​WATERSHED, in physical geography, the line separating the head streams 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 watershed, in a plain country of gentle slope (e.g. the central plain of Ireland) the watershed is often difficult to trace, as the headwaters of two different river systems may merge in marshes or lakes at the highest levels. In a mountainous country, where two streams, flov, 'ing 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.