Off-stream refers to a water body

Body of water

A body of water or waterbody is any significant accumulation of water, usually covering the Earth or another planet. The term body of water most often refers to large accumulations of water, such as oceans, seas, and lakes, but it may also include smaller pools of water such as ponds, puddles or...

 or system that is not located in a streambed or does not receive significant natural flows from the surrounding watershed

Drainage basin

A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...

. For example, instead of moving water between lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

s, an offstream pumped-storage hydroelectric

Pumped-storage hydroelectricity

Pumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps...

 system could use two large tanks of water. Another example is a reservoir that is not located on a streambed, and is supplied by a pipeline

Pipeline transport

Pipeline transport is the transportation of goods through a pipe. Most commonly, liquids and gases are sent, but pneumatic tubes that transport solid capsules using compressed air are also used....

, aqueduct

Aqueduct

An aqueduct is a water supply or navigable channel constructed to convey water. In modern engineering, the term is used for any system of pipes, ditches, canals, tunnels, and other structures used for this purpose....

 or an adjacent stream. San Luis Reservoir

San Luis Reservoir

The San Luis Reservoir is an off-stream artificial lake in the eastern slopes of the Diablo Range of Merced County, California, approximately west of Los Banos on State Route 152, which crosses Pacheco Pass and runs along its north shore. The reservoir stores water taken from the San...

 is the largest off-stream reservoir in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. Although it is located on a small stream, it gets the vast majority of its water from the California Aqueduct

California Aqueduct

The Governor Edmund G. Brown California Aqueduct is a system of canals, tunnels, and pipelines that conveys water collected from the Sierra Nevada Mountains and valleys of Northern- and Central California to Southern California. The Department of Water Resources operates and maintains the...

by pumping aqueduct water up-hill into the higher elevation of the reservoir.