Summary of Isotope Research on Sources of Perennial Flow in the San Pedro River
Some major points of the study are:
- The reaches of the San Pedro River that have perennial (year-round) surface water are critical to the existing rich riparian ecosystem. Management of the river basin with the best interests of the ecosystem in mind requires an understanding of the sources of the perennial water. Five reaches with perennial water between the international border and Redington were studied (see map).
- Measurements of stable oxygen and hydrogen isotopes enable us to distinguish three sources: (a) water flowing from upstream in or just beneath the river channel; (b) summer monsoon floodwater stored in the bed and banks of the river; and (c) water from the mountainous flanks of the valley.
- Perennial flow in the reach south of Sierra Vista (Area 1) is a combination of sources (a) and (c).
- All three sources contribute to the perennial reach near St. David (Area 2), where the source (c) is deep basin groundwater.
- In Cascabel, near Benson Narrows (Area 3), source (c) predominates. The water is probably from Red Rock Creek.
- At Cascabel, near Gamez Road (Area 4), sources (a) and (c) supplied surface flow that had disappeared by 2019. Source (c) was from Hot Springs Canyon.
- Perennial flow near Bingham Cienega at Redington (Area 5) prior to the early 2000s was of source (c) and appears to have flowed to the river through a limestone aquifer that has been quickly depleted under drought conditions.
- Impermeable sills of rock in the riverbed divide the valley into four separate groundwater basins that are becoming increasingly isolated from each other as perennial flow volumes decrease. Decrease in flow has occurred because of long-term drought in combination with pumping of groundwater.
- Increased urban growth upstream of Benson is likely to degrade the perennial reaches near Sierra Vista and St. David.
As we approach the one-year anniversary of working virtually, the WRRC Water RAPIDS team leans into collaborative watershed planning and research with our partners in the Cobre Valley from afar. In November 2020, the team co-convened the Third Annual Cobre Valley Water Forum alongside the Cobre Valley Watershed Partnership (CVWP) and Gila County Cooperative Extension.
UArizona Assistant Professor Courtney Crosson.