Reservoir is the word that identifies a large natural or human-made lake used to supply water.
Reservoir is a large natural or human- made lake used as a source of water supply. Reservoirs are very useful because they supply water when natural water bodies such as rivers, run dry (especially during droughts). Reservoirs help to control flooding and they are also used for fishing and boating. Valley-dammed reservoirs, bank-side reservoirs, and service reservoirs are the three main types of reservoirs.