Buddha’s Dam - The Rocks Don't Lie - page 5

strong monsoons delivered extra snow to the glacier's source on the high peak. As the gla-
cier repeatedly dammed the throat of the gorge, ice-dam failures generated catastrophic
floods that drained ancient lakes impounded in the valley upstream.
One day as we drove down the valley toward the gorge, one of my graduate students
relayed information from a guidebook he'd brought along. Local folklore told of a tradi-
tional kora—a Buddhist pilgrimage trek—that circled a small peak ringed by lake terraces.
Pilgrims walk the kora to commemorate how Guru Rimpoche brought Buddhism to Tibet
through defeating a powerful lake demon, draining its home to reveal fertile valley-bottom
farmland. It was a feat impressive enough to convert the locals. I began to think that an oral
tradition might record our glacial dam-break flood.
Suspicion moved beyond idle speculation when we got the radiocarbon dates from wood
fragments I'd painstakingly collected from the lake terraces. Radiocarbon dating uses the
ratio of carbon isotopes in once-living matter to determine how long ago it died. 1 The tech-
nique works because carbon-14 ( 14 C) decays to carbon-12 ( 12 C) at a known rate, and all
living things start with a 14 C/ 12 C ratio equal to that in the atmosphere from which the car-
bon was originally taken up by photosynthesis. Wood fragments from the higher terraces
of the older lake were almost ten thousand years old, dating from the tail end of the last
glaciation of what's popularly known as the ice age. Fragments from the younger lake were
only about twelve hundred years old—dating from around the eighth century AD .
This was about the time that Guru Rimpoche arrived in Tibet. Did the geologic story I
read in the landforms really support a Tibetan folktale? Or was it that the folktale told the
geologic story?
Two years later, in 2004, I returned to Tibet to explore the story of the lake-draining
flood. On our first trip we had hired a local farming couple to collect monthly samples of
river water. When I told the farmer's wife how we'd discovered that the whole valley was
once an ancient lake, she replied that yes, she knew about that. Caught off guard, I listened
to her. She pointed out a steep hillside across the valley and described how three boats had
been stranded there as the lake drained to reveal the farmland of the modern river bottom.
She told me she'd heard the story from the Lamas at the local temple.