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August 18, 2016

Streamflow and groundwater levels are declining in some locations along the San Pedro River near Sierra Vista, Arizona, according to a new report by the U.S. Geological Survey.

Future groundwater discharge into the San Pedro River and the adjacent river bank, or riparian area, will likely continue to decline in some areas. In other areas, however, water-management measures have resulted in stable or improving hydrologic conditions. 

Map of the Upper San Pedro Basin, the Sierra Vista Subwatershed, and the San Pedro National Conservation Area, southeastern Arizona.

Scientists looked at the Sierra Vista subwatershed, which includes a majority of the San Pedro Riparian National Conservation Area. This critical ecosystem provides habitat for half the bird species found in the United States. The 30-mile stretch of the San Pedro River and riparian area supports 389 species of birds, 84 species of mammals and 47 reptile and amphibian species. Groundwater is an essential component in sustaining the streamflow and habitat of the San Pedro River ecosystem.

The new USGS report represents one of the few studies assessing groundwater sustainability on a subwatershed scale. Scientists evaluated many aspects of the Sierra Vista subwatershed system, including regional and riparian area groundwater levels, streamflow levels in the San Pedro and Babocomari Rivers and rates of groundwater flow into springs. While some areas of the subwatershed appear to be experiencing sustainable conditions, others are not. The full USGS study is available online.

“Results from the northern part of the Sierra Vista subwatershed suggest unsustainable conditions currently exist,” said USGS hydrologist Bruce Gungle. “However, long-term monitoring is the first step in helping resource managers make informed decisions. Water-management actions have resulted in positive, sustainable effects in some areas along the San Pedro River, such as ending agricultural pumping in the Hereford area and recharging treated wastewater into the aquifer near Charleston.”

The Upper San Pedro Basin extends from around the town of Canena, Mexico to about 11 miles north of Benson, Arizona. The Sierra Vista subwatershed is an area within the Upper San Pedro Basin that is surrounded by the Huachuca Mountains and the Mule Mountains and Tombstone Hills.

“Pumping throughout the Sierra Vista subwatershed, which has been most intensive in the Fort Huachuca and Sierra Vista areas, captures groundwater that would otherwise go to the river,” said Gungle. “The hydrologic data indicate that pumping has resulted in decreased groundwater flow toward some reaches of the San Pedro River and a tributary, the Babocomari River.” 

This report was prepared in cooperation with The Nature Conservancy, the Bureau of Land Management, Cochise County, the City of Sierra Vista and the U.S. Department of Defense.

Photograph showing San Pedro Riparian National Conservation Area.
Photograph showing San Pedro Riparian National Conservation Area, residential development southeast of Sierra Vista, Arizona, and the Huachuca Mountains from Hereford Road, Cochise County, Arizona. Thomas Porter, USGS. Public domain

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