Albuquerque Basin  The Albuquerque Basin (or Middle Rio Grande Basin) is one of the largest and deepest of the structural basins in the Rio Grande rift. It contains the city of Albuquerque, New Mexico. Geologically, the Albuquerque Basin is a half-graben that slopes down towards the east to terminate on the Sandia and Manzano mountains. It has a semi-arid climate, with large areas that count as desert. From Paleo-Indian traces dating back 12,000 years, the climate used to be wetter and more fertile than it is today. The Rio Grande flows through the basin from north to south. Its valley has been irrigated for at least 1,000 years. Intense irrigation began in the late nineteenth century with new dams, levees and ditches and has caused environmental problems. In times of low water levels in the Rio Grande, Albuquerque relies on groundwater for its potable water supply. The aquifer is composed of deposits from the ancestral Rio Grande and the size of its annual resource follows fluctuations in weather and climate phenomena. There may be natural gas in the basin, but opponents to production fear the impact on the groundwater and on the quality of life. The Albuquerque Basin covers of New Mexico. The basin is bounded by the Sandia and Manzano mountains on the east, the Jemez Mountains to the north, the Rio Puerco on the west and the Socorro Basin to the south. It is north-south and east-west at its widest point. The Rio Grande, at an elevation of runs through the basin from north to south. To the east, alluvial fans and stream terraces descend to the river from the mountains that form the eastern boundary of the basin. West of the river the Llano de Albuquerque contains only isolated mountains and volcanoes, sloping gradually up to the Rio Puerco. Throughout the basin, which receives little rainfall, there are sand dunes and dune fields, some vegetated and some active. The basin is rich in Paleo-Indian sites, most of which are found on terraces or other uplands near to the Rio Grande. Some of the sites date back 12,000 years from the present. During the period of Paleo-Indian occupation the playas (dry lake basins) contained wetlands with more vegetation than in today's dry conditions. Remains of the Folsom people include flaked stone tools, pottery sherds and the bones of bison. Modern communities in the hydrological basin include, from north to south, Cochiti, Santo Domingo, San Felipe, Algodones, Bernalillo, Rio Rancho, Albuquerque, Isleta, Los Lunas, Belen and Bernardo.