Water is an integral part of the American Southwest. Erosion shapes the landscape and moisture stimulates growth and reproduction. Plant and animal species have adapted to the presence or absence of water in this arid region. Ancient cultures conserved and diverted water to support early agriculture. Euroamericans used water for agricultural development and settlement. More recently, large dams were constructed on western rivers, controlling the flow of surface water to support development of a growing regional population. More than 50 dams were constructed in the Southwest, including over 20 on the Colorado River. Water quality is affected by geology and land use, including ranching, agriculture, and development. The National Park Service inventories and monitors water resources to maintain or improve water quality and aquatic ecosystems. Scientists study the quality and quantity of surface water (e.g., lakes, reservoirs, rivers, streams) and groundwater.