Watershed
Watershed , noun
[Compare German wasserscheide; wasser water + scheide a place where two things separate, from scheiden to separate.]
1.
The whole region or extent of country which contributes to the supply of a river or lake.
2.
The line of division between two adjacent rivers or lakes with respect to the flow of water by natural channels into them; the natural boundary of a basin; -- called also divide and water parting.
3.
a point in time marking an important transition between two situations, or phases of an activity; a turning point.