State news agency Xinhua said the breach occurred on Friday afternoon in part of an embankment on the shore of Dongting Lake in central China’s Hunan province.
No casualties were immediately reported but the 226-metre (740-foot) breach forced nearly 6,000 people to evacuate from nearby areas, according to state broadcaster CCTV.
It published footage showing lake water flowing through a yawning gap in a dyke, inundating farmland and lapping at the rooftops of village homes.
The country’s road monitoring center said Saturday that traffic controls had been imposed on all roads leading in and out of Hunan’s Huarong county — home to around half a million people — “due to flood prevention needs”.
“During the control period, vehicles other than those assisting with flood prevention are prohibited from passing through,” the center said in a statement on social media platform Weibo.
President Xi Jinping “urged all-out rescue and relief work to safeguard people’s lives and property”, Xinhua reported Saturday.
The emergency management ministry dispatched over 800 people, nearly 150 vehicles, and dozens of boats to help block up the breach and assist with flood relief work, according to Xinhua.
The central government has earmarked an extra 540 million yuan ($74 million) in relief funding for Hunan and other disaster-hit areas, CCTV reported, citing the finance and emergency management ministries.
China is enduring a summer of extreme weather, with flooding across central and southern regions as much of the north swelters through rolling heat waves.
The Asian giant is the world’s largest emitter of the greenhouse gases scientists say drive global warming and make extreme weather more frequent and intense.
Nearly a quarter of a million people were evacuated this week in eastern China as rainstorms caused the Yangtze and other rivers to swell, according to state media.
Hunan was hit with deadly mountain floods and landslides last month, while deluges also left 38 dead in southern Guangdong province.
And the death toll from a tornado in eastern Shandong province on Friday rose from one to five, CCTV said Saturday, with dozens of others injured.