Reported by John Blackstone, CBS News San Francisco

SAN FRANCISCO (CBS SF) — Come spring, the American West’s vast water reservoirs are supposed to fill with melting snow. However this year, as in recent years, the large reservoirs of Lake Mead and Lake Powell in the Colorado River basin have seen declining water levels — an ominous trend that a new study warns could signal a looming megadrought.

“The persistence of the drought conditions, in the Colorado River basin especially, is essentially unprecedented in human history,” John Fleck, author of “Water is for Fighting Over,” told CBS News’ John Blackstone.

Fleck has spent years studying the Colorado River, a crucial source of water for much of the region around it. He said that Lake Mead and Lake Powell’s reservoirs have what he described as “big bathtub rings” around them, left behind as the water declines.

“There is less water in the system now than there was 20 and 30 years ago,” he said.

RELATED: Global Warming Drives ‘Mega Drought’ Across Western States, Study Finds

Fleck explained that a “wet year” every few years may seem like the drought is ending but those years are still comparatively lower than decades before.

“When we do get a snowpack in the mountains over winter, we are seeing less water make it into the rivers and downstream to the farms and cities and the fish and the ecosystems that depend on the water,” he said.

A team of scientists is researching megadroughts that have lasted as long as 40 years, using tree ring evidence going back 1,200 years.

“If they go back in time 500 years or so, there were these phenomenal droughts — in terms of both severity and in terms of length,” Park Williams, the scientist leading the research, said. “And until recently, those droughts have always been spoken about with almost a mythical-type character.”

Williams said the drought of the last two decades “developed the same way that the megadroughts did.”

However, the key difference now is climate change’s effect on weather conditions in the area, which largely depends on melting snowpacks to fill reservoirs.

“Without human-caused climate change, we would still have a drought,” Williams said. “But it wouldn’t be a serious as the one we’ve actually seen.”

NASA-run project SnowEx has researchers in the mountains of Idaho developing remote sensing equipment to get accurate snowpack measurements from space in order to determine how much water they will produce.

“It’s becoming more challenging for us to not only predict how much water is going to enter our reservoirs, but also the ability to store that water all the way through the end of the summer for agriculture and water resource purposes,” said Hans Peter Marshall, a scientist working on the project.

The shifting patterns and increased difficulty are some of the main reasons researchers are seeking to develop a space-borne approach to monitor the region’s water.

“We’re really needing an approach that maps the amount of water that’s stored within the snowpack,” he said.

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