Correct Answer!

This is the same principle used in 8 inch standard rain gages whereby the catching vessel diameter is about 2.5 inches. The sensitivity of the catching vessel is such that 0.10 inches of rain through the 8 inch opening produces 1.0 inch of water in the catching vessel. Similarly, by constricting streamflow in the horizontal direction, the stage - or vertical dimension - becomes more sensitive to increments of flow.