Las Cruces, New Mexico -- For nearly 2,000 miles, the Rio Grande River winds it way from the Rocky Mountains down to the Gulf of Mexico. As one of the country's longest and most iconic rivers, it provides drinking water and irrigation for more than six million people in three U.S. states.



But climate change is threatening that vital water supply.



For more than 130 years, one river gauge in northern New Mexico has tracked the pulse of the Rio Grande. "It's the oldest continuously operated gauge in the United States operated by the U.S. Geological Survey," USGS hydrologist Mark Gunn said.

When CBS News correspondent Michelle Miller and her team visited, the Rio Grande was beating along. But Gunn told us last July that its flow nearly flat-lined.

"Since we've been measuring since 1889, last year was the lowest discharge … in the history of this gauge," Gunn said, adding, "It was so low that we actually had to dig out this whole entire area and dig a channel into the river to be able to get water to go to the gauge."

"So it basically made your machine here malfunction," Miller said.



"Yes, it did," Gunn said.

The Colorado snowpack that melts into the Rio Grande is declining – 25 percent over the last 50 years – and University of New Mexico climatology professor David Gutzler said climate change is threatening to dry it up.

"I foresee dry spells getting drier, droughts getting more intense and water resources being put under more pressure," Gutzler said.

With that in mind, cities downstream have been preparing. Albuquerque's water authority has spent $6 million incentivizing desert-friendly landscaping. The city even sends every 4th grade class to the river for a lesson in water conservation.

We followed the Rio Grande 150 miles south to where it pools into the Elephant Butte Reservoir, New Mexico's largest.

The shrinking reservoir can be seen from space. But up close, you can see the bathtub ring left by higher water levels 25 years ago.

"We're sort of a microcosm of a lot of river systems in the world," said Phil King, a civil engineering professor at the University of New Mexico and an adviser to the Elephant Butte Irrigation District.

"So you're saying that the Rio Grande is the canary in the coal mine?" Miller asked.

"Sure. Think of it that way," King replied. "As things warm up, for a given level of precipitation, you get less water into your river and into your reservoirs."

That means less of it can be released to the 90,000 acres of farmland on the other side of this dam.

"It is now April … and we have not released any water from storage. We should have been running for a month and a half by now," King said.

Until water is released in June, parts of the Rio Grande will look dusty and dry, like the desert around it. The canals that deliver water to farms like Dixie Ranch will remain empty.

"So what happens upstream … directly correlates with what you see here?" Miller asked.

"Absolutely. Lots of snow, we'll have years of lots of water," Dixie Ranch owner Greg Daviet said.

Daviet has prepared his 310-acres of pecan trees for either scenario. He's added more groundwater wells for irrigation during drought and brought technology to the century-old farm to ensure that every drop counts.



"So we've developed a computer program for our farm," Daviet said. "And when I irrigate the tree, it can then predict when it'll need to be irrigated again."

Daviet doesn't deny that climate is changing. He just not ready to panic.

"Droughts come and go," he said, adding, "The worry would be that there's a future that I can't plan for, and I worry that it's coming. I believe that we can plan, that we can adapt, and that we can adjust to whatever conditions come. "

This year's above-average snowpack might offer some relief, but it doesn't mean the drought is over. One study published in the Ecological Society of America said by the end of the century, the flow in this part of the Rio Grande will drop by 50 percent.