With a sweep of the hand, he draws the 1,285-mile Yarlung Zangbo river, the main watercourse of Tibet, which flows east across the plateau before making a tight hairpin bend and turning south into India as the Brahmaputra. He marks two points on either side of the bend, only 25 miles apart but separated by an altitude of 7,300ft. To Mr Chen, secretary-general of the society, and his colleagues, the implications are clear. He confidently draws a straight line between the two.
Mr Chen's plan is to cut off the giant hairpin by a 10-mile tunnel bored through the Himalayas, and then a series of pipes or channels. A group of hydro-power plants would exploit the huge drop in altitude to generate 40,000 megawatts of electricity, or twice that of the Three Gorges Dam on the Yangtze, which has cost pounds 16bn and will displace more than a million people. In its most ambitious form, the new proposal envisages using the electricity to pump water from the Yarlung Zangbo and other rivers further east all the way across to China's arid north-west - a modest journey of 500 miles across the frozen Tibetan plateau.
"Man should change and recreate nature. The project can solve the drought in north-west China, and can alleviate flooding in south Asian countries, so why not do it?" asked Mr Chen, one of China's leading water resources experts.
The idea is so fantastical that it is difficult to take seriously Mr Chen's belief that it might actually get off the drawing board. The financial cost alone would seem prohibitive, and the environmental outcry would be deafening. But then China's forecast future demands for water and energy are also alarming, and the government's determined backing for the Three Gorges project has created an environment in which scientists are almost encouraged to put forward grandiose schemes for tampering with nature.
India and Bangladesh are keeping a wary eye on developments. The Brahmaputra ("son of Brahma") irrigates the north of Assam before flowing south through Bangladesh and emptying into the Bay of Bengal. Remodelling on this scale could have unimaginable results. "This is a grand idea," admitted Cheng Shengqui, deputy director of the Commission for Comprehensive Survey of Natural Resources. "[But] as the project would be so big, lots of consequences it will produce are still unknown. We don't know yet what kind of ecological effects it would have on the Tibet plateau, on north-west China, on China, on south and east Asia, and even on the whole world."
Mr Chen and his researchers have been working on his scheme for three years. Although stressing that "it is still in the stage of preliminary research", he added: "Before the year 2000 we want to reach a conclusion on this research." Only then might it be put forward for consideration by the central government.
Developing hydro-power is certainly part of Peking's plan for Tibet, the source of Asia's mightiest rivers from the Indus in Pakistan to the Yellow River south of Peking. But the biggest problem for Mr Chen is that this abundant electricity would be produced in a remote, undeveloped region which does not need it. Hence the initial plan to sell it abroad. "Part of it would be for China, but it is quite difficult to transport [because of loss during transmission]. So we could also provide India, Burma and Bangladesh with electricity. Maybe they will welcome electricity from China," said Mr Chen.
If they did not, then there is the extraordinary long-range irrigation proposal, which would be promoted to neighbouring countries for its promised flood-control benefits. "Every year, in the lower reaches of the Yarlung Zangbo, the Lancang and the Nu rivers there are floods. If the electricity was abundant, we could use it to send part of the water in these rivers to the north-west of China and solve the drought problem there," said Mr Chen. The technical question of whether it would actually be feasible to pump water across some of the world's highest altitudes does not seem to have been addressed in any detail.
The odds on the scheme winning central government approval seem very long, but 61-year-old Mr Chen is optimistic it will be built one day. And he can at least take heart from the fact that Sun Yat-sen, the founder of the Chinese Republic, first proposed a dam across the Yangtze way back in 1919. Construction of the Three Gorges project duly began - some 75- odd years later.