If you’re like me, and you didn’t study physics, then I picture myself standing barefoot in a fast moving, inch-or-so-deep stream. The ram pressure is highest right on some point on my ankle. I can feel it pelting my skin and watch it shooting off in all directions.
But as I stand there, I also feel that, slow, deeper drag, the way the rest of the water pulls on me.
Now I imagine down river a bit, the slow moving, deep, massive river now tumbles big stones along the bottom. The pressure on levees and barges on the surface is immense.
The faster water (or any fluid, including air, which is how planes fly) moves, the less pressure it has. It trades in pressure for velocity. If water moves more slowly, and there is more water, the higher the pressure.
That’s my over-simplification.