Is there an easy explanation for this flow pattern? I was just admiring the river formed after a storm and saw this flow pattern under the bridge.  Is there an explanation for this flow pattern?

 A: Yes there is, as follows.
If we disturb a body of water by moving an object through it, we create waves which propagate away from the object as it moves. Those waves will have a certain height and in the wake left behind by the object, a whole series of waves (peaks and troughs) is set into motion, with a well-defined spacing (the wave length) between them, one following the other, and all moving in the same direction.
Now imagine instead that the water is moving and the object stuck into it is standing still. the water can't tell the difference, and so waves will be set up in it just as before. But now the moving water carries the wave downstream and away from the object (in this case, the leading edge of a bridge pier) and if the speed of the water flow is the same (or nearly so) as the speed of the wave, then the wave itself appears to be stationary even though the water flowing by is in continuous motion.
This is considered to be what is commonly called a standing wave, and that's what you see along the sides of the pier, trailing away downstream of the leading edge.
