HOHOKAM
Figure 2. Phoenix Engineer Omar Turney’s 1929 map of Hohokam canals in Salt River Valley, Arizona composited with the Geological Map covering the same area. Legend: Qr - Holocene river alluvium, Qy - Holocene surficial deposits (Alluvial plains, fans, etc.), Qm – Late Pleistocene surficial deposits (alluvial fans, etc., with soil development), Tsv- Oligo-Miocene interbedded volcanic and sediments, Xg – early Proterozoic granite, Xms – early Proterozoic metasediments, Xm – early Proterozoic metamorphic rocks, Xq – early Proterozoic quartzite. Note that the Hohokam canal-builders had a preference for older alluvium with soil development (Qm).
by the 19th century or our era, these rivers were no longer “alive” all year.
Most of these hand-dug earthen canals are no deeper than a man is tall, and less than the width of a person’s outstretched arms. The Hohokam canals in the Tucson area range from less than a few hundred feet to a thousand feet long, whereas in the area around modern-day Phoenix, some of the 15 or so canals off the Salt River (see Figures 2 & 3) are eight times wider and twice as deep (Figure 4), because the soil there is easier to dig, and up to 20 miles long because the land is flat- ter. The over 500 miles of Phoenix-area canals are designed and constructed with a gradient of one to two feet per mile to sustain an erosion-safe flow velocity of about one to nearly three feet per second. They become narrower downgradient to maintain a nearly constant flow velocity. Consequently, the main canals when well-maintained may readily produce a base flow from shallow groundwater of about 25 to 75 acre-feet per day. The longest prehistoric canal identified along the Santa Cruz River (see Figure 2) is a 6.7-mile-long one that ran to the Marana Platform Mound site near the I-10 Marana Exit. The longest Hohokam canal known along the Salt River is about 27 miles long. Imagine the labor required to keep these canals alive, as they silt up quickly and need re-digging every other year or so. Only primitive hand tools made from stone, bones and trees, such as overhead-digging-sticks, were available for use - no metal drills, hoes, axes, hammers or shovels; no wheel barrows; no front-end loaders, backhoes or ditch-witches. No pulleys, gears or cables, and no theodolites or lasers, surveyor’s levels, tapes or chains.
Figure 3. Extent of Hohokam occupation. Author: Yuchitown, 2015. Creative Commons License.
www.aipg.org Jan.Feb.Mar 2021 • TPG 5
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