What interaction causes the water to spread as they pass through the rock opening?

The answer is: Diffraction

Diffraction refers to a phenomenon when water meet/encounters an obstacle that it cannot directly destroy. Rock is considered to be impermeable object because it does not have pores that make it able to absorb water. When water clash with solid rocks, it's particles will scatter and causes the water to spread as they pass through the rock opening.

sediment layers stop spreading when they reach a barrier or they run out of sedimentary material. lateral spreading is the movement of gently to steeply sloping, saturated soil deposits caused by earthquake-induced liquefaction. i hope this . : )