A glacial lake that overflowed in India’s Himalayan northeast, sending ice cold water crashing through a major dam and towns and killing at least 31 people, had been identified by state officials, researchers and environmental activists as dangerous years before the disaster. The flood began shortly after midnight Wednesday, when a glacial lake high in the mountains overflowed after a heavy rainfall. The waters crashed into the largest dam in the state of Sikkim, cracking its concrete before cascading through towns in the Lachan Valley below and sweeping away people, houses, roads and bridges.