by the mighty Ganges-Brahmaputra-Meghna river system. However, the hilly district of Bangladesh is located at the tipping point of four major sub-system of Asian monsoon. The monsoon climate system since its origin in the Neogene period, the region has witnessed significant variations in monsoon system. It is the key mechanism to geomorphological deformation of the hill topography of the country. The unique nature of the Tertiary hills and their evolution history at the margin of Indo-Burmese orogenic belt represent intense seismic activity. The tilting, warping and folding of the strata with anomalous drainage are the common indicators of deformation of the basin architecture. Among a variety of landscape in Tertiary landmass, two naturally created lakes- the Boga lake and Pukurpara lake are of immense beautiful treasures of the country. Despite significant progress has been made on the geological, seismic and hydro-metrological context of the region (Khan, 1991, Rashid, 1991, Banglapedia, 2006) there has been a very little progress to unveil the origin of these two lakes. It is the main aim of this paper is to study the origin of these two natural lakes in the context of regional earth system processes. AIM AND OBJECTIVES The main objective of the study is to reconstruct the history of origin of natural lakes, particularly the Boga lake and Pukurpara lake, in the hilly areas of Bangladesh. However, some specific objectives are; AN APPROACH TO THE ORIGIN OF BOGA LAKE AND PUKURPARA LAKE 33 i) to study the terrain of the Boga lake and Pukurpara lake, and their bottom topography, ii) to study the sedimentary sequence of these two lakes with the view to reconstruct the characteristics of monsoon during the geological past and iii) to establish the linkage between palaeo-earthquake and palaeo-monsoon to the origin of the lakes. STUDY AREA The study area is located in the hilly districts of Bangladesh where two natural lakes- Boga lake and Pukurpara lake are located (Figure 1). These lakes are not easily accessible due to undulating Tertiary hills topography and very poor communication system, Both the lakes bear the purity of nature, which bear significance of spiritual appeal to the visitors. There is very little human impact on the lake ecology, water is very stagnant and rain is the only source of water. Both the lakes bear the mythological stories on how they have been directing the cultural life settings of the different ethnic groups depending upon the lake environment. The present study is an attempt to understand the earth environmental system relating to the origin of natural lakes of hill regions based on the evidences derived from this two lake sites. i. Sample Site: Bogalake Bogakine lake (popularly known as Boga lake) is a natural lake in the hill area of Ruma Upazila, in Bandarban district. It is situated at 21o58'40"N to 21o58'57"N latitude and 92o27'55"E to 92o28'29"E longitude and is about 15 kilometers east of Ruma Upazila. The lake is partially parallelogram or oval in shape. It is bounded on three sides by undulating hills of upto 700m height, with an exception of plain land in the east. Situated on hill tops at an altitude of 372 meter the height of the lake is 165 meter from the nearest basement at Kamlapara. The slope of the hills is very steep in the east than that of other sides. The maximum water depth of the lake is 35 meter (Islam, 2005) and area is 15 acres (Debnath and Islam, 2015). The water of the lake is rain-fed and remains crystal clean. There are two villages, the Marmapara is located in the north of the down of the back slope and Bwam community lives in the south- eastern plain land. The lake is only accessible upto the basement by traditional vehicle (locally known as Chander gari) during the dry season, but during the monsoon only way to walk (7 hours from Ruma-the nearest town). 34 ORIENTAL GEOGRAPHER Figure 1: Location of the Study area. A. Bangladesh, B. Chittagong Hill Tract Districts, C. Boga lake and D. Pukurpara lake. AN APPROACH TO THE ORIGIN OF BOGA LAKE AND PUKURPARA LAKE 35 ii. Sample Site: Pukurpara Lake Raikkhyang lake, popularly known as Pukurpara lake, is located about 20 km (path distance) north-east of Boga lake at 22o01'12'N to 22o01'54'N latitude and 92o32'47"E to 92o33'15'E longitude in Belichhari Upazila of Rangamati District. It is very near to the Myanmar-India-Bangladesh border triangle. The only way to visit the lake is by walking (11 hours from Boga lake) through the undulating steep hills. The lake is located at 360 meter above the mean sea level, maximum water depth is 28 meter and the area is about 98 acres (Debanth and Islam, 2015). Pukurpara lake is semi-circular nut-shaped, has a length of 1.62 kilometer and occupies an area of 6 times bigger than that of Boga lake. The lake is surrounded by hills in all sides, except the southern side, which is the pain-land at lake level. The eastern hills are higher (500 m) than western hills (450m) and the lake topography appears as an elongated valley. The Raikkhyang khals flows along the eastern side of the lake parallel and is connected with the lake in the north with a sharp slope break of the cliff. The Tripura ethnic community live in the south-east and south-west side of the lake. Human intervention in the lake environment till now is negligible, and the water remains stagnant and very clean year round.