The cold freezes the Zanskar river, one of the tributaries of Indus, and the upper layer forms a hard layer of ice which is walkable. Its unbelievable that the gush and force of the same river itself is scary during summers, and to see it frozen and calm in the winters does make you calmer and excited at the same time ! This served as and still serves as a traditional way of connect between Leh and Zanskar for the locals. It was the documentary of National Geographic Channel which highlighted this and since then, Chadar trek has hit the popularity burst. Its rated as one of the best treks existing in the planet by many..