SPACEPHOTOS

The four north-central African countries of Chad, Niger, Nigeria, and Cameroon share a common feature—borders that extend into Lake Chad. Millions of years ago, Lake Chad encompassed more than 100 000 square miles (260 000 square kilometers) in central Africa. This low-oblique, color infrared photograph shows approximately 965 square miles (2500 square kilometers) of open water in the south basin (blue). Since the mid-1970s, only the south basin has had open water; many sand dunes cover the former lake floor, forming inlets into the remaining area of open water. Climatic changes, dynamic shifts in the Earth’s crust, and evaporation interacted to reduce the lake’s surface area. The Chari River, with its substantial delta on the southern side of the lake, supplies approximately 95 percent of the lake’s water. The Komadougou Yobe River of Niger and its ephemeral tributary streams enter the northern basin of Lake Chad along the western shoreline, but no large areas of vegetation exist in the north basin. In contrast, the south basin shows a large area of vigorous vegetation (reddish and darker areas).