When the reservoir is filled to its final height in 2009, will it be a flat body of water, measuring 175 metres above sea level all the way from the dam upriver to its tail 660 kilometres away near the major city of Chongqing?

Or will the reservoir still possess some of the slope that is characteristic of the Yangtze River in its natural state as it flows down from the Himalayas to the East China Sea, and be significantly higher than 175 metres above sea level by the time its backwater approaches Chongqing?

If so, many unsuspecting people living above the official "red line" for resettlement, who believed themselves to be in a safe zone, would in fact be put in danger by the rising reservoir in 2009. Many more houses and factories than expected would be submerged, and more farmland and infrastructure would be lost.

One contributor to the discussion on the bulletin board of the Changjiang Water Resources Commission (CWRC) site summed up the perplexing nature of the debate: "Experts’ opinions on the slope issue are so divided that we ordinary people have become confused. We don’t know who we should listen to, since all of them are well trained, knowledgeable and even powerful."

Zigui: 135 metres [Click on image to enlarge]

Currently, the height of the reservoir does increase with distance from the dam. Before the reservoir began storing water in June, this anticipated difference in water-surface elevation was clearly indicated on signs demarcating the initial "red line" for resettlement. For example, signs at Zigui, 30 kilometres upstream of the dam, read 135 metres [pictured above]; at Fengjie [see right], 165 kilometres upstream, 146.7 metres; at Yunyang, 225 km upstream, 148.4 metres [pictured below]; at Wanzhou, 300 km upstream, 150 metres.

Fengjie: 146.7 metres [Click on image to enlarge]

But all along the reservoir, the signs higher up the slopes suggest an unvarying final water level of 175 metres. And the participants in the CWRC bulletin-board discussion are not alone in wondering: Will this truly be the height of the entire reservoir after it is filled to its planned "normal pool level" in 2009?

Yunyang: 148.4 metres [Click on image to enlarge]

Confusion on the issue was already apparent in the late 1980s, when 14 teams of Chinese experts conducted a feasibility study examining different aspects of the Three Gorges project. They arrived at the same conclusion, that the dam should be built, but their arguments were based on different assumptions on the slope issue.

The resettlement team assumed the reservoir would be flat, that its final height would be the same for its entire length – a premise that helped to keep the resettlement figures and budget down. And thus a straight "red line" for resettlement was drawn, with signs posted the length of the reservoir indicating a final water level of 175 metres.

But two of the other expert groups, which looked at flood control and sedimentation, assumed the reservoir would have a hydraulic slope with a gradient of 0.007 per cent (a water-level difference of seven metres every 100 kilometres). This assumption helped these teams reach their confident conclusions about the reservoir’s flood-storage capacity (22.1 billion cubic metres) and ability to flush out sediment.

As the water began to rise behind the dam in June, leading Chinese water engineer Zhang Guangdou clearly distanced himself from the "flat reservoir" school of thought. He told the Xinhua news agency that the reservoir will not be as flat as some may have imagined. The depth of the reservoir will determine the size of the slope, he said, with the slope decreasing as the reservoir deepens. But even when the reservoir is filled to 175 metres at the dam site, there will still be a slope, though it will be smaller than at present, the June 4 Xinhua report quoted Prof. Zhang as saying.

On the same day, Pan Jiazheng, former vice-director of the Chinese Academy of Engineering and a chief designer of the Three Gorges dam, was quoted elsewhere as saying that the water level at the tail of the reservoir when it is filled to its final level might be "a little bit higher" than 175 metres because of "a slight slope."

But responding to an unnamed critic who has suggested the upstream reservoir level could be much higher than is being planned for, Mr. Pan had this to say: "It is absolutely wrong that the water level would be as high as 221 metres at the tail when the reservoir is filled to 175 metres because of a ‘water-surface slope.’ I won’t even bother to say more about this. The ridiculous claim was made by somebody who has no basic knowledge of water works."

However, in his remarks quoted on the Three Gorges Project Corp. Web site, Mr. Pan did concede that: "A real factor affecting the water levels in the tail area will be the sedimentation deposited and accumulated after a period of reservoir operation. But this issue was repeatedly discussed during the feasibility study."

Lu Qinkan, a leading member of that feasibility study’s flood-control group, who refused to sign the team’s final report, recently submitted a petition to central authorities warning about the danger of sediment building up and raising the water level near the tail of the reservoir. He predicted that after 20 years of reservoir operation, 150,000 to 200,000 more people than expected in the Chongqing area would have to move as a result.

But it remains unclear if yet more people could be forced to move for another reason: if the future reservoir turns out not to be as flat as the resettlement-group experts assumed.

And with inconsistent assumptions underpinning the scientific studies that went into the preparatory work for the dam, who can blame the Chinese citizens who took part in the recent discussion on the CWRC site for scratching their heads and wondering where the truth lies, and what the implications could be for those who may be living in harm’s way on the shores of the reservoir.