Many river-connected ground-water basins store large volumes of water that can't be pumped and used because the resulting depletion of the flow of the river would impair the rights of downstream users.   The Albuquerque-Belen Basin, for example, stores several hundred million acre-feet, but the Rio Grande is already over-appropriated, and an increase in pumping would cause still more depletion of the stream.   If the system is too large for channel-lining (to separate the river from the ground-water system) to be practical, the stored ground water must simply remain in place.   Another approach is to pump the non-renewable ground water, put to beneficial use only the portion that actually comes from storage, and return the balance to the stream.  Eventually, the entire amount pumped would be at the expense of streamflow—but pumping must continue so as to prevent the natural system from replenishing the volume in storage.  This situation is theoretically sustainable if the revenue derived from sale of the stored water has established a perpetual fund to support the continued pumping.   Public-policy implications include dependence on a continually shrinking supply, responsibility for the perpetual pumping, and commitment of future energy resources.  A hypothetical system in the Rio Grande valley would provide about 6.8 million acre-feet for municipal use (with 50-percent return flow) over 100 years, at a cost less than large-scale desalination.