Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 35Figure 4.4) divided it into 11 subwatersheds. Volumeof runoff, grouping of watersheds into connectionunits, and peak discharges for connection units wereprovided (Simons, Li & Associates 1982:Tables 4.22through 4.24). Peak discharge rates at four pointswere then presented for 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year storms (Simons, Li &Associates 1982:Table 4.28). The lowest rate was 910a/ft for a 2-year storm; the highest was 23,800 a/ft fora 100-year storm (e.g., covering the area from arroyorim to rim).To understand how these storms would affectagriculturalists with water control systems, this modelwas applied to two prehistoric systems (Lagasse et al.1984). During a typical rainstorm at Rincon 4 in thePenasco Blanco canal and head gate system used duringthe Pueblo adaptation, peak flow was estimated at0.09 em/so Discharge at the gate was 0.8 cm/s; therefore,it was capable of handling flow from a typicalsummer storm and smaller events. For longer storms,no peak higher than 0.08 cm/s is shown, with a peaktime between 30 and 50 minutes (Lagasse et al. 1984:Figure 11). At a second location in Werito's Rincon,a large reservoir was studied. It could collect waterand sediment from smaller storms without breaching,but larger storms would have brought too muchsediment, and water would have breached the walls.Would runoff from storms have been sufficientto water agricultural plots? Figures reported by H.Toil et al. (1985) and Lagasse et al. (1984) were usedby Gwinn Vivian (1992:52) to determine whetherPueblo gridded gardens would have been sufficientlywatered. His analysis suggested that current data onrunoff are insufficient to do more than allowspecUlation on the amount of water impounded byPueblo period water control systems. Their presence,however, testifies to a role within the subsistencesystem. Hopefully, Vivian will provide additionalinsights in his forthcoming evaluation of water controlfeatures in Chaco Canyon (in preparation, personalcommunication, 2003).Other water resources are available. On theplateau south of Chaco Canyon, Brand (1937c)documented a number of ponds; a spring (a reliableone on top of Chacra Mesa), seeps (located where theMenefee and Cliff House sandstones meet and clayprovides an impervious layer so that water flowslaterally); tinajas (holes or small basins); and charcos(puddles). Scour holes, located primarily in sidecanyons, also provide water. Waterholes on the mesabenches on the north would have filled during seasonalrains. Springs fed by seepage through the sandstonewould provide water on a more reliable basis.Historic white settlers found no permanent water tablein the canyon away from the underflow in theriverbed. There are a number of artesian basins southof the canyon, but seeps above the Mancos Shale wereprobably the most likely source of water. Some wellsproduced water that is hard and often quite salty(Brand 1937c:39-49).Windes (1987[I]:Table 2.3) summarized discharge-ratedata collected during 1985 and 1987 atseeps near Pueblo Alto and the precipitation ratescovering this period. These resources were probablyavailable prehistorically. He concluded that therewould have been a sufficient supply of water for anestimated population of about 100 to 200 people, butthat the water in the Gambler's Spring site (29S11971)was high in mineral and salts (Windes 1987[1]:37-42).One spring on Chacra Mesa has a good flow; it wasused historically by Navajo.An early study of water quality is reported byJudd (1964:10-12), who tested the water in the wellused by the National Geographic Society crew;floodwaters; Rafael's Well; and surface water nearKin Bineola. Results indicated that high percentagesof sodium were found, which suggests that the waterswould not easily penetrate the soils. The sample fromRafael's Well differed from the ones from the ChacoWash in that it contained a higher proportion ofcalcium. The water from Kin Bineola also containeda high proportion of calcium and was considered hardwater and usable for irrigation.Recent studies in the area indicate that specificconductance, which measures water quality, commonlyexceeds 2,000 "mhos (considered to be freshwater) but higher values (4,000 "mhos) are commonlyfound in the lower reaches. Stone et al. (1983:22)indicated that water with less than 1,000 "mhosnormally has sodium and sulphate as major constituents.Dam (1995) sampled water twice from wellno. 2 located in Chaco Canyon, somewhat near thevisitor center. In 1986 and 1987, the dissolved solidswere measured at 1,799 and 2,000 mg/L; these