Jeff Wynn and Mark Gettings, US Geological Survey

Abstract

In 1877 the US Army established Fort Huachuca in southeastern Arizona, in part to command the new southern frontier of the country but also to protect the Tombstone and Bisbee gold and copper mining districts. Shortly afterwards, the town of Sierra Vista sprang up outside the Fort's eastern gate. Water for the new settlers and soldiers came from wells dug by hand, and the groundwater was recharged by the late summer to early winter monsoon rains. About 20 miles to the east, the San Pedro River flowed northward from the Mexican border. During the 1920's agriculture sprang up along the river, and together with mines on the Mexican side of the border began to divert water from the San Pedro River. Gauging station records show that by the 1950's, surface water flow on the San Pedro River dropped to less than half its pre-1920's levels.

The San Pedro River is an area rich in biological diversity, and lies on a well-known migratory corridor for North American birds that winter in Central America; for example, 15 different species of hummingbird have been identified at nearby Ramsey Canyon. In 1988 the US Congress established the San Pedro River Riparian Conservation Area along parts of the river adjacent to Fort Huachuca. Currently, a controversy exists between environmental groups, the US Army, and the City of Sierra Vista about whether their withdrawal of water has impacted the surface flow on the river and the viability of the Conservation Area. Hydraulic modeling and well-production records (Corell, and others, 1996) suggest that recharge of the aquifer beneath Fort Hucahuca is about 7,000 acre-ft per year, while the total annual water withdrawal may be as high as 11,000 acre-ft. Most of those who have studied the issue agree that there is a significant ground-water withdrawal deficit in the vicinity of the Fort and Sierra Vista. Some isotopic evidence acquired during the early 1990's, however (D. Pool, USGS-WRD, Tucson, AZ, oral communication, 1997), has suggested that most of the water in the San Pedro River comes from the Tombstone Hills to the east, rather than from the Huachuca Mountains behind the Fort on the west of the river. There also appears to have been major withdrawal of water from the river by mines on the Mexican side of the border (Corell and others, 1996).