With a meandering length of around 4200km, the Mekong is the longest river in Southeast Asia, and some 50 million people depend on it for their livelihoods. In terms of fish biodiversity, the Mekong is second only to the  Amazon; but with regional energy needs ever spiralling, it is very tempting for a poor country like Cambodia to dam the river and make money from hydroelectric power. Even more tempting for Cambodia is the fact that the United Nations Development Programme (UNDP) and the Asia Development Bank (ADB) would pay much of the construction costs.

Overseeing development plans for the river is the Mekong River Com-mission (MRC), formed by the UNDP and comprising Cambodia, Thai­land, Laos and Vietnam. The odd one out is China, which has around 20% of the Mekong but feels it can do what it wants with the river. China has already completed the first dam project on the upper reaches of the Mekong, and many environmentalists fear that more projects will have an adverse effect down river.

China's dam projects are shrouded in secrecy, but there are thought to be 15 projects planned; one is operational now, another is likely to be up and running by the end of 2005 and another by 2009. Meanwhile, the MRC has plans for 11 dams for the Mekong in Laos and Cambodia.

Environmental concerns focus on a number of issues. For a start, even though the MRC dams planned for the Mekong will be small, it is thought they will flood some 1900 sq km and displace around 60,000 people. Secondly, there are worries about how the dams will affect fish migration - some environmentalists claim that the dams might halve the fish population of the Mekong and perhaps even Tonle Sap lake. Finally, and perhaps of most concern, is the importance of the annual monsoon flooding of the Mekong, which deposits nutrient-rich silt across vast tracts of land used for agriculture. Environmentalists say even a drop of 1m in Mekong water levels would result in around 2000 sq km less flood area around Tonle Sap lake, a result with potentially disastrous conse­quences for Cambodia's farmers. The opening of the first Chinese dam coupled with poor rains already saw the lake at one of its lowest levels in years during 2004, with fish stocks down by half. Imagine what happens when a dozen more dams come on line in a decade?

The Mekong is a huge untapped resource. It is probably inevitable that it will be harnessed to make much-needed power for the region. Local environmentalists hope that this can happen in the context of open dis­cussion and with foresight. Many fear, however, that long-term interests will be scrapped in favour of short-term profits.

Dams are planned for a few other rivers around Cambodia, although if you were to believe some of the ministry maps you would think the government was going to dam every bit of water in the country. The most realistic project is the damming of one of the great northeastern rivers that flows into the Mekong at Stung Treng. Tonle San, Tonle Sekong and Tonle Srepok contribute an estimated 10% to 20% of the Mekong's total flow at Kratie. However, the effects on the local indigenous population, Virachay National Park and fish stocks have not been considered in any detail yet - and perhaps never will be.