The Water Flow button is used to display the direction and degree of slope on the surface. The direction is indicated using shaded line. The blue end of the line is the "from" and the white is the "to" direction. In High Contrast, the grey end of the line is the "from" and the black end is the "to". The longer the line, the steeper the slope. A small "x" indicates a level surface. When the button is pressed, the following window displays.
Type in a Interval value between 5 and 10000 and press OK. The smaller the value, the shorter the lines and closer their proximity to one another. Higher values create longer, farther apart lines.
Below is and example of the displayed water flow.