A reservoir is an artificial lake where water is stored. A reservoir can also be formed from a natural lake whose outlet has been dammed to control the water level. Most reservoirs are formed by constructing dams across rivers. A dam is any barrier that holds back water; dams are primarily used to save, manage, and/or prevent the flow of excess water into s pecific regions. In addition, some dams are used to generate hydro power.
Dam https://goo.gl/images/fMfhcf
Reservoir https://goo.gl/images/yywqcy