Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 25that exists today contrasts with the lower area of theChaco Wash, which is more open and more remotefrom sources of silt and clay. To understandvariability among streams, DeAngelis assisted Hodges(1974), who studied 17 variables in 31 drainage basinsto devise two models for arroyo and washdevelopment in northwestern New Mexico andnortheastern Arizona. The first model predicted themagnitude of channel development at a selected site.The second aided in classifying the form of channeldevelopment. Hodges found that the geomorphicprocess responsible for creating and maintaining adrainage is dependent on the interactions and changesthat occur among several variables. Each basin isdifferent and has a unique history; therefore, causesfor drainage development may differ betweenneighboring basins. Thus, Bryan's correlation oferosional and depositional events across the regionwas not supported.The 0.91 to 1.2 mper 1.6 km gradient differencebetween the Chaco Wash and the uncut valley floorwould not be evident in the lower wash because coarsesands would not be able to maintain as great a slope.In the canyon between 1860 and 1925, whenovergrazing, aridity, and the increasing gradientexceeded the threshold necessary for arroyo-cutting,the Chaco Wash became entrenched. Vertical erosioncontinued to deepen the arroyo until a stable gradientwas achieved. Once entrenchment achieved a morestable condition, soil piping (water erosion in a layerof subsoil that results in the formation of tunnels andcaving) created incipient tributaries. Since 1925,lateral erosion or widening of the arroyo has beenmore dominant over vertical erosion (DeAngelis1972). After 1970, sedimentation is thought to be thecause of erosion (Brad Shattuck, personal communication,2003).Within the canyon, initial experiments withseismic refraction by Lewis and Shipman (1972) werenot successful in determining the depth of the bedrocksurface beneath the fill sediments, but Ross (1978)defined three seismic horizons that correlated fairlywell with data obtained from two core test holes. Anupper surface alluvium varied from 5.79 to 16.15 m(19 to 53 ft) with an average of 12.49 m (41 ft).Beneath this was a water-saturated alluvium, whichalso varied from 3.96 to 25.3 m (13 to 83 ft), with anaverage of slightly over 15 m (52 ft). Suballuvialbedrock (the Menefee Formation) varied from 17.06to 35.36 m (56 to 116 ft), with an average of27 .86 m(91.4 ft). The water table sloped to the west from 1 to5 degrees.The canyon floor is relative flat in cross-section,with some downcutting on the north side (Ross 1978).Gravels, sand, silt, and clays characterize the complexalluvial fill. Two depositional environments (analluvial fan and a meandering arroyo environment)were delineated. The presence of well-crystallizedkaolinite suggests alluvial fill from the sandstone wallsof the canyon; poorly crystallized illites and montmorillonitesare probably derived from the headwatersof the Chaco drainage basin. In the meandering environment,Ross defined five facies: 1) backwater-tochannelmargin, 2) mid-channel climbing ripple, 3)flat-bedded channel sand, 4) modified climbing ripple,and 5) channel lag deposits. These represent a rangeof water-flow regimes and load-carrying capacities.These facies also suggested more complex stratigraphythan the seismic data could discern.Research on Quaternary deposits in the canyonundertaken by D. Love (1980) provided a moredetailed analysis of events. Initially, Love (1974)reported a date of approximately 4,000 years B.P. onburned wood exposed in the bank of a collapsed wallin the wash just above its confluence with the GalloWash. Because the arroyo had filled 4.5 m above thelowest entrenchment level when the bum occurred andfilled at least 2 m above it, this fill represented apreviously unknown cycle of cut and fill. Because ithad occurred prior to major human use of the canyon,Love thought this degradation and aggradation mustbe explained by climatic fluctuations rather thanenvironmental degradation by canyon occupants. Ifso, later cycles were perhaps also climatically induced.Thus, he focused on the relationships between climateand geomorphic and sedimentologic conditions in theupper Chaco Basin (D. Love 1974, 1977a, 1977b,1983a, 1983b).After reviewing data on semiarid drainage basinsand considering the deposits and processes that wouldhave affected the upper Chaco Drainage Basin, D.Love (1977a, 1977b, 1980) indicated that the majorvariables that affect a geomorphic system include thephysical geography, bedrock geology, soils, vegetation,and climate. He also found that all runoff from