The first maps that track where the San Pedro River flows and where it is dry between Mexico to the Gila River are available. These maps establish an important baseline for evaluating the San Pedro, the last major free-flowing river in Arizona and Sonora, Mexico, & a vital source of water for nearby communities. One hundred and twenty (120) river miles were mapped, out of the total 174 miles that the river flows.
The San Pedro supports nearly half of the avian diversity in North America. About 250 species - including 1 to 4 million migrating songbirds - use the riparian habitat annually as they move between wintering grounds in Mexico and Central America & breeding grounds in the United States and Canada. It also provides habitat for 80 species of mammals, including the elusive jaguar.
The new maps reveal that overall, 39% of the river remains “wet” - that is, it flows year round - a rarity for river systems in the arid Southwest. However, some areas of the river have lost year-round flowing water & now struggle to support a wide array of plants and animals.
According to Holly Richter, director of The Nature Conservancy in Arizona's Upper San Pedro Program, the river’s future requires additional efforts on both sides of the international border, & these new maps can play an important role guiding these efforts.
“The fate of this unique river will be increasingly dependent upon regional water management decisions made by many different entities within both the United States & Mexico in the coming decade,” said Richter. “This mapping project complements more traditional types of hydrologic data, greatly enhances our overall understanding of the river system & helps inform decisions regarding water management issues affecting the San Pedro River.”
Created by a partnership of The Nature Conservancy in Arizona , the U.S. Bureau of Land Management (BLM) & the Community Watershed Alliance, the maps show where water remained in the river in June 2007, the month each year when river flows are expected to be their lowest. While the Conservancy & BLM initiated monitoring on portions of the San Pedro in 1999, this is the first year the maps include the majority of the river, beginning in Mexico & ending where it meets the Gila River. The maps also include portions of the Babocomari River, one of the largest tributaries of the San Pedro.
These maps show that approx. 51% of the length of the river in Mexico had water, 52% of the river within the Upper Basin had water, & 21% of the river in the Lower Basin had water. Overall, water was present in 39% of the length of the river for the 120 miles mapped.
Many factors, including human water demand & climate, can influence how much water is present in the river each year. These maps enable annual comparisons & insights into areas of flow improvement or areas of concern.
“These maps provide a snapshot in time during critical summer base-flow conditions,” said Brian Bellew, the BLM's Tucson field manager. “We now can study the relationship between surface & groundwater, & the response throughout the San Pedro watershed to changing conditions. This helps us make the best land management decisions we can, so that we can fulfill our obligation to conserve, protect, & enhance the valuable resources of the San Pedro Riparian National Conservation Area.”
Dozens of volunteers including local citizens, landowners, academic researchers, ranchers, Salt River Project employees & the Community Watershed Alliance hiked or rode horseback along the river during a single day to collect data on where the river flows. Participants used global positioning system (GPS) units to identify the locations where water begins & ends over a total of 120 miles of the river. In the Benson area, the University of Arizona NEMO Program partially supported this work through a grant.
“This is a wonderful opportunity to build community partnerships while collecting factual information that can be used by us all,” said Mary McCool, executive director for the Community Watershed Alliance.
The maps are accessible at The Nature Conservancy’s Arizona science site at, azconservation.org or the Community Watershed Alliance site at, cwatershedalliance.com.