The irrigated agricultural field patterns stand out in marked contrast to the naturally nonvegetated desert region of the southwestern United States and northwestern Mexico. This view provides a rather rare opportunity to observe a manmade boundary between the two countries. (Look for the abrupt changes in agricultural field sizes and the regular field orientation that creates an east-west line along the southern edge of the California Imperial Valley.) Two rivers, the Colorado and the Gila, are the major sources of water for this arid region. The Colorado River enters this photograph from the northeast, and the Gila River enters from the east; both rivers flow generally south-southwest. Two sizable lakes (the Salton Sea in southern California and Laguna Salada in Mexico) are observed. A swirling sediment plume, probably created by surface winds, is seen in the Salton Sea. A well-defined area of distinctive sand dunes, called the Sand Hills, is located just to the east of the field patterns in the Imperial Valley. Several fragmented mountain ranges trend in a southeast-northwest direction along the southern California and northern Baja California coasts.