The once quiet gorges between Lincang Prefecture and the Dali Bai Autonomous Prefecture in Southwest China's Yunnan Province is clamorous with construction.

Workers there are busy building the Xiaowan Hydropower Station on the Lancang River which runs through the gorges.

While the project is China's second largest, smaller only to the Three Gorges project on the Yangtze River, Chinese experts said that the impacts of the 292-metre-high dam to the lower reaches of the river will be modest.

Originating in the Qinghai-Tibet Plateau, the river runs 4,880 kilometers through China, Laos, Thailand, Myanmar, Cambodia and Viet Nam. Lancang is the name for the river's 2,160-kilometre upper and middle reaches running through China while the lower reaches outside China is known as the Mekong.

Since 1986, China has begun to build eight hydroelectric dams and two reservoirs on the waterway in Yunnan, where the Lancang traverses for more than 1,000 kilometers, to lift the backward region out of poverty.

The first, at Manwan, was finished in 1993. The second, at Dachaoshan, began in August 1997 and will be completed next year.

Lastly, with a total installed generating capacity of 4.2 million kilowatts, the US$2.7 billion Xiaowan project is scheduled to be completed in 2012.

Ma Hongqi, chief engineer of the Yunnan Lancang River Hydropower Development Co Ltd, the major developer of these projects, said that Chinese scientists made a comprehensive analysis of the Xiaowan project's potential impacts on the lower reaches of the Mekong River before construction of the dam began this January.

"We concluded that the Xiaowan project will have limited impacts on the lower reaches of the river," said the engineer at the Chinese Academy of Engineering.

"Instead, the dam project will help with irrigation and navigation in the lower reaches," Ma said.

He said that construction of the Xiaowan dam will not decrease the total amount of water reaching the countries in the lower reaches, for it is not a water-diverting project between different drainage systems.

After completion, the Xiaowan Dam will create a 15 billion cubic-meter reservoir with an area of more than 190 square kilometers. The reservoir can ease water shortages in the lower Mekong during the dry season which usually lasts from November to May. On average, the water flow to the lower reaches will increase by 39.7 percent.

This will help improve the efficiency of flood diversion projects along the lower Mekong, Ma said. The increased flow of water will help prevent intrusions of saline water from the South China Sea, he said.

In June 2001, the Mekong River was opened for commercial navigation simultaneously in China, Laos, Myanmar and Thailand.

Thus far only small vessels with a weight below 300 tons can reach Laos from China.

When the Xiaowan Dam begins operation, the increased flow of water in the dry season will benefit navigation on the river.

According to the expert, the Xiaowan reservoir will have a 1.1 billion cubic-meter capacity for flood adjustment. During the flood season, it will store more flood water rather than discharging it, which will decrease flooding in the lower Mekong regions.

"But the impact is actually very limited," Ma said.

Only 13.47 percent of the Mekong's flow at its mouth in Viet Nam originates in the Lancang River, and monsoon rains in the lower Mekong contribute much more precipitation than that in the Lancang drainage area.

The reservoir will cool the water flowing downward from April to September, Ma said, as still water in a reservoir is cooler than running water in the summer and is warmer in the winter.

The cooler water can cause some negative impacts to the environment. "It's not good for the growth of crops and warm-water fish species," he said.

However, the largest drop in water temperature at Xiaowan, which occurs in June, is estimated to be only 2.7 C (the average temperature is 23.1 C). The cooler water will flow from the dam over 550 kilometers out of the country and run through the 230 kilometers that includes tropical gorges between Laos and Myanmar.

"When the water reaches irrigation projects in the lower reaches, its temperature can return to normal," he said. "So the construction of the Xiaowan Dam will have little negative impact to the agriculture in the lower Mekong."

According to data from the Kunming Animal Research Institute at the Chinese Academy of Sciences, there are 127 species of fish living in the drainage areas of the Lancang River. Among them, only four belong to migratory species.

They migrate from the lower Mekong to the Puyuan River, a tributary of the Lancang River in Xishuangbanna, a region down of the Xiaowan Dam site, to spawn. Therefore the dam will have no impact to migratory fish, Ma said.

Only warm-water fish species living in the middle reaches of the Lancang River in China may suffer in the cooler water.

As the Manwan, Dachaoshan and Xiaowan dams block the Mekong River, the Mekong River's sediment reaching its lower reaches will be decreased by about 10 percent as well, helping prevent soil erosion along the Mekong River.

But this may decrease the food supply for fish living at the mouth of the river, he said.

China has also launched a comprehensive environmental protection project in the Lancang River drainage area with the aim of preserving the river's purity.

The project began with a major afforestation drive that began in 1999. The goal of the ongoing effort is to increase the forest coverage rate in the area from 38 per cent at present, to 50 per cent in 20 years.

According to the engineer, the latest statistics from hydrological monitoring stations along the river show that the water quality of the Mekong River meets the international standard for drinking water, indicating an absence of water pollution in the river.

"We are building three sewage treatment plants at the dam site to purify sewage from the construction," he said.

"We have confidence the river will be clean."

The article is supported by the IPS-Rockefeller media fellowship program, ''Our Mekong: A Vision amid Globalization.''

(China Daily September 16, 2002)