It's probably a combination of both, and maybe even the angle of the sun. I've seen advice to put virus-contaminated materials in full sunlight for a day to help reduce the contamination.

But the SODIS site does mention temperature.

"...otherwise the top 6" of every stream and lake would be sterile."

Only if it stayed in the same place long enough. I'm sure the water is always moving, due to current, wind, temperatures within the water source, etc. If molecules were like marbles, their constant movement would limit purity, as new bacteria would constantly be mixing with the cleansed parts.

Sue