High Quality Content by WIKIPEDIA articles! Off-stream refers to a water body or system that is not located in a streambed or does not receive significant natural flows from the surrounding watershed. For example, instead of moving water between lakes, an offstream pumped-storage hydroelectric 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, aqueduct or an adjacent stream. San Luis Reservoir is the largest off-stream reservoir in the United States. Although it is located on a small stream, it gets the vast majority of its water from the California Aqueduct by pumping aqueduct water up-hill into the higher elevation of the reservoir.