Here is another approach I have been playing with. I am far from sure it is a good idea, but I will see what people think. It starts with a series of images of the flowing water - I use Continuous High on the D500 for about one second. Then every second or third image (second in this case) is colour shifted to red, green and blue, then re-combined. The idea is that highlight areas where the ripples have moved on in the 200 or 300 msec between shots will be different colours.