An actual watershed is a higher piece of ground than its surrounding territory - eg a mountain-ridge - which effectively separates the directions in which rivers form. In England, the Pennines form a watershed, with part of the rainfall forming rivers that then flow westwards into the Irish Sea and the other part forming rivers that then flow eastwards into the North Sea. This 'image' is nowadays applied to 9 pm in terms of TV broadcasts. It is ssumed that children will all be in bed from then on and it is, therefore, 'safe' to show programmes with a more adult content.