220 climate and geography: The Americas was not plentiful Streams ran off the mountains, and there were occasional summer rains, but these were undependable The Ancient Pueblo therefore relied on snowmelt and on springs where erosion had caused seeps beneath layers of sandstone Although there are rivers in the region, the Ancient Pueblo did not make use of them; rather, they settled near small streams and tributaries, which were more easily diverted for irrigation purposes The Ancient Pueblo are perhaps best known for their cliff dwellings In the canyons erosion created bridges and windows in the rock In some parts of the region shale rock, which erodes more easily than sandstone, was hollowed out by erosion, leaving behind sandstone overhangs These dwellings remain, and tourists and historians continue to visit them in several U.S national parks The Ancient Pueblo provide a striking example of the interactions of climate, geography, and human populations during the medieval period From 700 to 1130 the population of the Ancient Pueblo communities increased rapidly This increase was probably the result in part of reliable rainfall, leading to increased fertility and decreased mortality Some historians use the term golden age to refer to the period from 900 to 1130, when a warm climate and regular rainfall led to larger communities, which in turn gave rise to trade, local pottery and architecture, better food-storage technologies, and the domestication of the turkey But after about 1150 significant climate change occurred in North America, with a 300-year-long dry period Ancient Pueblo farmers tried to stave off disaster by using dams and terraces to control and store water, but evidence shows that many people abandoned settlements under environmental stress Simultaneously, and unrelated to rainfall patterns, the water table in the region dropped, compounding the drought Ultimately, the Ancient Pueblo culture disappeared in the 12th and 13th centuries, although some historians argue that it did not actually disappear but was absorbed by other cultures Historians are also not in complete agreement about the reasons behind this apparent disappearance The collapse of the Ancient Pueblo probably came about through a combination of circumstances, including crop failure resulting from drought, the erosion of topsoil, and deforestation These environmental stresses, caused by climate change, weakened the Ancient Pueblo culture, rendering it helpless in the face of invasions from the north by Ute, Shoshone, and Paiute, who were themselves under stress because of the drought Archaeological evidence suggests, for example, that communities moved from canyons, where water was still adequate, to mesas, where water and arable land were scarcer, probably for defensive purposes Warfare became common as communities turned to raiding and theft for food and other supplies Climate change also had effects on Ancient Pueblo religious beliefs Many ceremonial centers were closed up with rock and mortar, and huge fires were set in underground chambers called kivas in the belief that the people could thereby appease the gods that controlled nature The Anasazi are the most intensely studied of the Pueblo peoples, but they were not the only culture that inhabited this area—a region sometimes referred to as Oasisamerica The Hohokam people occupied the desert lands of Arizona and the Mexican state of Sonora The area was extremely dry, but it was bounded by two rivers, the Colorado and the Gila To grow crops, they constructed canals, and to protect themselves from the intense sun, they formed underground dwellings during the earliest phase of the culture’s development, up to about 550 The culture lasted until about 1400, when it collapsed under the same climatic stresses that affected the Anasazi The Mogollan people, who lived in Arizona and New Mexico, attained their height from about the 11th through the 15th centuries They lived in mountainous regions that were not suited to agriculture, so they made their living through ceramics and minerals, which they traded with Mesoamerica to the south Mesoamerica The geography of Mesoamerica is as complex as that of the rest of North America Mesoamerica includes central Mexico, the Isthmus of Tehuantepec, the Yucatán Peninsula, the countries of Guatemala and Belize, and parts of Nicaragua, El Salvador, Honduras, and Costa Rica, although these last are often regarded as a transitional zone between Mesoamerica and Andean South America Despite this complexity, geographers and climatologists divide Mesoamerica into two broad parts One is the lowlands, which feature tropical climates The other is the highlands, which include both dry tropical and cold mountainous climates Overall, however, Mesoamerica offered a warm, temperate climate with abundant rainfall and arable soil The climate during the medieval period was similar to that in the 21st century, and some archaeologists have argued that it was even more hospitable, especially in the highlands The chief climatic problem that Mesoamericans faced on an ongoing basis was water—either too much or too little In the highlands water tended to be scarce Accordingly, highland farmers learned to channel water from mountain runoff to their fields One technique, used extensively during the middle of the Postclassic Period (ca 1150–ca 1350), was the chinampa, a word that comes from the Nahuatlan language and means “square made of canes.” The chinampa was a rectangular plot of fertile land made of silt, mud, and decaying vegetation in the shallows of highland lakes in the Basin of