Workers and volunteers mobilised to urge people to leave the area after receiving an imminent heavy rain warning on Saturday afternoon, the emergency management bureau in Pengzhou city said.
People could be seen scrambling to flee in videos posted on social media but some were caught when the torrent hit about 50 minutes later.
One man at the scene said several people were washed away, including some children, when the water flow in the lower reaches of the river suddenly increased in just 10 to 20 seconds, the state-owned China National Radio reported.
The Chengdu city government said seven people had died and three others were hospitalised with minor injuries.
Pengzhou is a tourist spot about 70 kilometres north of Chengdu, the capital of Sichuan province.
A video showed a helicopter rescuing a person stranded on a small outcropping by descending to just above the water and opening a door so the person could climb in.
Elsewhere in China, heavy rain flooded streets in the northwestern city of Xining on Saturday night.
Heavy to torrential rain was forecast for the northeast from Sunday to Monday afternoon, with 10 to 18 centimetres of rainfall expected in parts of Liaoning and Jilin provinces.
A heatwave was hovering over a wide swathe of southern China, with forecast highs on Sunday of 35C to 39C and possibly surpassing 40C in some places including Shanghai.
8°C