Professor Linda Cordell, director of the CU Museum, said many sites that appear to be abandoned are settlements that were relocated nearby. Understanding the frequent movements of prehispanic farmers involves looking at their environment and subsistence patterns, the accuracy of archaeological investigations and the peoples' social landscape, or "sense of home," she said.
A paper on the subject by Cordell, titled "Why Did Prehispanic Farmers Move So Often?" was presented at the American Association for the Advancement of Science meeting held Feb. 13 to Feb. 18 in Denver.
Southwestern peoples tended to move often -- locally or regionally -- when natural resources became depleted or the climate was changing -- by moving up or down in elevation or crossing over to nearby, unoccupied valleys, said Cordell.
The Greater Southwest is either desert or semi-desert and large portions of the region are at high elevations, she noted. Clever farming strategies allowed ancestral Pueblo people on the higher Colorado Plateau and adjacent highlands to overcome challenges like too little moisture and too short growing seasons for hundreds of years.
They used "ingenious devices" to conserve moisture and moderate temperature at ground level, including moving run-off water to agricultural fields, planting grid-based gardens, placing crops at the mouths of arroyos and in dunes and building check dams and terraces. They even spread gravel mulch over vast field systems that not only slowed run-off, but also moderated temperatures at ground level.
Since gravel mulch absorbs heat from the sun in the daytime and releases it slowly after sundown, they were able to extend the frost-free periods in these areas for several weeks. "Along the Chama River in New Mexico, Pueblo people even grew cotton, something not done in that location where the growing season is marginal even for corn," she said.
It is often difficult for archaeologists to "untangle" occupation histories at sites that were occupied for only a decade or two from those occupied for significantly longer periods, even with modern survey techniques, she said. The thousands of sites in the Southwest compound the problem.
Some American Indian groups, such as the Hohokam near present-day Phoenix, occupied sites for centuries using sophisticated canals to water crops, primarily maize. Nearly 350 miles of prehispanic canal systems in the region have been found, with individual canal segments that carried water over three miles.
"In contrast, the Anglo-Mormonsettlement on the Muddy River that began in 1864 and included a canal nine miles long was abandoned in just seven years, leaving behind homes, orchards, farms and 8,000 baskets of wheat," she said.
Huge Southwest settlements like Chaco Canyon in New Mexico, Canyon de Chelly in Utah and Mesa Verde in Colorado were depopulated in the late 1200s.
But rather than disappearing, most of the people moved south and southeast. There, many integrated into large villages that are ancestral sites of modern Pueblo peoples along the Rio Grande River, the Galisteo Basin, at Zuni, N.M., and at the Hopi Mesas of Arizona, she said.
The "sense of home" Cordell refers to includes the use of unoccupied places by modern southwestern peoples for ceremonies, teaching and to make offerings. Pueblo peoples, Navajos, Spanish and Anglo-Americans visit archaeological sites and 19th and 20th century ghost towns to remember their own ancestors and histories and to derive lessons for the living today.