It was approaching high water on the Arkansas River a year ago as we rigged our rafts under the full moon, preparing to depart not long before midnight.

The flows were going up in late June as the mountain snow melted and streamed into the rivers.

Most of us knew the flows would increase on our nighttime excursion, but didn't anticipate the extent of it.

But, the water did come up, and dramatically — sending us on a memorable ride through Browns Canyon on the Arkansas River — and I learned the hard way that Colorado's rivers and streams flow with more volume at night.

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.

Flows don't often change much, as shown by U.S. Geological Survey data, but it does change enough to note a difference. Colorado Department of Transportation officials said last week's flooding on Highway 9 in Blue River could change roughly 3 inches between the day and during the night.