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Important San Pedro River Subflow Adjudication Ruling!

By Order dated July 13, 2017, Maricopa County Superior Court Judge Mark H. Brain (presiding judge in the Gila River adjudication) affirmed the most recent effort by the Arizona Dept. of Water Resources (ADWR) to delineate the lateral extent of certain hydrogeologic areas known as subflow zones in the San Pedro River watershed in southeastern Arizona. A well located within a subflow zone is presumed to be pumping subflow, which is water that is hydraulically connected to the surface flow of a perennial or intermittent stream.

This ruling stems from technical reports and oral testimony presented before and during a four-day long hearing conducted in Phoenix in August-September 2015. It is the latest chapter in a multi-decadal effort to differentiate between surface-water and groundwater in Arizona. The San Pedro River, the Babocomari River and Aravaipa Creek were test cases for this issue; if courts bless a methodology there, it likely will be applied to other perennial and intermittent streams within much of Arizona where surface water rights are being adjudicated.

Adjudicating Subflow – More complicated than anticipated. Since 1988, the Gila River adjudication court has been involved in developing a test to determine whether a well is pumping subflow based on its location and certain hydrogeologic factors. In 2000, the Arizona Supreme Court held that a well located within the saturated floodplain Holocene alluvium (FHA) of a perennial or intermittent stream (known as a subflow zone) is presumed to be pumping subflow rather than groundwater. Since then, water users, lawyers, geologists, hydrologists, and judges have been wrestling with how to delineate the subflow zone for the San Pedro River. In general, the combatants have broken into 2 camps: the surface-water users, who argued for a wide subflow zone to maximize the number of wells that would be subject to the adjudication, and groundwater users, who argued for a narrow subflow zone so that fewer wells would be impacted. ADWR has occupied a difficult neutral position as technical advisor to the adjudication court – imagine barrages of slings and arrows from both sides. Delineating the lateral extent of FHA proved to be much more difficult than was originally envisioned by the Arizona Supreme Court, and has involved numerous reports from ADWR and several evidentiary hearings with multiple parties.

A Role for Geology and Geomorphology. The AZGS has provided substantial technical assistance in this process since 2007, when ADWR contracted with us to map the lateral extent of Holocene river alluvium and other geologic units along the river corridors. In 2009, we released geologic strip maps that accurately depict the geologic framework of the entire San Pedro River and its major perennial tributaries, Aravaipa Creek and Babocomari River (AZGS DM-RM-01).

Holocene river deposits
Holocene river deposits

Surface contacts between Holocene river alluvium and bounding geologic units were the primary data used by ADWR staff to define the lateral extent of FHA in maps released in 2009. After critical comments from technical experts and legal teams, this approach was rejected by the courts in 2012; the reasoning was that surface contacts underestimated the lateral extent of FHA in the subsurface (see schematic diagram below).

In response to that ruling, ADWR contracted with us again in 2013. This time, we visited and documented stratigraphic exposures that shed light on the lateral extent of FHA in the subsurface (AZGS OFR-15-02). We also developed an overview of the geology and geomorphology of San Pedro Valley, focused on the nature and timing of river and tributary erosion and deposition that has shaped the river valley (AZGS Special Paper 10). These geologic investigations provided a conceptual framework for understanding river behavior in the late Pleistocene and Holocene (the past 50,000 years or so), and provided important insights into how FHA is likely distributed in the subsurface of the valley.

The Result. There is unavoidable uncertainty when attempting to estimate the extent of FHA in the subsurface. With our assistance, however, ADWR staff developed a new, more realistic delineation of the extent of FHA that has passed the latest test of thorough judicial review. It is uncertain whether any of the parties will seek review by the Arizona Supreme Court of the Gila River adjudication court’s ruling.

Phil Pearthree