One of the purposes of retaining structures is to create large bodies of water, or reservoirs, that have a variety of functions, including land irrigation, power generation, water supply and flood control.

The retaining structures used to build reservoirs are called dams and dikes. A dam is built on the riverbed; it serves to hold back water and raise the water level of the resulting reservoir. Dikes are often built to increase a dam's effectiveness by preventing water from leaving the reservoir through secondary valleys. Daniel-Johnson dam	Large dams   Dam inspection