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.
Suds formed, a presume from natural elements.
Typical muddy flow as canyon drains and water levels drop.
More rings with debris deposits.