The San Pedro River

I've been working on a story about the San Pedro River for the past few months. I've gone out to different points along the river a few times this semester and have put in quite a few interviews with researchers and scientists at UA and up at ASU (boo!). Anyways, this is my first draft on the story, which I'm not sure what to do what in terms of who to submit it to. It's been run gathering information though. It's a pretty unique river in the sense that it flows upward from Mexico. It's interesting because at some points it will be dry, but if you keep hiking up down river it was then be wet and so on and so on because the river is fed by many mountain ranges in southeastern Arizona. From what's I've been hearing it's in pretty bad shape due to a growing demand on ground water. Our state better find a way to slow down demand on the water supply or it looks like we're in trouble. Apparently mother nature in Arizona is already feeling the effects...

The San Pedro River, one of the last undammed and free flowing rivers in the Southwest is dying due to climate change and increased demand on the ground water supply, a strong indicator that Cochise County and southeastern Arizona may be drying up.

Thomas Maddock, head of the University of Arizona’s Hydrology Department was asked to file a report on the state river in the 1980s by Cochise County, using the health of the San Pedro as a model to measure the health of groundwater.

In 1990, Maddock and his colleagues predicted the system would dry up between 10 and 20 years.

“It dried up in about 15.” Maddock said. “It isn’t in the best of health.”

Other researchers agree the San Pedro system is going through a dramatic change.

Juliet Stromberg, ASU Life Sciences associate professor and an ecologist who specializes in riparian ecosystems such as the San Pedro River has seen a response to decrease in stream flow and ground water level through changes in vegetation and riparian ecosystem since 1993.

“There is reason to be concerned. We’re documenting a decline in base flows,” Stromberg said, who monitors the river at a series of established sites along the river. “Part of river is becoming intermittent that was perennial.”

In response to the changes in stream flow permanence and depth of the ground water, Cottonwood willows, tall high-canopy trees that fill the valley are being replaced by other shorter and more shrub-like species that require less water, Stromberg said.

“As the water level drops, ecology fluxes,” Stromberg said. “It’s transitioning from a forest structure to a scrubland.”

In other words, the change in plant life along the river shows that stream flow and the ground water are receding.

Thomas Meixner, associate professor in the Department of Hydrology and Water Resources at the University of Arizona has seen other rivers in Arizona become severely altered by people, including the Gila and Santa Cruz rivers.

“We know how this story ends if no action is taken,” Meixner said.

“There’s a really good comparison photo that people often show of Tucson, where in the 40s the Santa Cruz River has this nice, green, Mesquite Bosque, lots of trees, and now you go and it’s just a harsh desert landscape,” Meixner said.

“The concern is that the San Pedro could end up like the Santa Cruz—a river no more.”

Data from the Arizona Department of Water Resources show that the River’s annual flow saw a major drop in the 1960s and again in the 1990s.

Maddock attributes recent changes to climate change and groundwater pumping.

“I think the climate’s probably going to hit it the hardest, although, ultimately, the groundwater pumping will be the real issue,” Maddock said.

Since 1990, the population of Cochise County has risen from 97,624 to 129,006, a 32 percent increase, not including the growth of Fort Huachuca, Arizona's biggest military installation.

Ground water supplies have been stressed due to population growth in the county and Sierra Vista, he said.

“A major city is growing between point of recharge and discharge. What do you thinks going to happen?” Maddock said. “Ground water pumping exasperates climate change issues.”

For each a cup of water pumped from the ground to support Cochise County’s growing population roughly a cup of water disappears from the river as a result, Maddock said.

Now, with the river looking more and more like a wash, researches worry that the ground water eventually won’t be able to support more growth.

The problem likely will not stop with the San Pedro - one of the reasons why it has become one of the most heavily researched river systems in the Southwest.

“I think it’s a problem that’s going to be faced all over the world,” Maddock said.

Climate change has causes increased temperature and less precipitation in the Southwest, he said.

“You have less precipitation, that mean less water in the river, less recharge in the mountains, less groundwater,” he said.

With the river as one of the last rivers left in the Southwest and with the valley being a major migratory corridor for hundreds of bird species, researchers are keeping a close on the valley.

“There aren’t many rivers that haven’t been dammed or severely damaged by people.” Stromberg said. “Although its been heavily used by agriculture, it’s more or less a natural river.”

Although that’s changing due to increased demand on the ground water, researchers say the fate of the river isn’t set in stone.

“It depends on what people do for one thing. What growth is like how people change actions in terms of recharge and pumping,” Stromberg said.

“Effects from climate change remain uncertainties but just how severe the change will be depends on how humans manage that.”

But Meixner said getting people to recognize groundwater pumping as a problem might prove difficult.

“One of the struggles convincing people that the pumping will, sort of, take out the river, if you will, is that it won’t happen today,” he said. “It’s going to be a very subtle, gradual shift.”

The river is already changing though, and showing signs that it is not in the best of health.

“The river isn’t running as often,” Maddock said.

Fewer portions of the river maintain a perennial flow. Or in other words, less of the river runs all year long.

Meixner says the flows are also lower, and less of the streambed is wet year round.

“As a result of the streambed being wet less of the year…ultimately, some of the trees that will come in will die,” Meixner said. “There’s a lack of young tree systems.”

Although changes in the river over the past 20 years has researchers concerned, the recent shift isn’t the first major transformation the valley has seen in recent history.

When the ground water was full at the turn of the last century the valley was a marshy area. During the last 100 years, intense droughts, cattle grazing, farming and ground water pumping has transformed the valley to what it is today.

“There’s been a major shift in vegetation from 100 years ago,” Stromberg said.

“Picture this area, demitted by vegetation by drought and cattle. Then floods cut a big gully widening the channel. That area then became full in with cottonwood willows in wetter sections of the valley.”

Now, the current state is in flux again.

Dave Meko work at the University of Arizona Tree Ring Laboratory, the world’s largest laboratory of its kind. By studying and measuring the rings of trees, Meko and his colleagues can look back in time to weather conditions in and around the San Pedro river valley.

Meko and his team concluded that in the mid 1990s, the amount of water that reached the valley took a sharp dive due to lack of rainfall in the mountain ranges that feed the San Pedro.

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

“When you put them together, droughts and a demand for water, it puts a big stress on the system. 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,” Meko said. “That is threat a lot more than human encroachment than climate variations. In the San Pedro the effect of man is an even bigger concern than climate.”