Rock Rings (from December 2014)

I have a new puzzle to sort out. Last December, I examined some rocks in a nearby canyon that where covered by silt, but had rings at various former water levels. The rings are levels on the rock that got cleansed, while the rest of the rock is covered with a fine layer of sediment as the water receded. What mechanism would cause the selective cleaning?

My guess is that as the water slowly receded, it would deposit sediment on the rocks. Then, if the rain resumed, we might get a freshened surface layer of water that would cleanse the rocks. The puzzle is how an increase in rain would remain less silty than the rest of the flow. It seems logical: the first rains loosen most of the material that becomes silt, and later rains loosen less. But the water has to flow down the canyon, so any turbidity would be stirred up by the freshened flow. But it seems reasonable that an increase in flow with fresher water at the surface could clean the rocks and leave clean rings.

Other photos:

Suds formed, a presume from natural elements.

Typical muddy flow as canyon drains and water levels drop.

More rings with debris deposits.

jg