FEATURE ARTICLE
- Aug.
30, 2004
Death of the San Pedro: Not if, but when
by Tony Davis
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."
