In a pluvial period associated with Wisconsin glaciation the closed basin of the Estancia Valley in New Mexico held a lake which, at its maximum extent, was 150 feet deep and had a surface area of 450 square miles. This basin, with a mean elevation of about 6,000 feet, has at present an annual precipitation of about 14 inches.

Estimates have been made of the Pleistocene precipitation necessary to maintain this pluvial lake. Instead of the present annual average of 14 inches it has been variously estimated that the precipitation must have been between 20 and 24 inches. Lakes existed during Pleistocene time in many places in the western United States that are now true deserts - with a precipitation of less than 4 inches - and there is abundant evidence that early man lived on the shores of these lakes. He must have adapted himself to the increasing aridity; this adaptation can be seen even at present in the form of floodwater farming practices, which have been highly developed by the Hopi Indians, particularly in northeastern Arizona.

A gradually changing climate is only one, and not the most important, of the changing conditions to which man must gradually adjust in his particular relation to the use of water. The changes in his own culture in conjunction with changes in population density are usually even more important determinants of man’s use of and attitude toward his water supplies. In a desert area of Central Arizona, near Florence, the remains of irrigation systems developed by the aborigines to irrigate the alluvial valley floor with water diverted from the Gila River, which was at that time perennial, have been mapped and partially excavated. Irrigated agriculture was not practised nearly so extensively in the arid portions of the United States as in Persia, India, and many Mediterranean countries, nor was the general culture of indigenous American tribes so highly developed. Even in the simple cultures of the American Indians patterns of adjustment to a changing climate and to a changing culture and population level can be discerned. These patterns include, however crudely, the development of irrigated agriculture, floodwater farming, water storage for both stock and community use, spring development, and even efforts at rain-making through the offices of prayers, rattles, and dances. These same patterns, more complex to be sure, can be seen to have characterized the adjustment of modern culture to the limited water supplies of the arid climates, even including the prayers and rattles.

An aspect of the development of American culture in the arid areas is probably typical and may have a counterpart in certain of the underdeveloped areas in other parts of the world at the present time. The local civilization of the arid climate usually does not develop to a very high level in situ. The indigenous cultures are usually transfused with new bursts of energy and knowledge by the incursion of other cultures which have developed in other climes. The cultural advances in the Fertile Crescent of Mesopotamia were gradually influenced by the barbarian invasion, which added much to, as well as detracted from, the locally developing society. Similarly the spurt of civilization which has characterized the arid parts of the United States since 1846 was determined by the superposition of a culture from the eastern United States on the essentially Spanish culture which had been developing since the initial exploration of the southwestern desert in 1630.