The smooth, round rocks will have a clear layer of water flowing over them with very little turbulence until the water reaches the down stream side of the rock. That smooth flow over the rock is like the lifting air over the top of your canopy. The turbulent water behind the rock is form drag, the wake your canopy leaves behind as it cleaves through the air. Moss, irregularities in the surface, and roughness at the upstream edge is parasite drag - you can see it. Now look at a jagged rock.
Flow separation is written all over it, all rough water and no smooth layer.
No smooth flow, no lift. No lift, no control. As you drive down the highway, put your hand out of the window. We'll look at that soon. But before we do, let's take a look at the different canopy designs on the drop zone so we can understand why they are built the way they are, and what we can expect them to do.