Watershed

[Panic Button]

Where does 'watershed' come from?

[Clanad]

Online Etymology Dictionary says: "Watershed: line separating waters flowing into different rivers," 1803, from water + shed. A loan-translation of Ger. Wasser-scheide. Fig. sense is attested from 1878. Meaning "ground of a river system" is from 1878.

[Quizmonster]

C's description above is excellent as far as geography is concerned...ie basically a mountain-ridge where - once rain falls on it - the water will obviously flow down one side or the other.
However, I wonder whether you mean 'watershed' in the sense used on British television. This is set at 9 pm, the idea being that programmes suitable only for adult audiences will be broadcast after that deadline. The concept is the same as the geographical one...ie it is a dividing-line.

[heathfield]

Interestingly, (to me, at any rate), - the German word scheide means a division or borderline (amongst other things) and so applies to the geographic division of the waterflow.
Yet the English word 'shed' can mean a falling off (from) - as in 'dogs shed hair', and so could describe the water 'falling off' the hillsides.
Hmmm. Curious.

[emi-elektrik]

They arent much help if your wondering about television after 9 o clock. i dont have a clue, but i was thinking about it the other day.