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Know the West

Death of the San Pedro: Not if, but when


Note: this is a sidebar to a main story about the political struggles over protecting the San Pedro River.

New evidence has surfaced that pumping in the Sierra Vista area may already be reducing groundwater flow to the San Pedro River.

Water levels in seven monitoring wells on U.S. Army property have dropped by roughly a half-foot per year since 1995, according to recently released U.S. Army Corps of Engineers data. The wells range from less than one mile to seven miles west of the river, on Fort Huachuca’s property.

The dropping levels, measured from 1995 to 2003, are reducing the rate at which water flows toward the river, says Don Pool, a United States Geological Survey hydrologist in Tucson. If that trend continues, groundwater discharges to the river from the aquifer will eventually decrease, reducing the river’s flow.

But when? "That’s the $64 question," says Mark Anderson, associate Tucson district office chief for the U.S. Geological Survey. So far, officials say they haven’t seen water level declines in 60 to 70 monitoring wells closer to the river.

Anderson says that pumping in the Sierra Vista area, about 16 miles north of Mexico, is probably not yet imperiling the river to the south. The first declines would most likely be felt at the San Pedro Riparian Conservation Area’s lower end, north of the largely abandoned village of Charleston, where the river flows intermittently, he says.

But if the Sierra Vista area’s population doubles as predicted over the next 25 years, pumping could affect the river directly east of town. In that northern end, the water table is a little more than nine feet underground. Some willow trees are already showing signs of stress that could be caused by pumping or drought, says Julie Stromberg, an Arizona State University associate plant ecologist; another three-foot drop could kill many of them. The Arizona Department of Water Resources says that water-level declines have caused the loss of wetland plants and prevented cottonwood seedlings from surviving in parts of the northern end of the river.

The Corps data means the river’s demise is at hand, warns Robin Silver, the Center for Biological Diversity’s board chairman. "We can’t wait around and do more studies," he says. "We now need to control the excessive groundwater pumping."

But Anderson says most aquifers near population centers in the United States are dropping, and "declining water levels are relatively common across the West."

The San Pedro’s flows have been dropping for many decades, according to the survey. An unfinished study, due for publication in six months, found that the river’s summertime minimum flows fell from 50,000 acre-feet to less than 10,000 acre-feet annually between 1910 and 2000. Anderson says it’s not certain why. In addition to groundwater pumping, decreased surface runoff from rainfall and from "return flow" from irrigated farm fields may be causes.

Then there are all those water-sucking trees. Over the past 50 years or so, tree growth has increased along the river, possibly due to the decline of agriculture, removal of livestock and natural downcutting of the river’s channel, Anderson says.

Though the scientific picture is complex, Anderson says that doesn’t mean action isn’t needed to reduce the groundwater pumping deficit. One option for the Sierra Vista area would be to find a basin to pump from that is farther away from the river, he says.

"With pumping, you may not see the effects for years," Anderson says. "It’s just that at some point, the price will be paid."