Watershed?

[~ vortex ~]

Why do they call the guideline for parental TV control a "Watershed"?
eg.. Programs for adults are broadcast after the 9 o'clock 'watershed'

[Quizmonster]

A watershed in geographical terms is usually a mountain range...ie it is the point from which rain, once it has fallen, runs off in opposite directions. In TV terms the same idea is involved, because before the "turning-point" one type of programming is permitted and after it another type is possible. It's a dividing-line, in other words.