Geology and Geomorphology of the San Pedro River, Southeastern Arizona- special report released

AZGS geologist Dr. Phil Pearthree is testifying this week at

a new round of San Pedro River Subflow hearings to decide the extent of the subflow zone, in response to a long-standing judicial determination that river subflow and groundwater are different resources.

Phil is explaining our findings and conclusions regarding the geology and geomorphology of the San Pedro River, and the implications of our work for the potential extent of Holocene river deposits in the subsurface. The objectives of our work were to provide basic data and interpretations to ADWR to assist them in their efforts to delineate the lateral extent of the Holocene river alluvium, which is used as a proxy for the extent of the subflow zone.

AZGS released a report on geology and geomorphology of the San Pedro River earlier this summer.

The San Pedro River flows through the rugged landscape of San Pedro Valley from the U.S. – Mexico border to its confluence with the Gila River at Winkelman (Figure 1). The river has incised (downcut) moderately to deeply into the valley floors that were accumulating sediment until the past few million years or less. River incision has in turn driven incision of all of its tributaries and erosion of the surrounding landscape. The inception of river incision resulted in a dramatic narrowing of the extent of river deposits, and this trend has continued to the present. As the river has incised, it has carved an erosional trough in older basin-fill deposits and bedrock, and has left behind remnants of older river deposits (river terraces) that record previous levels of the river. During the Holocene (the past ~10,000 yrs) the river has operated within a relatively narrow corridor bounded by eroded basin deposits, older river terrace deposits, tributary deposits, and bedrock, so all Holocene river deposits are restricted to this corridor. The primary purpose of this paper is to summarize the geologic setting and the geomorphic development and evolution of San Pedro River during the middle and late Quaternary. The latter point is particularly important for understanding the physical setting of Holocene San Pedro River deposits. This work was undertaken to assist Arizona Department of Water Resources staff in their efforts to delineate areas associated with river subflow in the San Pedro River Watershed.

Ref: Pearthree, P.A. and Cook, J.P., 2015, “Geology and Geomorphology of the San Pedro River, Southeastern Arizona,” Arizona Geological Survey Special Paper #10, 23p, http://repository.azgs.az.gov/uri_gin/azgs/dlio/1633