When damming a creek, your core trench will determine the success of your dam. What Scott is saying is that you must dig down, through layers of porous soils in the creek bottom, to get to solid soils to create a foundation for your dam.
A core trench is the best and most effective water barrier, to prevent dams from leaking. Lots of discussions on this site about core trenches.

Teach a man to grow fish...
He can teach to catch fish...