Waterfall flow control

Question:

Hi, how do i make it so that the waterfall weir flows over a rock with a nice even current and without the water going all over the place?

Answer:

With many waterfall a Spillway is used. A spillway has a basin that fills up and then the water flows out over the lip of the spillway. The basin is a very important aspect when trying to make a nice natural looking flow. The lip of the spillway also controls the flow in a directional sense and as a means of creating an even flow.

In order to achieve the nice, even flow that are looking for you are going to need to these two things.