New Mexico Tech scientists think they've found the largest spring yet in the upper Rio Grande, a few miles south of the Colorado border near Ute Mountain.

The scientists have dubbed it the Lava Tube Spring and say the spring's crater is 12 feet deep and bubbles 6,000 gallons of water a minute into the Rio Grande. It is the largest of the 170 springs and seeps the scientists have documented so far in the 80-mile river reach from the Colorado border to the Embudo measuring gauge. They estimate the Lava Tube Spring contributes 10 percent of the total flows from springs and seeps in the stretch.

The Rio Grande is fed by mountain snowmelt and springs beginning at its Colorado birthplace and gains water from tributary rivers as it courses into New Mexico. Scientists have known for a while there was some surplus water added to the Rio Grande that wasn't coming from Colorado or the tributaries. "We're trying to figure out where that other 130 (cubic-feet-per-second flow) is coming from and what are the geologic conditions," said Paul Bauer, associate director and principal geologist with the New Mexico Bureau of Geology and Mineral Resources at New Mexico Tech. "It's been known for a long time there's accretion, basically additions to the river from springs and seeps. But no one had tried to figure out where and why."

So Bauer, hydrologist Peggy Johnson, and other scientists started floating parts of the upper Rio Grande two years ago and documenting the springs. The scientists love floating and studying rivers even when it isn't official, but some of these trips tested them. In some places, like the Upper Taos Box, where the canyon is steep and the rapids are Class 5, they had to scramble down to the river carrying inflatable "duckie" kayaks and their water sampling gear. They had poison ivy and other challenges to deal with. "It was absolutely brutal," Bauer said. "It is a formidable field area."

But Bauer's not complaining. "It's a really cool project," he said from his Socorro office.

Bauer and the rest of the team are delighted at what they've documented so far. Geochemical testing overseen by Johnson on 37 springs has found some of the water is 15,000 years old. Some is only 30 years old. The testing can help scientists figure out what is feeding the spring, such as recharge from nearby mountains, and how long it takes to reach the river.

Bauer and Johnson, his wife, floated over the large Lava Tube Spring last fall. "The river was too high to evaluate the spring," Bauer said via e-mail. "This year we returned at very low water to check it out."

When they returned, the science team inserted 1.5-inch-diameter pipe into the spring's crater. A 5-gallon bucket was fixed on top of the pipe to collect spring water as it rose above the river through the pipe. "We were able to get the spring water to rise as artesian flow nearly 4 feet above the river," Bauer reported.

Other scientists measured the river flows above and below the spring, and calculated how much water the spring was adding. "Our best guess for the origin of the spring is that groundwater is flowing through a collapsed lava tube in the Servilleta Basalt. At river levels of more than about a couple 100 cfs, the spring disappears from view."

The preliminary report on the springs in the Upper Rio Grande will be published online early next year, Bauer said. The study, started in 2007, was funded by The Healy Foundation through Taos County and by the Interstate Stream Commission.

Contact Staci Matlock at 986-3055 or smatlock@sfnewmexican.com.