Many regions in southern China remain on high alert for thunderstorms, heavy rain and flooding. The affected area stretches from Tibet in the west to Shanghai in the east and incorporates almost the entire basin of China's longest river, the Yangtze. Such are the levels of rainfall that the river's middle and lower reaches are now coming under severe pressure. This is having an impact on the structure of its dams, including the world's largest, the Three Gorges Reservoir.