All navigation in the Three Gorges section of the river will be halted on April 10 for 67 days. At the end of that period, on June 15, the reservoir will have been filled to 135 metres above sea level, the permanent five-step shiplock allowing boats to get through the dam will be put into operation, and shipping will resume.

Once the reservoir is filled, the backwater will reach as far as Fengdu county in Chongqing municipality, 430 km upstream of the dam. Officials are concerned that navigation between Fengdu and Yichang, the site of the dam in Hubei province, will be difficult "and dangerous" after June 1.

"Filling the reservoir will pose major problems and even significant threats to shipping on the river," the news agency said in its March 24 report. "With the rising of the water, mountain slopes, cliffs, rocks and the mouths of Yangtze tributaries will be inundated. This could lead to new submerged reefs and dangerous shoals, and undercurrents in some sections of the river. Passenger boats, cruise ships and freighters will face the unprecedented threat of such undercurrents downstream of Fengdu."

Furthermore, Yangtze officials will have no chance to readjust the main shipping channel before navigation resumes, the news agency said, warning of "a high probability of accidents in the river section below Fengdu, with ships hitting rocks, running aground and colliding."

The Chongqing maritime affairs bureau is working out an emergency rescue plan that will include monitoring stations set up at every port along the river from Fengdu to Wuhan, and lifeboats to patrol the stretch between Fengdu and Wushan, the news agency said.

Officials in the Wuhan-based Changjiang [Yangtze] Channel Administration Bureau told the Changjiang Daily (Changjiang ribao) that they face two major problems. First, it will be extremely difficult to design a new channel and reposition all the navigation marks as the water rises as much as 70 metres from its current level during the filling of the reservoir.

Second, navigating the river below the dam could also become much more difficult after the water is impounded in the reservoir. "While the reservoir is being filled, the runoff in the river below the dam will fall as low as 3,410 cubic metres per second. This is not good news for shipping,"the newspaper said. (In a normal year, the runoff would be about 18,000 cubic metres per second.)

"If the decline in the runoff below the dam is so steep and so rapid, there would be no way to flush away sediment, and siltation-induced shoals could form and impede river traffic in a dramatic way. Thus, it could be very hard to maintain "business as usual" even on the river below the dam," the newspaper said.