WATERSHED  , in

physical geography, the line separating the headstreams tributary to two different river-systems or basins . Alternative terms are " water-parting " and " divide." The crest of a mountain ridge forms the most clearly marked water-shed; in a plain country of gentle slope (e.g. the central plain of Ireland) the watershed is often difficult to trace, as the head-waters of two different river systems may merge in marshes or lakes at the highest levels . In a mountainous country, where two streams, flowing in opposite directions but having their sources adjacent, are both gradually eroding or cutting back the land at their heads, a pass is formed . In such cases, where one stream erodes faster than the other, the stronger may ultimately " behead " the weaker, and " capture " some of its waters,. whose flow is diverted from one basin to another .