Tonlé Sap Ecosystem is threatened by Development of Hydropower Dams on the Mekong River - Future Directions International

Background

Every year, between May and October, floodwaters from the Lower Mekong Basin reverse the flow of the Tonlé River in Cambodia. That pushes sediment-rich waters into the Tonlé Sap Lake and its surrounding floodplains and forests, which support complex food webs, migration patterns and agricultural production. The process, known as the ‘Tonlé Sap pulsing system’, provides the region with the nutrients that sustain the largest remaining area of natural wetlands connected to the Mekong River. It supplies the food resources needed to support the three million people that live around the lake and in its surrounding area.

During the wet season, Tonlé Sap expands to 60 times its normal size, to become the largest fresh-water lake in South-East Asia. There are concerns, however, that this phenomenon will not occur this year, as the regional ecosystem has come increasingly under threat from dry conditions and also upstream hydropower dams that affect sediment flow and water levels.

Comment

The energy sourced from hydropower infrastructure is integral to the development of both urban and rural areas in the Mekong Basin. Since 2000, direct investment, mainly from Chinese companies, has fueled the rapid expansion of hydropower projects in Laos and Cambodia. Rising energy prices have also encouraged China, Thailand and Vietnam to expand their own infrastructure along the Mekong.

Currently, plans exist for the construction of eleven dams on the Lower Mekong. The problem is that estimates indicate that they will trap 96 per cent of the current sediment flows. The complex food web in the area is sustained by the annual sediment distribution from the receding water levels, which fertilise the floodplain forests and agricultural land. Sediment concentration and composition also determines the degree of light penetration in the river, which regulates algal growth, the amount of nutrients in the water and the availability of oxygen. The nutrient and sediment deposits in the floodplains and forests also support the spawning and breeding of fish and amphibians, while flooding carries fish eggs and larvae.

The mainstream dams on the Mekong and along Cambodia’s tributary rivers have already reduced connectivity to Tonlé Sap by 31 per cent. If development continues, it is expected that connectivity will decline by at least another 30 per cent. The developers of upstream hydropower dams have proposed releasing water during the dry season, but that would prevent the Tonlé Sap from draining out, thus affecting sediment distribution. If less water is released during the wet season to control the harmful effects of monsoon floods, then a loss of habitat in the floodplains and forests will harm biodiversity. The flood waters also carry decomposing organic matter into the lake, which provides the most important source of nutrients for the aquatic species that live in the lake and underpin the food web.

Changes in water levels would harm the highly productive fishing industry. Presently, the rapid flooding of the river increases fish stocks, while the rate at which the river recedes allows fishers to catch the fish with ease. The Tonlé Sap fishing industry has an annual catch of 455,000 tonnes of fish (out of a total of 2.4 million tonnes in the Mekong Basin), contributing to Cambodia’s overall fish harvest valued at US$600 million ($880 million). The preservation of fish stocks is dependent on the annual floods that reverse the flow of the river; studies have proven that high flood levels result in higher annual fish catches in the lake. As 90 per cent of the lake’s population relies on agriculture or fishing for their livelihood, a reduction in harvests would force families to migrate to urban areas in search of employment.

The Mekong River Commission, compromising representatives from Laos, Thailand, Vietnam, and Cambodia, released a study on the sustainable development of the Mekong River with a focus on mainstream hydropower projects. The report found that hydropower development would eliminate migratory fish in large parts of the river by 2040 and reduce fish biomass by 35-40 per cent by 2020. Vietnam was the only member to endorse the report, demonstrating the lack of political concern for the developing crisis facing the region’s biodiversity and food security.

The inability of countries along the Mekong to agree on a regional water management plan, has allowed the continued exploitation of their segment of the river. Safeguards are needed to both manage the effects of hydropower dams on downstream countries and to prevent further damage to the region. Investing in alternative energy sources would reduce the environmental and socio-economic costs that hydropower is likely to impose on the Mekong and its tributaries, while still meeting the demand needed for development. If Cambodia is to maintain the Tonlé Sap pulse it must invest in regional partnerships and policy to protect the fragile ecosystem.

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