Many of the older canals that cross the valley follow the same path as the Hohokam ones. To what degree is that true?Ī: The earliest canals that Anglo-Americans used in the lower Salt River valley were dug-out Hohokam canals. Q: I have read that many of the CAP canals that pass through the Phoenix area make use of the Hohokam ancient canals. They likely had to be replaced annually or after flood events. Some kind of plug or gate may have been placed in the opening when river water was not needed, such as during canal cleaning. An opening was at the center to allow water to flow through. Around this was fill (pottery pieces, rocks, and other items) sandwiched between retaining walls and faced with interwoven branches on the sides of the canal. Those gates had logs or thick branches placed vertically to act as support posts. The stratigraphy in large main canals shows that the Hohokam were likely regulating water flow into the irrigation systems. Q: Wondering about the mechanics of headgates, what are these made of? How managed? Did flooding damage temporarily/permanently?Ī: Hohokam headgates were long destroyed by the time Anglo farmers began to irrigate. These kinds of dams were constructed into the twentieth century in parts of the Phoenix Basin. Q: Any indication of damming to manipulate river levels to facilitate irrigation?Ī: The Hohokam likely used weir dams that were made from boulders and other fill items with a wickerwork of logs and branches. The flow of the Salt River is influenced by the El Niño Southern Oscillation (ENSO). Gary Huckleberry and others have discussed the Salt and Gila rivers being subject to different climatic regimes. The streamflow reconstruction for the Gila River seems to have more issues with long periods of low flow (droughts) than the Salt River.
Water became more problematic during the early Classic Period (1100/1150–1300 CE), but the cause seems to be that farmers in the valley had reached the upper limits of what could be irrigated consistently, in terms of the number and size of irrigation systems that may have operated. Q: How do extreme drought events also factor into the timing of social changes during the important transitions that you have pointed out?Ī: My water sufficiency analysis indicated that some water reached the lower Salt River valley through time, based on the tree-ring work by Donald Graybill and others. Keith Katzer is the first to my knowledge to propose an extremely large flood significantly shaping Hohokam society this early in the Hohokam history. How did you determine the high levels of floods for the Salt?Ī: Jonathan Fuller (U of A thesis 1987) determined that a very large flood likely occurred around 900 CE based on flood deposits in Hohokam canals and the height of flood deposits in the Salt River canyon that appeared to have the same dating (slackwater analysis). Record levels on the Colorado River was 500,000 CFS. Q: The level of flooding on the Salt is extraordinary to me. Approximately every 14 years, floods of 100,000 cfs occurred, damaging earthen canals. Those floods destroyed weir dams and headgates. As a point of reference, before the construction of the Roosevelt and other dams along the Salt River, annual flooding was 8,000–12,000 cfs. The late Colonial/early Sedentary extremely large flood may have been as much as 400,000–450,000 cfs.
The floodwaters extended two to three miles from the riverbed. If the 1891 flood provides insights for the extremely large flood, then it is conceivable that the Riverview and Scottsdale systems located their intakes where they did because the river had dramatically shifted its main channel.Ī: The peak of the 1891 flood (February 19 th) was 300,000 cubic feet per second or cfs. 900 CE), a flood deposit is found covering a canal. In a few cases, like the late Colonial/early Sedentary extremely large flood (ca. It is extremely difficult to separate individual flood or channel shift events in the depositional history of rivers, including the lower Salt. The archaeological record provides us with a general view of the past, so we rely on finding patterns or exceptional events. River aggrading, as it is called, appears to have been significant along the middle Gila River in the past, but is less well understood along the lower Salt. Q: Is there any evidence of that changing channels in the Salt River that may relate to how canals were constructed and maintained?Ī: The lower Salt River is known to have shifted parts of its main channel historically, because of flooding. Explore additional content related to The Flow of Water and Time: Irrigation Longevity and Social Change among the Lower Salt River Hohokam, Chris Caseldine’s November 10, 2020, Archaeology Café Online presentation.