Three floodwater storage areas are under repair in north China's Inner Mongolia for the most severe Yellow River flood threat caused by ice blockage in 40 years as the spring thaw began on Tuesday.

The three storage areas in Erdos City, on the southern bank of the Yellow River, China's second longest, can divert 100 million cubic meters of ice and water, said Zheng Chunmao, Inner Mongolia Autonomous Region's top flood prevention official.

More than 100 residents living in the storage areas are ready to evacuate if the areas are commissioned after the repair of their embankments are completed in three days.

Sections of the river freeze and thaw at different times. When an ice run flows into a frozen section it can become blocked. If the blockage persists, water levels may rise and cause flooding and dam bursts, threatening lives and property. The ice-run phenomenon takes place at the start of winter and spring.

The river's ice flood was the heaviest in 40 years for Inner Mongolia. The river's watercourse in the region is currently holding some 1.8 billion cubic meters of water, 50 percent more than in a normal year. Low-quality dams in certain sections worsened the situation.

About 2 million out of 7 million people living near the river in the region are estimated to be threatened by dam bursts due to the thawing ice blockages.

More than 70,000 people in the region have been on standby for rescue and disaster relief efforts along the 720 kilometer frozen stretch of the river.

By Wednesday, around 30 km of the stretch had thawed.

The regional government has allocated 15 million yuan (US$2.1 million) to fund the flood prevention work.

"The ice blockage prevention of the Yellow River has entered a critical period," said Vice Minister of Water Resources E Jingping during an inspection tour to the region on Tuesday.

He demanded "relevant departments closely monitor the ice run situation and prepare enough materials" in case of a dam burst.

The section of the Yellow River in Inner Mongolia began to melt on Tuesday after a full thawing of the upper stretch in neighboring Ningxia Hui Autonomous Region.

On Monday, the water began to be diverted at 50 cubic meters per second to the Hetao area in Inner Mongolia to relieve floodwater pressure.

To help ensure a smooth thaw, the upper Liujiaxia Reservoir in Gansu Province has reduced sluicing water flow from 400 cubic meters per second to 240 cubic meters per second.

The Inner Mongolia stretch was forecast to be fully thawed by late March.

The 5,464-km-long Yellow River originates in Qinghai Province in the northwest and flows through Sichuan, Gansu, Ningxia, Inner Mongolia, Shanxi, Shaanxi, Henan and Shandong before emptying into the Bohai Sea.

(Xinhua News Agency March 13, 2008)