From the Summit Daily News (Janice Kurbjin):
It’s not because the moon somehow heats the snow more than the sun. Experts say it’s because Colorado’s waterways are largely fed by snowpack high on mountain peaks. It takes until about mid-afternoon for the higher elevations to warm up enough to start melting snow, and it takes even longer for that water to flow down the hillside into rivers and streams.
“During the daytime, the water that melts up on the higher slopes melts at about (3 p.m.),” Colorado Department of Transportation spokesman Bob Wilson said. “It takes several hours for that water to make its way down from 13,000 feet to about 9,000 feet. By the time we get to nighttime hours it’s making its way down the mountain” to the streambed.