Brown water, clumps of trees and occasional vehicles swamped low-lying areas of the town, washing away remains of corpses, family mementoes and valuables left under the rubble.
Wang Guiru, 43, whose wife died in the quake alongside his father and mother-in-law, said he had hoped eventually to look for their remains.
"Now I guess we can never go back," he said stoically. "This is fate. We have to learn to face up to realities."
Further downstream near Mianyang, the river had turned into a torrent several hundred metres across, bursting banks in places.
"I never thought it would rise so quickly," said Fu Anyun, who stood watching with her extended family of about 12 in a ridge above the river.
"We've been waiting and waiting, but when it finally came it was still a shock. Now all we can do is watch and see if our homes our flooded or not."
The water level at the Tangjiashan quake lake, formed by China's most devastating earthquake in decades, dropped by 13 metres (43 ft) on Tuesday, Xinhua news agency said.
The Tangjiashan lake, the largest of the more than 30 created when landslides triggered by the quake blocked the flow of rivers, has so far prompted the evacuation of more than 250,000 residents downstream in case the mud-and-rock dam bursts.
SOLDIERS EVACUATED
Helicopters were evacuating remaining soldiers and experts from the top of the dam, which was no longer safe with cracks appearing as the flow of water accelerated by more than tenfold, Xinhua said.
It attributed the abrupt increase in water discharged from the lake to "two massive blasts on Monday evening which broke through the bottleneck" in a sluice opened by soldiers.
The torrents had further widened and lowered the sluice, Xinhua said.
"The flow downstream has increased dramatically, but the dam hasn't collapsed," Zhou Hua, spokesman for the lake relief operation, told Reuters. "The channel has widened but this isn't a collapse.
"... So far everything is happening within expectations. As things are, we don't expect to have to evacuate any more."
Rescuers found a relief helicopter, carrying 19 quake survivors, medical workers and crew, that crashed near the epicentre of the quake on May 31. There were no survivors.
Mianyang hosts tens of thousands of quake refugees from Beichuan, where more than 15,000 people died in the May 12 tremor. Road blocks kept traffic away from nearby Jiangyou, but it was not clear if the whole town was cut off.
The flooding brought more heartache to people displaced by the earthquake in which nearly 87,000 people either died or are missing. Many said valuables were now lost for good.
"It began flooding early this morning," said shop assistant Zhu Yunhui, 37, who lost loved ones in the quake and said she had kept many tens of thousands of yuan in her home. "Now we can never go back. This is heartbreaking."
