User:Saravask/Chaco

 

Chaco Canyon lies within the San Juan Basin, which sits atop the vast Colorado Plateau.. The Chuska Mountains to the west, the San Juan Mountains to the north, and the San Pedro Mountains to the east are the closest mountain ranges; ancient Chacoans relied upon their dense forests of oak, piñon, ponderosa pine, and juniper to obtain timber and other resources. The canyon itself, located within lowlands circumscribed by runs in a roughly northwest-to-southeast direction, and is rimmed by flat massifs known as mesas. Large gaps between the southwestern cliff faces were critical in funneling rain-bearing storms into the canyon, boosting local precipitation.^[1] The principal Chacoan complexes, such as Pueblo Bonito, New Alto, and Kin Kletso, are at elevations of 6,200 to 6,440 feet (1,890 to 1,963 m).

The canyon floor, which slopes downward to the northeast at a gentle grade of 30 feet per mile (6 meters per kilometer), is bisected by the Chaco Wash, an arroyo that only infrequently carries water. Of the canyon's aquifers, the largest are located at a depth that precluded the ancient Chacoans from tapping their groundwater; only a few smaller, shallower sources supported springs.^[2] Surface water is virtually non-existent except in the guise of storm runoff flowing intermittently through arroyos.

As the Chaco Wash flowed across the Chacra Mesa, it cut into it, creating the canyon. The mesa itself comprises sandstone and shale strata dating from the Late Cretaceous;^[3] they are of the Mesaverde Formation.^[4] The canyon and mesa lie within the "Chaco Core", distinct from the wider Chaco Plateau, a relatively flat region of grassland with infrequent and interspersed stands of trees. The Continental Divide is 15.5 miles (25 km) west of the canyon. Geological characteristics and different patterns of drainage differentiate these two regions both from each other and from the nearby Chaco Slope, Gobernador Slope, and Chuska Valley.^[5]

During the Cretaceous period, after the supercontinent of Pangaea had split apart, the region that now Chaco Canyon became part of a shifting transition zone between a shallow inland sea—the Western Interior Seaway—and plains and low hills to the west. A sandy and swampy coastline repeatedly shifted east and west, variously submerging and uncovering the canyon's portion of what is now the Colorado Plateau.^[4]

An arid region of xeric scrubland, the canyon and wider basin average 8 inches (20 cm) of rainfall annually. Chaco Canyon lies on the leeward side of extensive mountain ranges to the south and west, resulting in the rainshadow effect that leads to the prevailing lack of moisture there.^[6] Chaco Canyon experiences four distinct season; rainfall is most likely between July and September; May and June are the driest months. Northward excursions of the intertropical convergence zone may bring unusually high amounts of precipitation. Orographic precipitation, resulting from moisture wrung out of storm systems ascending mountain ranges around Chaco Canyon, is responsible for most precipitation in both summer and winter; rainfall increases with higher elevation.^[3]

The Chaco Canyon area is also characterized by climatic extremes: recorded temperatures range between −38 °F (-39 °C) to 102 °F (39 °C).^[7] The region averages less than 150 days without frost per year. The local climate can swing wildly from years of plentiful rainfall to extended droughts, and is heavily influenced by the El Niño-Southern Oscillation phenomenon.^[7]