The water in the Three Gorges reservoir is becoming cleaner as the reservoir fills, say local authorities tasked with monitoring the quality of the Yangtze River water as it flows into the huge dam.

Zang Xiaoping, director of water quality monitoring for the Yangtze River valley, said that a two-month-long study showed that by Sunday afternoon, the water in the 150 kilometer area between the Three Gorges dam and Wushan Mountain was cleaner than before.

The water quality had been unaffected by the flooding of the reservoir, and acidity and alkalinity showed little difference from records of past decades.

Zang said that the reservoir had begun to fill naturally before the official flooding began and mud and sands in the water had gradually settled, cleaning the water.

There will be 18 staff at the center monitoring water quality until mid-July.

The water level in the reservoir will rise by four to five meters daily and is expected to reach a height of 135 meters by June 15.

Construction of the Three Gorges project, the largest in the world, began in 1993, with an estimated total investment of 180 billion yuan (US$21.8 billion).

The water level is expected to reach a height of 175 meters by 2009 when the whole project is completed, with the reservoir's storage capacity planned at 39.3 billion cubic meters.

By 2009, 26 power-generating units with a combined capacity of 18.2 million kilowatts will be operational.