Yellow algae emerged in a major river in central China after days of lower than normal water levels due to less rainfall recently and affected water supply in at least two cities.

The large amount of algae began to emerge in filtering ponds of three plants of the state-owned Zhonghuan Water Company. The build up led to frequent blockages in the ponds, according to the Xiangtan City-based company in Hunan Province.

Usually, the filtering ponds were washed every 24 to 36 hours. At present, they had to be washed each two hours in the most serious situation, said Yuan Cheng, Zhonghuan Water deputy general manger.

Large areas of algae could be seen in the section of the Xiangjiang River passing through Xiangtan.

Yuan said the city was experiencing water shortage and its urban Hedong District only had a supply volume half its usual amount. Apartments in high-rise buildings were suffering from low pumping pressure.

Hunan's main Xiangjiang River saw its water level hit a record low 26.82 meters at 8 a.m. on Friday in Xiangtan, 0.04 meter below the previous low.

The combination of the low water level, its high content of ammonia and nitrogen, and the high temperatures of recent days had contributed to the algae outbreak, according to experts.

After the summer flooding season, the river drainage area only had precipitation of 16 mm, 64 percent less than the average in the same period in recent years, according to the provincial hydrological resource bureau.

Algae was also affecting water supply in Zhuzhou City downstream. Monitoring by local environmental authorities showed the amount of algae was 10 times greater than usual, affecting the city's water clarifying plants in different degrees.

The plants had increased the amount of chlorine put into the water to clean it before the filtering process, a Zhuzhou Tap Water Company official said.

Polluting factories along the river were being closed in such cities as Changsha, the provincial capital, to reduce pollution.

Two hydroelectric facilities, one upstream and the other midstream, began to discharge more water downstream at 8 a.m. on Friday, according the Hunan Provincial Flood Control and Drought Relief Headquarters.

(Xinhua News Agency October 25, 2008)