2 The map of the Tonle Sap Lake area, showing the private fishing lot areas and the flooded area during exceptionally high-flood year of 2000 

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... In terms of hydropower development, more secure electricity production, increased water availability and predictability for agriculture, and income gained from the export of hydroelectricity represent obvious and often-stated examples of positive impacts, and ultimate reasons, for such projects. At the same time, however, there are also a variety of environmental, social, and economic impacts that the water development causes to water-related ecosystems, and consequently on livelihoods and industries dependent on them. Numerous impact assessment processes have been undertaken also in the Mekong River Basin by actors at various levels. A great majority of these assessments indicate that the planned water developments in the basin are likely to cause remarkable changes for the availability of water-related resources – most importantly fish – and, consequently, for the livelihoods and food security of millions of people (see, e.g., MRC 2006a, 2009c, 2010 ; IUCN and IWMI 2007a ; MRCS/WUP-FIN 2007 ; Dugan 2008 ) . Yet, the estimates about the actual magni- tude of such impacts remains varied, with different assessments providing widely differing estimates on the potential environmental, social, and economic impacts. Particularly basin-wide assessments have several challenges related to their comprehensiveness and overall reliability (see, e.g., Mirumachi and Nakayama 2007 ; MRCS/WUP-FIN 2007 ; Wyatt and Baird 2007 ; Keskinen 2008, 2010 ; Kummu and Sarkkula 2008 ) . Various kinds of computational models provide one way to simulate the potential changes in the river system due to different kinds of developments. 4 Models are generally used to improve understanding of cumulative and aggregate effects, to provide forecasts, and to help to quantify different scenarios. These in turn are help- ful for long-term planning of water resources development as well as for the assessment of water-related impacts. There has, however, also been active discussion about the challenges linked with the models and their results, related for instance to their transparency, reliability, and the possibilities for misuse (see e.g., Sarkkula et al. 2007 ; Käkönen and Hirsch 2009 ) . This chapter draws on the findings from the hydrological modeling and impact assessment work carried out in the Lower Mekong Modelling Project (WUP-FIN) under the Mekong River Commission (MRCS/WUP-FIN 2007 ) . The hydrological models of the WUP-FIN Project used the basin-wide scenarios developed within the Decision Support Framework (DSF) of the MRC as their starting point to simulate the changes in flow regime with foreseen hydropower developments in subbasin scale. In addition, environmental and socioeconomic analyses were carried out to understand better the consequent environmental, social, and economic impacts that such flow changes are likely to have (see MRCS/WUP-FIN 2007 ; Sarkkula et al. 2007 ) . However, as the scenarios used in the impact assessment of the WUP-FIN Project were developed already several years ago, even the most radical scenario, i.e., so-called High Development Scenario, 5 included only Chinese mainstream dams and some Lower Mekong Basin tributaries dams. Consequently, the estimates presented in this chapter can be considered to be relatively moderate, and the actual cumulative impacts of the current hydropower development plans – including several dams for the Lower Mekong mainstream – are likely to be much bigger in terms of changes in both water quantity and water quality. This section discusses the projected impacts of large-scale hydropower development in the Mekong Basin by presenting examples of the impact estimates on water quantity, water quality, and ecosystem productivity. By doing this, we aim to highlight two important issues. First of all, the existing estimates already in some, relatively simple water-related indicators such as water levels and sediments point toward remarkable potential changes due to hydropower development. Second, the examples illustrate that the actual impacts to systems as complex as floodplains or fisheries are much more difficult to estimate, since the impacts to these systems are felt through a combination of several impacts, both direct and indirect. In addition, due to the critical social and economic importance of the floodplains and the fisheries, the physical and ecological impacts need to be closely connected with broader social and political dimensions – a process that is still at a very early stage in the Mekong. Consequently, we hope that the findings presented in this chapter are useful also when studying and discussing the ongoing impact assessment processes and their results, including those within the MRC ( 2009b, c, 2010 ) . Most of the discussion on the potential impacts presented in this chapter focuses on the Tonle Sap Lake system that forms a particularly important economic, social, and environmental resource for the entire Mekong Basin and for Cambodia in particular (Fig. 6.2 ). Overall, the Tonle Sap Lake and the resources it supports form a central source of livelihoods and food for well over a million people living in the lake and its floodplains (Keskinen 2006 ; Keskinen et al. 2007 ) . The significance of the Tonle Sap extends, however, much further, as it is estimated that up to half of Cambodia’s population benefits from the lake’s resources (Bonheur 2001 ) . The Tonle Sap is known for its extraordinary flood pulse system 6 with a remarkable but nevertheless rather regular seasonal variation in the lake’s water volume and level (Lamberts 2006 ; MRCS/WUP-FIN 2007 ) . The main driver of the flood pulse system is the Mekong River and its floods: during the wet season the water level in the Mekong mainstream rises faster than the water level in the lake. As a result, part of the floodwaters run to the Tonle Sap River, causing the river to reverse its flow back toward the Tonle Sap Lake. The lake thus loses its only outlet, and the floodwaters extend to large floodplain areas surrounding the lake. An exceptional and highly productive floodplain ecosystem has been formed based on this flood pulse system, and the Tonle Sap is considered to be among the world’s most productive freshwater ecosystems and fishing grounds (Rainboth 1996 ; Lamberts 2001, 2006 ) . This productivity is epitomized by the immense fish catches of the Tonle Sap Lake and the Tonle Sap River. Taken together, the unusual flood pulse system and immense aquatic production of the Tonle Sap make it perhaps the single most vulnerable area to major changes in water quantity and quality of the Mekong River (see, e.g., Lamberts 2008 ; Kummu and Sarkkula 2008 ) . The Tonle Sap is also exceptional for a lake of its size, as due to its exceptional flood pulse system, the impacts of any environmental change are felt as a combination of changes in its own basin and that of the Mekong River. The actual “impact basin” of the Tonle Sap Lake is thus not merely the lake basin (86,000 km 2 ), but the entire Mekong River Basin upstream from the Tonle Sap (680,000 km 2 ). This, naturally, makes the assessment of potential impacts to the area a particular challenge – and at the same time very much a regional issue as well. Different Cumulative Impact Assessment (CIA) studies have looked at the impacts of the planned hydropower development to water quantities of the Mekong (Adamson 2001 ; ADB 2004 ; DHI 2004 ; World Bank 2004 ) – and more such studies are currently being carried out (see, e.g., MRC 2009b, c, 2010 ) . The estimates of these assessments are, however, relatively inconsistent due to the different assumptions used and the differences in the models and assessment tools themselves (Keskinen 2008 ; Kummu and Sarkkula 2008 ) . The three earlier CIAs discussed here (Adamson 2001 ; ADB 2004 ; World Bank 2004 ) indicate that planned development in the upper parts of the Mekong Basin will alter the water levels downstream and, consequently, in the floodplains. The dry season water levels are subject to rise and flood season water levels to decrease. Such changes would mean that the future flood amplitude will be smaller, leading to decreased extent of the floodplains and, consequently, to less potential spawning habitats to fish and other aquatic animals. Further, due to the smaller flood amplitude, less water will also enter to the floodplains from the mainstream. The floodplain ecosystems need both the dry and the wet periods, and the increased low water levels would therefore permanently change the floodplain ecosystems. In the case of the Tonle Sap, the analysis of the dry season water level rise due to Mekong upstream development has in the different CIA studies been estimated as follows: • 0.15 m increase: Estimate based on the MRC’s basin-wide CIA under the Integrated Basin Flow Management (IBFM) process using the MRC’s Decision Support Framework modeling tools (World Bank 2004 ) • 0.30 m increase: Estimate based on the analyses of the downstream hydrological impact of the Chinese cascade of dams (DHI 2004 ; Adamson 2001 ) • 0.60 m increase: Estimate based on the basin-wide CIA conducted within the Nam Theun 2 environmental impact assessment study using MIKE Basin model (ADB 2004 ) The impact of the estimated water level rises for the dry season area of the Tonle Sap Lake is presented in Fig. 6.3 . 7 The estimated rise of 0.60 m in dry season water level, as simulated by ADB ( 2004 ) , would result in the permanently inundated area of 3,200 km 2 . This would lead to the increase of the permanent lake area by nearly 1,000 km 2 (40%) when compared to from the current situation 8 (Kummu and Sarkkula 2008 ) . This kind of a rise in the lake’s dry season water level, and the consequent exten- sion of the permanent lake, would result in varied impacts to the Tonle Sap and its ecosystem. Some of these impacts would be largely positive, including improved navigation possibilities due to higher water levels. The most ...