The Doon Valley lies between two intermittent ranges of the Himalayas, the Outer Himalayas (a.k.a. the Sivalik Hills) and the Lesser Himalayas, know locally as the Mussoorie Range. It is bounded on all sides by mountains, with northern range running from Kalsi in the west to Muni Ki Reti in the east with Mussoorie at the centre in a semi-circular arc; and southern range running at south from Paonta Sahib in the west to Haridwar in the east. The valley also forms a watershed between the Yamuna and Ganges river systems. In fact, the Yamuna and Ganges are closest to each other as they pass the Doon valley, with the Yamuna forming the western boundary and the Ganges the east. It runs 75 km long from west to east.