CHENGDU, China — Rain grounded helicopters and added urgency to attempts on Thursday to drain an earthquake-created lake that was threatening to flood disaster victims in central China.

The new Tangjiashan lake above the city of Beichuan has been rising by several yards a day after being formed by a landslide triggered by the magnitude 7.9 earthquake that struck May 12.

Some 158,000 people have been evacuated downstream from the new lake and officials have pledged to warn other residents nearby in case of flooding so they have time to flee.

With roads to the area cut off, helicopters have airlifted heavy equipment there to dig drainage channels. Heavy rain prevented aircraft from flying Wednesday, state TV reported, but workers were continuing efforts to clear debris despite the weather.

In the past three days of round-the-clock work, troops have dug a 50-yard wide channel running 300 yards long, CCTV said.

Rain was forecast to continue in the area through Thursday before subsiding later in the day, according to the China Meteorological Bureau.

The death toll from the quake is expected to rise above 80,000. The confirmed number of dead was 68,516, the Cabinet said Thursday.

Many of the dead were children, although the exact number was not known. Parents have become increasingly vocal about the poor quality of the construction of schools destroyed in the earthquake.

Dozens of parents gathered Thursday outside a destroyed primary school in the town of Wufu and said they would take their complaints to higher officials if the local authorities failed to find who was responsible for the building's collapse.

"This building definitely had quality problems," said Yang Mantang, 40, whose 12-year-old son died. "If it were like the buildings left behind, my son and other kids wouldn't have died. They just shouldn't have died."

Meanwhile, a former Chinese professor who says he was detained for 10 days for articles he wrote criticizing the government's response to the earthquake has been freed.

Guo Quan, who was released Wednesday, is the first known case of someone being detained for quake-related criticism. Other detentions reported by state media have been of people accused of spreading rumors of future quakes.

The criticism came as China enjoyed rare praise for the relative openness of its earthquake response and media coverage of the disaster.

The articles by Guo said the Chinese government ignored warning signs before the quake, and that officials should have immediately responded to the danger of lakes that now threaten to burst. He also questioned the safety of nuclear facilities in the area.

At least one of his articles was published by The Epoch Times, a U.S.-based newspaper linked to the banned Chinese sect Falun Gong.

Guo, reached by phone Thursday in the central city of Nanjing, said police told him his detention was mainly for his quake-related articles.

Calls to the media office of the Public Security Bureau in Nanjing rang unanswered Thursday.

Guo, an expert on Chinese literature, lost his teaching duties at Nanjing Normal University after founding the China New Democracy Party last year.

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Associated Press writer Cara Anna contributed to this report.