The Middle San Pedro: Benson Sub-area

[From Near Fairbank to The Narrows]

Below: looking southeast toward the Mule Mountains from near Fairbank, Arizona: gallery forest of the Babocomari Stream drainage in the foreground, running from lower right to middle-left; San Pedro River Cottonwoods in the farther distance, both rivers running from right to left. (July 2002)

Below left: the San Pedro River floodplain photographed (looking north) in July 2002 at the terrace site of the ruins of Santa Cruz de Terrenate, the Spanish Fort built in 1775. See that link for a brief history of this substantial, high-walled enclosure (complete with both a chapel and a triangular bastion for cannon emplacements), a presidio which the Spaniards founded as a permanent base for subduing roving predatory Apache bands who were then dominating the area. Instead, the Apaches routed them using guerrilla tactics without loss to themselves, forcing the Spaniards to abandon the entire post after only 4 years. (Note the railway in the foreground, which dates from the 1880s mining era., and The river floodplain a short distance beyond it.) (Click on each image to enlarge it.)

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The western bajadas of the San Pedro River south of Benson stream out of the Whetstone Mountains in long straight lines of alluvium (see image below, lower-central to left). This photo also nicely shows the tilted fault-block shape of these Basin-range mountains:

The view above is from Kartchner Caverns State Park, looking from the bajada toward the southwest, where the escarpment tilts down from its apex, toward the west. (Click on the image above for a close-up of the tilted formations.) An informative video presentation of the geological and hydrological history of this part of the San Pedro River Valley may be viewed at this link: Kartchner Caverns Hydrology.

Below: the "pink cliffs" surrounding Benson (here, viewed from the west-side terrace, looking east across the San Pedro River floodplain, with the Little Dragoon Mountains in the background; taken near sundown in October 2002). These pink cliffs are sedimentary remains of the St.David Basin Lake of Pliocene times: (before the Gila River captured the San Pedro Valley basins, which had previously contained several interior-basin lakes) .(Click on image below for a closeup view from the base of the terrace.)

The sediments originally deposited in this extensive lakebed (see the "silt and clay" layers in the cross-section dispayed in the USGS diagram above) consist of as much as 900 feet of clays, silts, and freshwater limestone, and they tend to confine the vertical movement of ground water in the basin, creating aquifer conditions that sustain the distinctive artesian wells found near the town of St. David. (See Thomas Blakemore, cited in Geology Credits.)

The city of Benson lies on the western edge of the river, and forms the main vehicular intersection of the area. The largest city in the Middle San Pedro Watershed (with a population of ca. 4900 in 2005),its numbers may well increase to more than 50,000 by 2025. Groundwater is its primary source of water and hence anticipated increases in groundwater pumping will soon threaten quality of life (including riparian habitat). Sources of decline in streamflow of the San Pedro River here also include changes in upland land-cover and riparian vegetation. Quality of water remains good for domestic, governmental, and commercial uses (though there a 9-square mile area of contaminated ground water just SW of St. David (now designated as an EPA Superfund site). [Blakemore].As in the St. David area, the basin fill around Pomerene contains clay deposits from the ancient lake, so artesian wells are found here too, and these are much used by local farmers.

Though the city of Benson would not be directly affected by the building of either Alternative "D" or "K" on their northern edges, the indirect, longer-term impacts would of course be huge. We are not ourselves in a position to assess them, but we hope that citizen groups in and around the city will send us such assessments for inclusion in this page. For information on a community that would be directly affected by the building of Alternative "K", see this side-page for discussion including the relation of "K" to the Cascabel Road near Benson:

Side-link: The Community of Pomerene