What is a reservoir?
A reservoir is a humanmade lake that is created when a dam is built on a river. River water backs up behind the dam creating a reservoir. Reservoirs are built to store freshwater and function as a direct source of water supply as well as to balance supply across periods of high flow input (snowmelt runoff for example) and periods of time associated with low or no water flow conditions.
Source: UGS
Click here for more information from the Utah Geological Survey regarding reservoir storage conditions in Utah.