The Xin'anjiang Reservoir opens three of its nine floodgates to discharge water at about 9:30 am on June 21, 2011, Hangzhou, Zhejiang province, at risk of overflowing.[Photo/Xinhua]

HANGZHOU - Authorities on Tuesday ordered the operators of major reservoir at risk of overflowing, amid torrential rains in East China, to start discharging water from it.

The Xin'anjiang Reservoir, built on the Qiantang River in 1959 with a maximum water-holding capacity of 21.6 billion cubic meters, opened three of its nine floodgates at about 9:30 am Tuesday, said officials with flood control and drought relief headquarters of Zhejiang province.

The water level of the reservoir had risen to 107.18 meters, or 0.68 meters above the warning line by Monday night. It may take 30 to 40 hours before the water level falls below 106.7 meters, officials said.

The operator of the reservoir, the largest in eastern China, had not been forced to discharge water from it since 1999.

Rain-triggered floods, the worst since 1955, have hit 10 cities of Zhejiang since last Saturday. More than 4.41 million local residents have been affected. Dikes near the city of Lanxi city were reported to have come close to overflowing due to surging water levels of the Lanjiang River, a tributary of the Qiantang River.

Rain-triggered floods have swept large swaths of East and South China since early this month, leaving 175 dead and 86 missing so far, the Ministry of Civil Affairs said Monday.