spoiler - possible puzzle answer
***possible spoiler of puzzle question***
(see previous toot in this thread)
I think it has something to do with the molecular adhesion between the water molecules and the molecules of the face of the dam wall. When the stick provides an alternate adhesion surface for the water to follow, it follows the path outward away from the dam wall due to the inertia of the flowing water and the cohesion of the molecules of liquid water. Then the cohesion of the water flowing outward pulls the adjacent water away from the dam water, widening that flow patterns, recursively across the dam.
That's my guess.