Floods caused by heavy rains in north-eastern China stranded tens of thousands of residents without power Wednesday, as the worst flooding in more than a decade continued to besiege areas of the country.

Floods this year have killed at least 823 people with 437 missing and have caused tens of billions of dollars in damage, the State Flood Control and Drought Prevention reported. More heavy rains are expected for the southeast, southwest and northeast parts of the country through Thursday.

About 30,000 residents in Kouqian town were trapped after torrential rains drenched the northeastern province of Jilin on Wednesday, the official Xinhua News Agency reported. Water began flooding the town after the nearby Xingshan Reservoir and the Wende and Songhua rivers overflowed.

Flooding has hit areas all over China. In Wuhan city in central Hubei province thousands of workers sandbagged riverbanks and checked reservoirs in preparation for potential floods expected to flow from the swollen Yangtze and Han rivers, an official with the Yangtze Water Resources Commission said Wednesday. He was surnamed Zhang but refused to give his full name, as is common with Chinese officials.

“Right now, the Han river in Hubei province is on the verge breaching warning levels,” Mr. Zhang said.

The Han is expected to rise this week to its highest level in two decades, Xinhua reported.

Though China experiences heavy rains every summer, flooding this year is the worst in more than a decade, as the flood—prone Yangtze River Basin has seen 15 percent more rain than in an average year, Duan Yihong, director of the National Meteorological Centre, said in a transcript of an interview on Wednesday posted on the Xinhua website.

“Rains should begin to slow down in August, but it is hard to predict now what exactly will happen, said Duan. “We have to be vigilant and closely monitor the weather ... do a better job of forecasting.”

Thousands of rescuers in central China’s Henan province searched for survivors on Wednesday after a bridge collapsed from heaving flooding in the Yi River over the weekend, killing 37 people with 29 missing, Xinhua reported.

Floods have also put China’s massive Three Gorges Dam to the test. On Wednesday morning, the dam’s water flow reached 1.96 million cubic feet per second (56,000 cubic meters), the biggest peak flow this year with the water level reaching 518 feet (158 meters), Xinhua said, about 10 percent less than the dam’s maximum capacity.

Chinese officials for years have boasted the dam could withstand floods so severe they come only once every 10,000 years. The dam is the world’s largest hydroelectric project and was also built to end centuries of floods along the Yangtze River basin.

More In: International | News