The possibility of flooding from the Tangjiashan "quake lake," caused by China's May 12 earthquake,increased Thursday even as water levels rose steadily to the point where engineers believe they may be able to open a drainage sluice.

The water level at Tangjiashan Lake in Beichuan County, Mianyang City, southwestern Sichuan Province, had risen to 738.81 meters by 5 p.m. on Thursday, 0.9 meters higher than that of 5 p.m. Wednesday. It was still 1.19 meters below the drainage sluice, according to the lake control headquarters.

The Sichuan Provincial Meteorology Bureau had forecasted thurderstorm for the next two days. The rainfall on Tangjiashan Lake was likely to reach 15 mm to 35 mm, according to the forecast.

Five rescue venues, each with one or two boats, were set up in Jiangyou City in the lower reaches of the lake. Traffic control has been imposed since Thursday afternoon in many townships of the city to keep people within the safe area.

Premier Wen Jiabao arrived in Mianyang Thursday afternoon to oversee drainage of the Tangjiashan Lake by helicopter.

"Now is a critical moment for the Tangjiashan quake lake, and the most important thing is to ensure there are no casualties," he said.

Emergency teams had been hoping to drain the lake on Thursday, but unforeseen factors, such as fresh landslides and higher than expected rainfall, had added to the risk of the lake breaching its banks and flooding downstream areas, said a spokesman with the lake control headquarters.