UPSTREAM IMPACTS ON LOWER MEKONG FLOODPLAINS: TONLE SAP CASE STUDY

UPSTREAM IMPACTS ON LOWER MEKONG FLOODPLAINS: TONLE SAP CASE STUDY MATTI KUMMU Laboratory of Water Resources, Helsinki University of Technology, Tietotie 1E, 02150 Espoo, Finland. E-mail: matti.kummu@iki.fi JORMA KOPONEN Environmental Impact Assessment Centre of Finland Ltd (EIA Ltd), Tekniikantie 21B, 02150 Espoo, Finland. JUHA SARKKULA Finnish Environment Institute (SYKE), Mechelininkatu 34a, 00251 Helsinki, Finland. Abstract The Tonle Sap Lake and floodplains in the heart of Cambodia contain the largest continuous areas of natural wetlands habitats remaining in the Mekong system, while being the largest permanent freshwater body in Southeast Asia. Tonle Sap is a crucially important source for food and living in Cambodia. More than one million people live in the immediate surroundings of the Tonle Sap Lake and wetlands, being the poorest ones in Cambodia, and highly dependent on agriculture and fisheries. An important phosphorus source of the mainly phosphorus limited aquatic ecosystem is the sediment carried by the flood. Mekong Upstream developments, like construction of dams may lead, and has already led, to significant trapping of sediments and nutrients. This would dramatically reduce the productivity of the Tonle Sap system. The 3D mathematical model has been set up for Tonle Sap Lake for simulating water levels and currents, inundation of the floodplain, suspended sediment transport and sedimentation, dissolved oxygen as well as nutrients and algal growth (chlorophyll-a) to understand the ecosystem processes and the possible changes in it due to the upstream development. Keywords: Mekong, Upstream impact, Tonle Sap Lake, impact assessment modelling, sediment INTRODUCTION The Mekong is one of the largest rivers in the world (Figure 1). Altogether, more than 70 million people live in the basin, of which 45 % in its lower part (Cambodia and Vietnam). Rapid economic growth, especially in countries such as China and Thailand, has increased the pressure to increase hydropower production and other development plans as well. These upstream developments may have significant affect to the downstream countries, as Cambodia and Vietnam. The Cambodian floodplains and Mekong Delta receive over 90% of the available water resources and 95% of the total suspended sediment flux from the upstream countries. Hence, this part of the basin is directly dependent on the conditions of the Mekong and any changes upstream would affect dramatically the living conditions there. In the Mekong basin, any significant changes in the flood regime and the suspended solid concentrations may have a noteworthy influence on the productivity of the Lower Mekong Basin (LMB). The trapping of sediment may also cause severe problems in the downstream countries due to the unbalanced sediment concentration, which increases bank and bed erosion. Moreover, the LMB countries (Thailand, Lao PDR, Cambodia, and Vietnam) may lose important floodplains and controlling the flow may disturb the ecosystem balance. The increasing dry- and decreasing wet-season flow can, however, be seen respectively as helping to overcome water shortages and contributing towards flood protection. 1 2 Figure 1: The Mekong Catchment presented with the constructed and projected dams in the mainstream and measurement stations. Source for the data: MRC [1]; source for the basin information: Meade [2], Milliman and Meade [3], MRC [4], and MRC [5]. Tonle Sap Lake and floodplains in Cambodia contain the largest wetlands habitats in Mekong, while being the largest freshwater body in Southeast Asia. Mekong Upstream developments, like construction of dams may lead to significant trapping of sediments and nutrients and reduce the productivity of the Tonle Sap system. The 3D mathematical model has been set up for Tonle Sap for simulating water levels and currents, inundation of the floodplain, suspended sediment transport and sedimentation, dissolved oxygen as well as nutrients and algal growth (chlorophyll-a) to understand the ecosystem processes and the possible changes in it due to the upstream development. The work is part of the MRCS/WUP-FIN Project, made for the Mekong River Commission by the Finnish Environment Institute in consortium with the Environmental Impact Assessment Centre of Finland and the Helsinki University of Technology. It is funded by the Development Cooperation Department of the Ministry for Foreign Affairs of Finland. 3 UPSTREAM DEVELOPMENT The Mekong is one of the largest rivers in the world. Altogether, more than 70 million people live in the basin, of which 45 % in its lower part (Cambodia and Vietnam). Rapid economic growth, especially in countries such as China and Thailand, has increased the pressure to increase hydropower production. Thus, China has already finished two hydropower dams for the Mekong mainstream and six more are under construction or projected [6-8]. Also, Thailand, Laos, and Vietnam have built and planned several dams on the Mekong tributaries [9-11], China is blasting rapids for a planned shipping channel [12]. An increasing number of irrigation structures in the tributaries and mainstream have been constructed and projected, by Thailand mainly, but also by Laos and Vietnam [13]. Probably the biggest threat for the Lower Mekong Basin is the Lancang1 cascade of dams in Yunnan, China. It is a huge project, including altogether eight hydropower dams, which takes the advantage of an 800 meter drop over a 750 kilometre river section in the middle and lower sections of the Yunnan stretch [8] (Figure 1). The hydropower potential in Yunnan is unquestioned, but there is a major concern about the negative impacts of the dams on the LMB [e.g. 14, 15-18]. The major worries are: • increasing flow fluctuations downstream, • increasing average downstream dry-season flows, • decreasing wet season flows, • decreasing flux of nutritious sediments crucial for fisheries and agriculture production, • geomorphological changes as bank erosion and bed degradation due to sediment trapping. • Proponents argue that, besides hydropower, the dams offer flood control, more assured dryseason flows, increasing navigation options, reduced saline intrusion and the creation of extra irrigation opportunities for downstream countries [8, 13]. The first dam in the Mekong mainstream was completed in 1993 in Manwan, China, while the second was completed in October 2003, in Dachaoshan (China), [6]. In total, China plans to build six more dams on the Mekong mainstream. The Xiaowan dam, the second biggest dam in China after the Three Gorges (dam height 292 m and reservoir length 169 km), is under construction since 2001. The total storage volume of the constructed and projected reservoirs in the eight-dam cascade would be over 40 km3, while the total yearly discharge from the UMB on the Chinese border is 73.6 km3. Hence, the reservoirs of the cascade would be able to store more than half the annual discharge of the UMB. UPSTREAM EFFECTS ON LOWER MEKONG BASIN The upstream development may have severe impacts on the downstream countries. The operation of the reservoirs leads to increasing dry season flows and decreasing flood peaks. Furthermore, it may shift the flooding period forward and increase the fluctuation of the water level in the LMB. The most sensitive areas for the flood level changes are the floodplains in Cambodia and Vietnam, as the Tonle Sap Lake and Mekong Delta. The sediment trapping due to the reservoirs and dams in the UMB may increase the bank and bed erosion in the LMB area and at the same time reduce the natural supply of nutrients, which again affects directly the biological productivity. Based on the MRC data set used in the analysis, the SS flux from the UMB at Chiang Saen was, before the Manwan dam, 68.5 million tons/year (MT/year), while it is now only 35.1 MT/year. If the whole cascade of dams were built in Yunnan, it would theoretically trap some 94-98 % of the suspended sediment load coming from China. In other words, only 2 to 5 million tons of SS would annually reach the LMB if the six under-construction or projected dams were finalized [19]. The suspended sediment coming from China is said to be very fertile [14, 17, 18]. However, the dams are trapping a large portion of the sediments and perhaps also nutrients and will anyway decrease biological productivity. Fish and other aquatic species are adapted to sediment-rich and turbid conditions of the Mekong; possible changes would interrupt the feeding and spawning conditions and perhaps lead to declining biodiversity and productivity [14]. The dams also block the natural biodiversity corridor of the river, which may seriously disturb fish, especially those that migrate, as well as the habitats of other 1 in China the Mekong is called Lancang 4 aquatic species. The reduced sediment and nutrient deposition will reduce the natural soil fertility and hence lower the yields in rice fields which may increase the adoption of artificial fertilizers, which would lower the economic viability of the area [14]. But again, our present knowledge permits only a general discussion of the effect, not quantitative estimation or analysis. TONLE SAP CASE STUDY The Tonle Sap Lake in Cambodia is the largest permanent freshwater body in Southeast Asia. Cambodian floodplains including Tonle Sap floodplains contain the largest wetland habitats in the Mekong system (Figure 2). The area is globally unique and the lake has an extraordinary hydrological system: in the wet season, the Tonle Sap River changes its direction and flows to the Tonle Sap Lake instead of from the lake because of the flooding of the Mekong River. The lake functions as a natural flood water reservoir for the Mekong system. The area of the lake varies between dry and wet seasons from 2500 km2 up to about 15,000 km2, while the depth increases from less than one meter up to 7–9 m [20]. The Tonle Sap ecosystem is believed to be one of the most productive inland waters and one of the most fish-abundant lakes in the world [21]. A dominant feature of the Tonle Sap system is that the sediment flux to the Tonle Sap Lake in the flood season (June-September) is manifold compared to the outflow flux in the dry season (OctoberMay). This means that the Tonle Sap Lake and floodplain ecosystem is retaining more than 80 % of the amount of the sediments it receives from the Mekong River and the tributaries and utilizing this material in the ecosystem processes. The Mekong River is responsible for the main part (ca. 70%) of the sediment load to Tonle Sap. The annual sediment flux to the lake is on average 7 million tons [20, 22]. Figure 2: Tonle Sap Lake. Dry season lake Dry season lake (~1.5 m above the mean sea level [a.m.s.l.]) is presented as dark blue while the flood plain of the year 2000 flood is presented as lighter blue (~10.3 m a.m.s.l.) [20]. It is hypothesized that sediments carried by the Mekong waters to the Tonle Sap Lake bring in the essential nutrients that feed into the lake’s food webs [20]. The higher the flood the more sediments and nutrients is brought in [23]. The Mekong Upstream developments like construction of dams have already led to significant trapping of sediments and nutrients [19] and may reduce the fertility of the Tonle Sap system. Also, any significant changes in the flood regime may have influence in the productivity of Tonle Sap system. The 3D mathematical model has been set up for the Tonle Sap Lake and its floodplains for simulating the hydrodynamics, inundation of the floodplain, suspended sediment transport and sedimentation to 5 understand the ecosystem and geomorphological processes, and the possible changes in it due to the upstream development. The model has been developed for the scenario simulations and impact assessments to assist in the maintenance of sustainable conditions in the lake and its surroundings [24]. The model was run with a “dam trapping” scenario, where the flood of 2000 was used but the sediment load from the Mekong halved. This can represent the effect of the regional developments utilizing Mekong water, such as extensive damming of tributaries and the mainstream, which may lead to excessive upstream trapping of sediments. It can be seen that the “dam trapping” scenario would mean a dramatic reduction in the net sedimentation of the Tonle Sap and consequently, in the supply of sedimentbound nutrients to its floodplain, which maintains its biological productivity (Figure 3) [24]. Figure 3: Comparison between 2000 calculated sediment concentrations and net sedimentation (left), and dam trapping scenario (right) [24]. CONCLUSION The major impacts of the trapping of sediment by reservoirs in the UMB on the LMB are increasing flow fluctuations, increasing average downstream dry-season flows, decreasing wet season flow, and decreasing the flux of sediments crucial for fisheries and agriculture production. Also, the disturbed sediment balance in the mainstream of the Mekong will increase the bed and bank erosion; this might have severe impacts on the LMB countries The Mekong Upstream developments, like construction of dams and reservoirs may lead to significant trapping of sediments and nutrients and reduce the fertility of the Tonle Sap system. An important phosphorus source of the mainly phosphorus limited aquatic ecosystem is the sediment carried by the flood. Any significant changes in the flood regime and the suspended solids concentration may have influence in the productivity of the lake Tonle Sap. With the help of mathematical models, it is possible to better understand the ecology of the Tonle Sap Lake and also predict the possible changes in the lake ecosystem due to the human impact and possible upstream developments. Most of the impact and causes of the upstream development are little known and poorly understood in the Lower Mekong Basin. More studies and research and above all open information flow are urgently needed in the area to permit quantitative estimations of effects such as decreasing biological productivity, as well as of impacts on fish habitat and bank erosion. REFERENCES [1] 1. MRC 2004. Databases of Mekong River Commission. Mekong River Commission Secretariat, [2] [3] [4] [5] Vientiane, Lao PDR. 2. Meade, R.H. 1996. River-sediment inputs to major deltas, in Sea-Level Rise and Coastal Subsidence: Causes, Consequences, and Strategies., J.D. Milliman and B.U. Haq, Editors. Kluwer Academic Publishing: Dordrecht. p. 63– 85. 3. Milliman, J.D. and Meade, R.H. 1983. World-wide delivery of river sediment to the oceans. Journal of Geology 91, 1–21. 4. MRC 1996. Lower Mekong Hydrologic Yearbook. Mekong River Commission Secretariat, Phnom Penh, Cambodia. 467 pages. 5. MRC 1998. Annual Report 1998. Mekong River Commission, Phnom Penh, Cambodia. 6 [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] 6. Dore, J. and Yu, X. 2004. Yunnan hydropower expansion - Update on China's energy industry reforms & the Nu, Lancang & Jinsha hydropower dams. Chiang Mai University's Unit for Social and Environmental Research & Green Watershed, Kunming, PR of ChinaAvailable online at http://www.rwesa.org/document/YunnanHydropower.pdf. 7. McCormack, G. 2001. Water margins: competing paradigms in China. Critical Asian Studies 33 (1), 5-30. 8. Plinston, D. and He, D. 1999. Water resources and hydropower. ADB TA-3139 Policies and Strategies for Sustainable Development of the Lancang River Basin. Asian Development Bank, Manila. 9. AMRC 2003. Hydropower Development in the Se San Watershed. Australian Mekong Resource Centre, Sydney. Available online at http://www.mekong.es.usyd.edu.au/case_studies/sesan/index.htm. 10. IRN 2002. Damming the Sesan River: Impacts in Cambodia and Vietnam. Briefing paper 4. International River Network, Berkeley. Available online at http://www.irn.org/programs/mekong/gmskit/04.sesan.pdf. 11. IRN 2002. The Nam Theun 2 Hydropower Project in Laos - Another World Bank Disaster in the Making. Briefing paper 5. International River Network, Berkeley. Available online at http://www.irn.org/programs/mekong/gmskit/05.namtheun2.pdf. 12. IRN 2002. Navigation Project Threatens Livelihoods, Ecosystem. Briefing paper 2. International River Network, Berkeley. Available online at http://www.irn.org/programs/mekong/gmskit/02.navfactshet.pdf. 13. Hori, H. 2000. The Mekong: Environment and Development. United Nations University Press. 398 pages. 14. Blake, D. 2001. Proposed Mekong dam scheme in China threatens millions in downstream countries. World Rivers Review, pp. 4-5. 15. Pearce, F. 2004. China drains life from Mekong river. New Scientist 182 (2441), 14. 16. Pearce, F. 2004. Where have all the fish gone? Independent.co.uk. 21 April 2004. Available online at http://news.independent.co.uk/world/asia/story.jsp?story=513488. 17. IRN 2002. China's Upper Mekong dams endanger millions downstream. Briefing paper 3. International Rivers Network, Berkeley. Available online at http://www.irn.org/programs/mekong/gmskit/03.uppermekongfac.pdf. 18. Roberts, T. 2001. Downstream ecological implications of China's Lancang Hydropower and Mekong Navigation project. Available online at http://www.irn.org/programs/lancang/. 19. Kummu, M., Sarkkula, J., and Varis, O. 2004. Sedimentation and Mekong Upstream Development: Impacts to the Lower Mekong Basin. in 2nd IAG Yangtze Fluvial Conference. Shanghai, China. 20. WUP-FIN 2003. Modelling Tonle Sap for Environmental Impact Assessment and Management Support. Water Utilization Program - Modelling of the Flow Regime and Water Quality of the Tonle Sap. MRCS / WUP-FIN Project. Mekong River Commission, Phnom Penh. 21. Bonheur, N. 2001. Tonle Sap Ecosystem and Value. Technical Coordination Unit for Tonle Sap, Ministry of Environment, Phnom Penh, Cambodia. Available online at www.mekoninfo.org. 22. Kummu, M., Koponen, J., and Sarkkula, J. 2005. Modelling sediment transportation in Tonle Sap Lake for Impact Assessment. in Proceedings of the 2005 International Conference on Simulation & Modeling, SimMod05. Bangkok, Thailand. 23. van Zalinge, N., Loeung, D., Pengbun, N., Sarkkula, J., and Koponen, J. 2003. Mekong flood levels and Tonle Sap fish catches. in Second International Symposium on the Management of Large Rivers for Fisheries. Phnom Penh, Cambodia, 11-14 February 2003. 24. Sarkkula, J., Kiirikki, M., Koponen, J., and Kummu, M. 2003. Ecosystem processes of the Tonle Sap Lake. in Ecotone II - 1 workshop. Phnom Penh/Siem Reap, Cambodia.