On June 30, 2009, the Arizona Department of Water Resources (ADWR) issued its "Subflow Zone Delineation Report for the San Pedro River Watershed." The report was completed per the request of the adjudication court in connection with the ongoing In re the General Adjudication of All Right to Use Water in the Gila River System and Source.

Overview and Deadline for Objections

The report presents a series of maps that delineate the subflow zones for the San Pedro and Babocomari Rivers and Aravaipa Creek. In addition, the report describes the criteria and analytical process used by the ADWR in compiling the subflow zone maps. As directed by the adjudication court, the scope of the report is limited to delineating the subflow zone and does not set forth proposed water right attributes for any individual water right claim or use.

Objections to the report must be filed by December 28, 2009, and must be limited to ADWR’s findings regarding the lateral extent of the subflow zone. After considering the objections, the adjudication court will approve a map that delineates the subflow zones within the San Pedro River Watershed.

Once the subflow map is approved, ADWR will apply the adjudication court's "cone of depression" test to wells located outside the lateral limits of the subflow zone and examine all water rights claims to determine de minimis water rights in the San Pedro River Watershed. ADWR will then publish a "Supplemental Final San Pedro River Watershed Hydrographic Survey Report," containing its findings and proposed water rights attributes on a claim-by-claim basis, including wells withdrawing subflow, "cone of depression" analyses, de minimis water rights, and all other new or updated information.

ADWR Subflow Zone Criteria and Analysis

As directed by the adjudication court, the subflow zone is defined as the saturated floodplain Holocene alluvium. "Holocene" describes a geological period that began approximately 10,000 years ago and continues today. In determining the subflow zone for the report, ADWR examined hydrologic criteria, geologic criteria, and hydrogeologic criteria. ADWR also followed the procedures outlined in its 2002 "Subflow Report" to determine the location of perennial and intermittent streams, the lateral extent of the floodplain Holocene alluvium, and the saturated portion of the floodplain Holocene alluvium.

Hydrologic criteria considered by ADWR include the following:

  • perennial, intermittent, and ephemeral streams, as defined by the adjudication court's June 30, 1994, order
  • historical and current documents, scientific reports, mapping projects, aerial photography, and field investigations to locate perennial, intermittent, and effluent-fed streams
  • predevelopment streamflow conditions, including predevelopment streamflow identifiable for a year or range of years immediately prior to regular diversion or depletion of streamflows caused by human activity

The report provides a map with a graphical representation of perennial and intermittent streams. ADWR's hydrologic analysis based on this criteria describes the natural streamflows of the San Pedro River Watershed before development of the area and then describes the change in the streamflows over time, including diversions for early settler irrigation ditches and an ore mill. ADWR also considered 1935 aerial photographs of the watershed and surrounding communities. From this analysis, ADWR developed and included various graphs of predevelopment streamflow conditions.

Geologic criteria considered by ADWR include National Resources Conservation Service soil survey maps that delineate the lateral extent of the floodplain Holocene alluvium. ADWR, at the instruction of the adjudication court, limited its analysis to the floodplain Holocene alluvium. Other deposits or materials within the floodplain are reported, for example, sediment from the Pleistocene age, but the criterion for determining the subflow zone was the floodplain Holocene alluvium. The geologic criteria examined by ADWR enabled it to determine the lateral extent of the floodplain Holocene alluvium. ADWR's geologic analysis examines the sedimentary basins in the watershed and how channel conditions have changed over time. An examination of various soil samples was necessary to distinguish Holocene alluvium from other materials, such as Pleistocene alluvium. The analysis resulted in a series of geologic maps, included in the report.

Hydrogeologic criteria considered by ADWR required the assumption that the entire lateral extent of the floodplain Holocene alluvium is saturated for the purpose of delineating the subflow zone. In addition, per the instruction of the adjudication court, ADWR excluded any tributary aquifers, areas of basin fill recharge, and the alluvial plains of ephemeral streams from the subflow zone. The hydrogeologic analysis includes a discussion of the potential impact of hydrostatic pressure on the subflow zone.

In addition, in the event ADWR could not delineate the subflow zone of a specific area based on the above criteria or analyses, then it could use any other appropriate criteria for the specific area. ADWR developed criteria apart from those described above and analyses for a limited number of locations within the San Pedro River Watershed.

Ultimate Determination of the Subflow Map

ADWR prepared the subflow zone delineation maps by combining the geologic maps that show the extent of the floodplain Holocene alluvium with the graphs that compare evidence of predevelopment streamflow conditions. To map the subflow zone, the lateral extent of the floodplain Holocene alluvium was adjusted by 100- and 200-foot setbacks to account for side recharge.

ADWR divided the subflow zone into various quadrangle maps to show greater detail. The entire subflow zone for the San Pedro River is depicted on 21 quadrangle maps, and the Babocomari River and Aravaipa Creek subflow zones are depicted on three quadrangle maps and six quadrangle maps, respectively. These maps, at times, depict a subflow zone that might be larger than expected. For example, in the detailed quadrangle map for the area in and around Benson, Arizona, the delineated subflow zone extends underneath a portion of the city.

The Gila River  Adjudication and Your Rights

The Gila River Adjudication began in the mid-1970s when the Salt River Valley Water Users' Association filed a petition to determine its rights within the Salt River Watershed. Other large water users followed with similar petitions, and in 1981 the Arizona Supreme Court consolidated all the adjudications under the caption Gila River Adjudication. More than 60,000 claims have been filed in the Gila River Adjudication, which is to determine the rights of all persons to use the waters of the Gila River system and its sources pursuant to A.R.S. § 45-251 et seq. The Gila River Adjudication covers more than half the State of Arizona. A similar, but somewhat smaller adjudication, is taking place in the Little Colorado River Watershed in northern Arizona.

The report could adversely affect your rights to pump water within the San Pedro Watershed if ADWR includes your current or future wells in the subflow zone. All surface water, including subflow, is subject to the adjudication court’s jurisdiction.   If your well is included in the report and you are not already participating in the Gila River Adjudication, the court may serve a summons to advise you that your water use is within the court’s jurisdiction. If you do not file a Statement of Claimant in response to the summons, you may lose your right to pump the water you are currently using. This can happen even if you already have received a permit, certificate, or other approval from ADWR.

If your wells are included in the report, you have until December 28, 2009, to file an objection to the report. It is vital to participate in any proceedings that may affect your water rights.

For more information, please contact any member of the Environment and Natural Resources Group.

 


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