Bhutan's valleys are carved into the Himalaya by its rivers, fed by glacial melt and monsoon rains. Much of the Bhutanese population is concentrated in valleys and lowlands, separated by rugged southward spurs of the Inner Himalaya.: Despite modernization and development of transport in Bhutan, including a national highway system, travel from one valley to the next remains difficult. Western valleys are bound to the east by the Black Mountains in central Bhutan, which form a watershed between two major river systems, the Mo Chhu (Sankosh River) and the Drangme Chhu. Central valleys are separated from the east by the Donga Range. The more isolated mountain valleys protect several tiny, distinct cultural and linguistic groups.