A shallow stream running between banks with shady trees leaning over the water.
San Pedro River The San Pedro River southeast of Phoenix, Arizona. © Harold E. Malde

Stories in Arizona

San Pedro River: 23 Years and Hundreds of Volunteers

Mapping water in the San Pedro is a labor of love.

2021 Wet-Dry Mapping

This year was one of the hottest driest years in the San Pedro River watershed. Still volunteers managed to map the 130 miles of the San Pedro River plus its tributaries. 

Last year, the river was somewhat wetter than the year before. Fifty-nine percent of the river—along 29.4 miles—in the San Pedro Riparian National Conservation Area were wet, an 8 percent increase over the previous year. To see maps of where there is water, please visit AZconservation.org. This year’s results will be posted this fall.

Citizen Scientists

Each year, more than 100 volunteers have worked along 300 miles of the San Pedro River and its key tributaries to map where the river has water and where it doesn’t.

The citizen scientists are equipped with GPS technology, cameras and plenty of water. They are rewarded with wildlife sightings and scenic river stretches. For many, it is an annual event and a labor of love for a threatened desert river that is almost unmatched on the continent for its importance to migratory birds and wildlife.

Group Effort

Dozens of partner organizations coordinate this massive effort that covers more than 300 miles of main stem and tributaries within the San Pedro River Basin. Partners include: The Nature Conservancy, Bureau of Land Management, National Park Service, Arizona Game and Fish, Community Watershed Alliance of Benson, Cascabel volunteers, and Mexican partners and landowners among others, and many local volunteers.

The effort extends from San Pedro River headwater streams in Mexico to its confluence with the Gila River, a key tributary of the Colorado River. Volunteers also map key tributaries of the San Pedro, including the Babocomari River, Aravaipa Creek, Hot Springs Canyon and many other smaller streams.

Water Snapshot

In 1999, volunteers mapped the first 50 miles of the U.S. length of the upper San Pedro River through the BLM’s San Pedro Riparian National Conservation Area. By 2007, the effort had expanded to mapping from the headwaters in Mexico all the way to the Gila River confluence, collecting data on an average of 130 of the 170 mainstem miles annually.

Most of the wet river miles occur in those first 50 U.S. river miles where much of the TNC’s water conservation and program work has been focused, and where the data shows reaches of the San Pedro gaining and maintaining wet river miles.

Since 2007 the percent of wet river miles across the watershed has ranged from 23 to 45 percent. Wet river miles at the higher end of that range followed several notable years where large floods coursed through the river.

In 2018, our 20th year of mapping, 23 percent of the river had surface water, the lowest number of wet river miles in our 20-year record. This low point is consistent with climate, water and drought data recorded throughout the Colorado River Basin. Records indicate early 2018 was the third driest in the Colorado River Basin historic record.

Two decades of mapping, however, shows that our conservation actions on the upper San Pedro are having a positive impact. The mapping information, along with USGS research, has helped TNC and its partners design an innovative regional recharge effort to collect stormwater and other water sources and put it back in the underground aquifer to replenish river flows.

Maps of past wet-dry mapping are available at azconservation.org.