Vegetation and man in the Basin of Mexico

Abstract

The Basin of Mexico (Fig. 1) lies at an elevation of about 2,000 m. Surrounded by mountains on the east, south and west, it receives drainage from all directions, but drainage from the north is less. As a consequence of continued input and no outlet from the Basin, water accumulated in the past in the valley to form 5 large, shallow lakes during the dry seasons. During the period of historical observation, these at least partially coalesced, particularly during heavy rainy seasons. Generally, the level of the lakes at the time of Spanish Conquest is stated to have been 2,240 m. In the historic period, a drain was opened to the north to reduce the lakes to their present condition. Colonial period accounts (Rojas et al., 1974) indicate that various causeways constructed prehistorically were still functional in providing overwater access to Tenochtitlan (Mexico City) and in separating the waters of Lake Texcoco (Fig. 1) into an eastern saline body and a western fresh water area as well as separating lakes Xochimilco and Chalco. Fresh water from the mountains to the west and south provided a flow which made possible the utilization of the lake water for intensive farming with the development of the unique chinampa system (Armillas, 1971). The lakes of the Basin of Mexico made the area very attractive to prehistoric people who found game animals plentiful and useful plants from a number of different associations within easy reach (Lorenzo et al., 1975). With development of cultivation, undoubtedly around the margins of the lakes initially, the Basin became the site of one of the earliest urban populations in the Americas. However, in order to understand the development of a burgeoning population it is necessary to provide the biological background in which this developed.