San Pedro River Geology – Implications for water law

The Arizona Geological Survey has just released a 78-page report and six maps of the Quaternary geology of the San Pedro River system. According to the survey, “The geologic maps, which encompass a two-mile wide swath centered on the river, provide foundational geologic data, i.e., context, for deciphering the past 10,000-year history of the San Pedro river system. Stakeholders – water managers, civil authorities, ranchers, wildlife biologists, archeologists, environmentalists, and the Arizona public – should find these maps indispensable in considering the physical and biological character of the San Pedro riparian corridor.” Portions of Aravaipa Creek, and the Babocomari River are included in the study.


The report goes into great detail defining the characteristics of deposits associated with the river system. This helps define the floodplain and the deposits which carry the subflow of water along the river beneath the surface.

The mapping was done principally to provide better information to the Arizona Department of Water Resources. Since the 1930s, there has been a legal controversy about whether or not the subflow through the river sediments is part of the river system or the groundwater system. Each is adjudicated differently. The geological survey provides no opinion, just the science that hopefully will aid ADWR and the courts in coming to a decision.

Besides geology, the report provides interesting archaeological information about artifacts from inhabitants along the river from almost 12,000 years ago to historic cultures.

Some excerpts from the report (my comments in italic):

River deposits commonly consist of two fairly distinct phases: channel deposits dominated by sand and gravel, and overbank deposits found on floodplains and terraces that are composed of fine sand, silt and clay with minor gravel. River channel deposits are distinguished from tributary deposits based on the presence of well-rounded pebbles and cobbles composed of diverse rock types derived from upstream areas along the river. Tributary deposits typically have less diversity of rock types, gravel tends to be more angular, and in many areas, tributary deposits have less silt and clay. Overbank deposits associated with the river typically are thicker and more laterally extensive than fine-grained tributary deposits, although deposits of large tributaries may be quite similar to river deposits.

 Several archaeological investigations have been conducted of latest Pleistocene Paleoindian sites in the upper San Pedro Valley. These sites include the remains of large mammals such as mammoth, and spear points and other human artifacts, indicating the presence of big-game hunters in association with late Pleistocene fauna. Radiocarbon ages obtained from these sites date to about 12,000 years ago.

The Paleoindians referred to are the Clovis people, so named because their distinctive spear points were first identified near Clovis, New Mexico.

The mountains delineating the San Pedro drainage basin generally are north-to-northwest-trending fault-block ranges of diverse lithology formed by extensional forces during the Basin and Range disturbance from about 25 to 8 million years ago. As the basin was forming it was synchronously being filled with sediments eroded from adjacent mountain blocks.

 These sediments later became filled with water during the glacial epochs of the last two million years.

Initially, these sediments were shed into one or more closed basins in the San Pedro Valley, resulting in a playa to pluvial lake-like environment (analogous to modern Willcox Playa) represented by extensive playa, lacustrine, and marginal playa to distal fan facies presently exposed in the lower San Pedro Valley.

(Geology speak for the next paragraphs: Pluvial = referring to the action of rain; Lacustrine = referring to lakes; Fluvial = referring to rivers)

These lake beds are the tan to pink layered sediments seen along I-10 near Benson, and along Route 80 between St. David and Tombstone.

Prior to the late 1800’s the San Pedro River was a relatively low-energy, unentrenched fluvial system with extensive marshy reaches, or cienegas.

During early historical times the San Pedro was known to be home to beavers, antelope, and numerous species of fish. In some areas a distinct channel was absent and marshy grasslands dominated. Beginning in the 1870s there was modification of the floodplain due to ranching, mining, and the subsequent construction of railroads. Many of the cienegas and beaver ponds were purposely drained to reduce the mosquito population in an attempt to prevent the spread of malaria. By about 1930 most of the San Pedro had been transformed from a low-energy, unentrenched fluvial system with a broad floodplain to a higher energy, in some places deeply incised channel.

Although these factors may have contributed to the widespread arroyo cutting observed along many rivers in the southwest, including the San Pedro, multiple episodes of arroyo cutting and subsequent backfilling occurred prior to the land-use changes associated with the most recent episode of incision and can only be attributed to natural variations in climate, vegetation, and wet-dry cycles.

Prior to historical entrenchment of the San Pedro River, vegetation in the valley consisted of tall range and marsh grasses largely void of woody shrubs and mesquite. Since the development of modern San Pedro Valley conditions, mesquite and acacia have become common on the piedmont and dense mesquite bosque is common on pre-entrenchment floodplains.


If you are interested in geology, archaeology, water supply, or your position on the floodplain, this is worth a read.

The report and maps may be downloaded, without charge, from AZGS website here:

The file DM-DF-1 contains the report. Six other files DM-DF-1A through DM-DF-1F contain preview maps and detailed, high resolution maps of six segments of the river system.

If you want more information on water rights adjudication, start at this ADWR site:

A gentle slope leading from the base of a mountain to a region of flat land.


  1. There is a really good 11″ x 17″ map available at:

    Now I need to find one for the west side of Pima county so I can see the names of the rest of the mountain ranges I look at every day.

  2. Unfortunately, the mapping answers the wrong questions. Many aquifers are hydrologically connected to rivers, and pumping directly or indirectly affects river base flow. Failing to analyze the entire hydrogeologic system fails to protect rivers or provide accurate and fair adjudication. (Adjudication based on current water law is patently unfair as it’s not based in reality.) The information provided, while interesting, fails to further any real protections for our state’s water resources.

    1. Your statement “many aquifers are hydrologically connected to rivers” is true for temperate climates, but not the desert.

      Most of our natural recharge of aquifers comes from monsoon rains flowing out of the mountains. This water seeps into the ground near the mountain fronts and flows underground through permeable horizons on top of the pediment, to the aquifer in the central valleys.
      Recharge from stream beds is minimal because most of the rivers are underlain by impermeable clay or caliche. Water in streams generally cannot penetrate to the main aquifer. Instead, this water either continues flowing northward or makes shallow, perched aquifers which contribute nothing the our pumped water supply.

  3. I was researching this site to see if there was any mention regarding the effects the May 6th 1887 earthquake originating from Mexico had on the San Pedro. The only account I have unearthed so far is a repost of a news report that can be accessed from the following source:  
    I vaguely remember someone telling me that the San Pedro ran underground as a result of the quake, but I have not been able to verify this or any of the other eyewitness accounts in the article.

    Any thoughts?

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