Record high water levels are putting the capacity of China's massive Three Gorges dam to the test after heavy rains across the country, compounding flooding problems that have left more than 1,200 people dead or missing.

The dam's water flow reached 56,000 cubic metres per second (1.96 million cubic feet), the biggest peak flow this year, with the water height reaching 158 metres (518 feet), the official Xinhua news agency reported. This is about 10% 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.

Floods this year have killed at least 823 people, with 437 missing, and have caused damage worth tens of billions of dollars, according to the state flood control agency. More heavy rains are expected for the south-east, south-west and north-east parts of the country.

Thousands of workers sandbagged riverbanks and checked reservoirs in Wuhan city in central Hubei province in preparation for potential floods expected to flow from the swollen Yangtze and Han rivers, an official with the Yangtze water resources commission said. "Right now the Han river in Hubei province is on the verge of breaching warning levels," said the official, who gave his name as Zhang.

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

Though China experiences heavy rain every summer, flooding this year is the worst in more than a decade, as the flood-prone Yangtze basin has seen 15% more rain than in an average year, Duan Yihong, director of the National Meteorological Centre, said in a transcript of an interview 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 after a bridge collapsed from heaving flooding in the Yi river over the weekend, killing 37 people with 29 missing, Xinhua reported.

In the southern province of Sichuan rescuers searched for 21 missing after rain triggered a landslide that buried 58 homes.