Earlier I made the statement that I think there 4 layers in a painting of water with rapids, but I was just looking over a painting from the series and I think a 5th layer might be worthy of it's own category - underwater reflected light from air bubbles. This would make the list:
Stream bottom (underwater rocks)
Reflected/refracted light from underwater bubbles
Reflections on water's surface
Bubbles/foam on top of water's surface
You can see this near white water where bubbles are forced under the surface and often vivid greens are apparent (from the sunlight (yellow) mixed with the blues of the water).
This is definitely going to take some time to master ...