Answer. Posted on Thursday, February 12, 2015.
The water is falling and it travels faster the further it falls. There is a continuity equation in fluid physics, A1 * V1 = A2 * V2, where A1 is the first cross-section of the stream of water, A2 is the second cross-section, V1 is the velocity at the first cross-section and v2 the velocity at the second cross-section.
We have a higher velocity at the bottom of the stream and according to the continuity equation, the second cross-section is going to have to be smaller in order for the flow rate to remain the same.