Culture and Ecology of Chaco Canyon and the San Juan Basin

------------------------.34 Chaco Project SynthesisGillespie (1985) reported that during the 1960sand 1970s the average annual precipitation in Chacowas 220 mm (8.5 in), but there was considerablevariation from year to year. The lowest amount was8.5 cm (3.35 in); the highest 35.0 cm (13.75 in). Inslightly over half the years, the median was lower thanthe mean. Characteristically in Chaco Canyon,August is the wettest month with 35 mm (1.37 in) ofrain. In July, September, and October, Chacoreceives approximately 25 mm (1 in). The driestmonth, June, receives approximately 10 mm (0.38 in)of rain. Because precipitation in Chaco Canyon variesspatially, the key constraint is the location of raingauges. Thomas Windes currently monitors raingauges throughout the canyon.Temperatures in the canyon vary diurnally, butthe average temperature is 9.9 degrees C (49.8 degreesF). The highest average temperature occurs in July(22.9 degrees C [73.2 degrees F]) and the lowestoccurs in January (minus 1. 7 degrees C [29.0 degreesF]). Since 1960, the frost-free period has spannedmore than 100 days, with half of the years betweenthen and now having fewer than 100 days. Otherpublished figures are somewhat higher, but these arebased on data collected prior to the changes in weatherstation location in 1960. The growing season can bemeasured in two ways: dates with freezing temperatures,and the number of frost-free days. Furtherevaluation is needed to determine which data set ismore representative (see discussion in Windes1993:36-43). There also has been a change in theregional climatic pattern, with recent years havingshorter frost-free periods. Additionally, one must askwhat is being evaluated. If a killing frost occurs atminus 1. 1 degrees C (30 degrees F) rather than 0degrees C (32 degrees F), the growing season is 13days longer. Gillespie (1985) asked how critical thisdifference is, especially if prehistoric peoples used afast-maturing com; he suggested that we consider thisa potential limitation on agriculture rather than abarrier to it.HydrologyThe amount of water available at anyone time,its flow rate, and its quality would have affected anypeople living in the canyon. Based on seven years ofdata, Fisher (1934) calculated that the Chaco Riveraveraged 379,926 acre ft/year and 30,751 ha (75,985acres) of land could be watered. He thereforesuggested that about 10,000 people dependent onagriculture could exist in Chaco Canyon.Recently, Kernodle (1996) provided generalinformation on 12 hydrostratigraphic units that arepart of the San Juan Basin water system. As measuredin its lower reaches, the Chaco River has a water flowfrom 0.283 to 0.849 m 3 /s (10 to 30 ft3/ S during nonstormflowperiods. Springs were considered theprobable major contributors to this flow. Until a deepwell was drilled into the Gallup Sandstone in 1973,water from the Menefee Formation supplementedwater from shallow alluvial deposits to provide waterfor domestic use and livestock for Chaco CultureNational Historical Park and the surrounding area(Kernodle 1996:40). Kernodle reported that mostwells have a low, but steady, flow because water rateis limited by leakage of water from shales and silt inthe lenses of sandstone sitting above (i.e., the CliffHouse Sandstone in Chaco Canyon). Stone et al.(1983:33) suggested a transmissivity of about 1858cm 2 /day (2 fe/day) in areas where sandstone is lessthan 60.96 m (200 ft) thick. Kirk Vincent (BradShattuck, personal communication, 2002) is currentlyreviewing data on ground water levels and the historyof their drop.Simons, Li & Associates (1982:Tables 4.15-4.20) documented the results of a computer simulationof storm events and their results for the Gallo Wash.Total water runoff ranged from 20.9 a/ft for two-yearstorms to 578.1 a/ft for over-lOO-year events. Theamount of runoff and the peak discharges wereconsidered small for the type of storm and size ofwatershed, probably because a large percentage of thewatershed is overlain by soils having high permeabilitythat can rapidly infiltrate rainfall. If thearea contained a higher percentage of less permeablesoils, the magnitude and peak rate of the runoff wouldgreatly increase. Also, the watershed is fairly largeand the probability of a thunderstorm covering theentire area with intense rainfall is small (Simons, Li &Associates 1982:4.31). Suspended sediment concentrationswere thought to be large, ranging from 9,600ppm for a two-year event to 113,000 ppm for an over-100-year event.Extrapolating from these data for the entireupper Chaco Basin, Simons, Li & Associates (1982: