Landslides caused by Monday's quake had blocked the Qingzhu River in Qingchuan county, creating two lakes with fast-rising water levels, an official at the Guangyuan disaster relief center said. Officials, worried that one or both of the lakes could burst, forced the evacuation of 10 villages and townships downriver.
Journalist Diego Laje, one of those ordered to evacuate, said a stampede of thousands of people began as military officers warned them to leave a low-lying area near the river.
The official at the relief center said the one lake has an estimated 11 million cubic meters of water and the other has 1.5 million cubic meters.
Chinese government experts were considering blasting holes to allow controlled releases of the water, which was quickly rising, the official said.
A strong magnitude 6.0 aftershock shook the region near the border of Sichuan and Gansu provinces on Sunday, the U.S. Geological Survey said in a preliminary report.
There was no immediate information on the impact of the aftershock.
Thousands of aftershocks have plagued the area since Monday's primary earthquake.
Elsewhere in Sichuan province, rescuers continued to search for more survivors, buoyed by news of the rescue of two men more than 110 hours after the quake.
A German tourist was pulled out alive from under rubble in Taoguan village Saturday, 114 hours after being buried, China's official news agency Xinhua reported. The man's identity remained unknown, Xinhua reported.
Xinhua also reported that soldiers dug out a 52-year-old man who was buried in ruins for 117 hours.
Xinhua said Saturday that the man had been buried under a collapsed building in Beichuan, not far from the epicenter of Monday's earthquake.
A German Foreign Ministry official said that country's government had no information about the German man or the rescue. The Xinhua report said more than 20 Chinese soldiers helped rescue the tourist Saturday morning.
The man was taken to a nearby medical center, but his condition was not immediately clear, Xinhua reported.
The discovery of the tourist added urgency to China's massive rescue operations.
"Saving lives is still the top priority," Chinese President Hu Jintao said Friday, as he arrived in the quake zone. Hu described the current situation as "the most crucial phase" of the rescue effort. "We must race against time to overcome all difficulties."