Meko and his team recorded a sharp dive in the amount of water that reached the valley in the mid-1990s due to a lack of rainfall in the mountain ranges that feed the river.

Looking at variations in tree rings, Meko has seen a history of dry and wet phases around the San Pedro River, including droughts in the '50s, '60s, '70s, and in present day.

Although variations in climate are normal, researchers are concerned that the combination of droughts and the water demands of the nearby population could have an extreme impact on the system.

"When you put them together, droughts and a demand for water, it puts a big stress on the system," Meko said. "The riparian zone exists because of the river and shallow ground water. The San Pedro will respond strongly to the combination of increased pumping and reduced stream flow. With the San Pedro, the effect of man is an even bigger concern than climate."

Since 1990, the population of Cochise County has grown from fewer than 98,000 to more than 130,000. Those numbers do not include the growth of Fort Huachuca, Arizona's biggest military installation, which has increased missions and personnel as Congress reduced the number of bases across the country.

Groundwater supplies have been stressed due to that growth, especially in Sierra Vista, said Thomas Maddock, head of the University of Arizona Department of Hydrology and Water Resources.

"A major city is growing between mountains that recharge the system and the river itself. What do you think is going to happen? Groundwater pumping exacerbates climate change issues."

Maddock estimates that for each cup of water pumped from the ground to support Cochise County's growing population, roughly a cup of water disappears from the river.