Peru's coast is a bleak, often rocky, and mountainous desert that runs from Chile to Ecuador, punctuated by fifty-two small rivers that descend through steep, arid mountains and empty into the Pacific. The Costa is a strange land of great dunes and rolling expanses of barren sand, at once a desert but with periods of humidity as high as 90 percent in the winter from June to September, when temperatures in Lima average about 16 degrees Celsius. Temperatures along the coast rise near the equator in the north, where the summer can be blazingly hot, and fall to cooler levels in the south. If climatic conditions are right, there can be a sudden burst of delicate plant life at certain places on the lunar-like landscape, made possible by the heavy mist. Normally, however, the mist is only sufficient to dampen the air, and the sand remains bleakly sterile. These conditions greatly favor the preservation of delicate archaeological remains. The environment also facilitates human habitation and housing because the climate is benign and the lack of rain eases the need for water-tight roofing.

Humans have lived for over 10,000 years in the larger coastal valleys, fishing, hunting, and gathering along the rich shoreline, as well as domesticating crops and inventing irrigation systems. The largest of these littoral oases became the sites of towns, cities, religious centers, and the seats of ancient nations. Although migration from the highlands and other provincial regions has long occurred, the movement of people to the Costa was greatly stimulated by the growth of the fishing industry, which transformed villages and towns into frontier-like cities, such as Chimbote. In the early 1990s, over 53 percent of the nation's people lived in these sharply delimited coastal valleys (see table 4, Appendix). As the population becomes ever more concentrated in the coastal urban centers, people increasingly overrun the rich and ancient irrigated agricultural lands, such as those in the Rímac Valley where greater Lima is situated, and the Chicama Valley at the site of the city of Trujillo. Although the region contains 160,500 square kilometers of land area, only 4 percent, or 6,900 square kilometers of it, is arable. By 1990 population growth had increased the density of habitation to 1,715 persons for each square kilometer of arable land (see table 5, Appendix). Throughout all the coastal valleys, human settlements remain totally dependent on the waters that flow from the Andes along canals and aqueducts first designed and built 3,000 years ago. Here, uncontrolled and unplanned urban growth competes directly with scarce and vitally needed agricultural land, steadily removing it from productive use.

Data as of September 1992