PUNAKHA, BHUTAN—Fed by a separate glacier, the Thorthormi has bulked up to alarming size and is in danger of swamping a third body of water, the Raphstreng. In a nightmare scenario, the two lakes could merge, punch through the natural but unstable moraine dams holding them back, and go cascading into the valley, picking up debris as they thunder downhill.
A 2002 study estimated that such a rupture could send 14 billion gallons of water barreling toward Punakha, though not all of it would reach the valley. Still, that is more than double the amount released in the 1994 deluge and about the same volume that plunges over the top of Niagara Falls in five hours.
Hauling major equipment up there, let alone getting it to work in the thin, frigid air, is so tricky that digging the channels to siphon off water from the lake will have to be done mostly by hand. Weather conditions allow for work barely six months of the year.
Not that there is much choice.
"Either drain it or get people out of the way," Namgyel said.
Officials hope also to install sensors as part of an early-warning system to alert residents in Punakha in case of a breach. In 1994, the floodwaters probably took several hours to reach the valley, but no one had any idea they were coming.
Gembo Tshering, a teacher, had just sat down to breakfast when the disaster struck.
"I heard the roar of rushing water," recalled Tshering, 52. "When we looked, the level of the Mo Chhu [the local river] had gone way up."
As he and others watched, the water kept rising. Across the way, a knot of people huddled on a patch of high ground. Logs that had been sucked into the maelstrom battered the banks and everything else in their path. Fields, homes and livestock were swept away.
Especially grievous to many in this devoutly Buddhist country was the effect on Punakha's 17th century dzong, the tall, whitewashed monastery-cum-fortress on the riverbank that once served as Bhutan's seat of government. The complex was surrounded by the tumbling waters, the force of which severely damaged one of the dzong's oldest temples.
Monks and novices in their burgundy robes clambered onto the rooftops to see what was happening. For three days they were marooned in their island monastery.
Kezang, a grizzled senior monk who uses one name, put his faith in divine protection.
"I wasn't particularly worried because of the blessing and the power of this place," he said, smiling through teeth stained red by betel-nut juice. "It was fearsome, but I wasn't afraid."
Scientists are not so sanguine. A flood of larger proportions emanating from above Punakha would be far more devastating now that new infrastructure, new hydroelectric projects and even a new town lie in its path.
Despite Bhutan's record as one of the world's most environmentally vigilant nations, it has no choice but to confront and plan for problems incurred by the actions of others, experts say.
"There is a sense of helplessness," said Tshering of the U.N. Development Program. "But at the same time, you can't sit back and do nothing about it."