Overview

THE SAN PEDRO RIVER BASIN

“For the space of an afternoon we’re sheltered from the prickly reality of the desert where we live. Most human visitors to the San Pedro appreciate it for about the same reasons they value gold: It sparkles, and it’s rare.”

-Barbara Kingsolver [1]

The San Pedro River Basin straddles the United States-Mexico border, extending from its headwaters in northern Sonora, Mexico to the southeastern corner of Arizona. Flowing north for approximately 150 miles until it joins the Gila River, the San Pedro is not a large river by any means. At points it is only three feet across, if the flow is visible at all. However, despite its small scale, it’s ecological impact is huge. The river hosts almost 400 species of birds, about half of all the species seen in North America, at some point in their lifetimes. Eighty-three species of mammals and forty-seven species of reptiles and amphibians rely on this freshwater oasis. The San Pedro River is second only to rainforests in Costa Rica in biodiversity counts in North American.[2]Amid the water-parched scrub of most of the Southwest, “it’s a sparkling anomaly for sunstruck eyes, a thread of blue-green relief.”[3]

The ecological significance of the San Pedro was federally recognized in 1988 with the creation of the San Pedro Riparian National Conservation Area that covers about a third of the river’s length.[4]Along with the protection of the riparian corridor, Congress also reserved “a quantity of water sufficient to fulfill the purposes of the San Pedro Riparian National Conservation Area”.[5]

However, despite this legal protection, the San Pedro River is drying up.

Groundwater withdrawals in the San Pedro Basin to support mining, farming, and development are depleting the underlying aquifer at an alarming rate. The San Pedro River relies on both groundwater discharge from the aquifer as well as surface discharge, so groundwater withdrawals from towns like Sierra Vista, (ten miles from the river) can significantly reduce the river’s flow.[6]

At the current rates of depletion, the San Pedro could face the same fate as many other Southwestern rivers, such as the nearby Santa Cruz in Tucson, that have lost their permanent flow due to dams, irrigation, land-use change, and extraction of groundwater. Demand in the Upper San Pedro Basin alone is expected increase from 18,000 acre-feet in 2005 to 24,000 acre-feet in 2030. Most of this demand, 84%, is from municipal and private use, while industrial and agricultural uses accounted for about 8% each in 2005. Agriculture in the Upper Basin has been decreasing since 2005 and is projected to continue to decline to about 5% of demand by 2030 due to the retirement of irrigated lands. Although artificial recharge is projected to increase from 2005 to 2030 due to municipal effluent recharge, projected increases in municipal and industrial use suggest that the annual overdraft will increase from 6,325 to 9,365 acre-feet per year.[7]Over this 25-year period, that would mean an estimated 160,000-234,000 acre-feet reduction in the aquifer. While that accounts for less than 0.01% of the total aquifer estimated volume, the heavy pumping of groundwater in certain areas creates cones of depression that lower the level of groundwater in the surrounding area. This is the case in Sierra Vista, where the cone of depression has reduced the depth to groundwater by -15 to -20 feet, which has caused some portions of the historically perennially flowing San Pedro to go dry.[8]

The debate around groundwater resources in the San Pedro Basin are decades old, yet fiercely ongoing. In August of 2018 the Supreme Court of the State of Arizona issued a decision (Silver et Al. v. Pueblo Del Sol Water Co. et al.) allowing for continued development within Sierra Vista, “even though the court acknowledges the area’s future water supply is uncertain.”[9] As Chief Justice Bales points out in the dissenting opinion “Arizona law still adheres to the legal fiction that surface water is hydrologically separate from groundwater,”[10]Although the San Pedro Conservation Areas’ water rights are guaranteed by federal law that established the conservation area, the law at this time is unable to fulfill this obligation, allowing for the continued depletion of the aquifer.

[1] Kingsolver. “The Patience of a Saint -- San Pedro River”. National Geographic. April 2000. http://www.eebweb.arizona.edu/Courses/Ecol406R_506R/SanPedro_NG_Kingsolver_smaller.pdf
[2] Glennon, R. J. (2002). Water follies: Groundwater pumping and the fate of America’s fresh waters. Washington, D.C.; Washington, D.C.: Island Press; Island Press. pg 53
[3] Kingsolver. “The Patience of a Saint -- San Pedro River”
[4] San Pedro River. Center for Biological Diversity. “San Pedro River” https://www.biologicaldiversity.org/programs/public_lands/rivers/san_pedro_river/
[5] Silver et Al. v. Pueblo Del Sol Water Co. et al. (Ariz. 2018). Retrieved from https://law.justia.com/cases/Arizona/supreme-court/2018/cv-16-0294-pr.html pg. 26 ¶62
[6] Glennon, Robert. Water Follies. pg 55.
[7] Arizona Department of Water Resources. Upper San Pedro Basin Water Resources Draft Plan Appendix. 2009. Pg. 50.
[8] Ibid.
[9] Valdez, L. (2018, Aug 10,). This Arizona supreme court ruling could hurt a lot more than sierra vista's water supply. AZ Central Retrieved from https://www.azcentral.com/story/opinion/op-ed/lindavaldez/2018/08/10/sierra-vista-supreme-court-water-ruling-change-Arizona-groundwater-law/962314002/Commons Theory

[10] Silver et Al. v. Pueblo Del Sol Water Co. et al. (Ariz. 2018). pg. 26 ¶61