The worst flooding in six decades in China is displacing millions and straining the Three Gorges Dam. WSJ’s Jonathan Cheng explains how the natural disaster is hampering the country’s efforts to restart an economy already battered by the coronavirus pandemic. Photo: Hector Retamal/Agence France-Presse/Getty Images BEIJING—Two months of unusually powerful rains in central and southern China have swelled the Yangtze River, triggering the worst flooding in decades and raising concerns about the Three Gorges Dam, the world’s largest hydroelectric facility. Last week, several days of fresh rain brought the water level at the dam, located several hundred miles upstream from Wuhan in central Hubei province, to its highest point since construction was completed in 2009. …