One of the great uncertainties about building high dams in the Himalaya for hydropower and flood control is the threat that they would pose to the plains in the event of a major earthquake. The grandeur of Himalayan peaks and their stupendous height deludes observers with an image of permanence. This is actually a gigantic pileup resulting from the collision of the Eurasian and Indian plates that began 50 million years ago. India continues to bulldoze under the Tibetan plateau, creating tremendous tectonic tension under the mountains. Most areas of the Himalaya where future dams are planned are rising or slipping at between 10-20 mm a year.
The major Himalayan rivers are older than the mountains and have their headwaters in the Tibetan plateau, behind the main chain. The rivers rose as the mountains were formed, cutting deep gorges and they store great amounts of potential energy—which is what makes them so ideal for power generation. As the mountains rose, they formed a monsoon trap giving the southern slopes one of the highest precipitation rates on the planet. The annual rains turn this steep and seismically unstable mountain range into a crumbling, shattering mass that erodes faster and washes down more sediment than any other mountain system.
It is the debris from the erosion of the young Himalaya that filled up the Tethys Sea and turned it into what is now the Gangetic plains. This process of mass wasting that deposits debris in the plains continues, so the notion that floods in northern India and Bangladesh can be 'controlled' is wishful thinking. The rainfall volume, sedimentation levels, and the size and frequency of earthquakes in the Himalaya far outstrip parameters laid out in engineering textbooks prepared for comparatively docile climes. Many of our specialists have been trained for technological solutions based on case studies that greatly underestimate the Himalayan dimensions of cloudburst, glacial lake outburst floods, and earthquakes in this part of the world.
The rock strata bent by the enormous forces beneath the Himalaya trigger thousands of small tremors every year. But every once in a while there is a major crack as the pressures are too much for the elasticity of the rocks, and the strata snap. When that happens, there is a magnitude Richter 8+ earthquake. Geologists now agree that there occurs a high intensity earthquake once every 100 years along any section of the Himalayan chain.