Water resources management often must deal with several conflicting water uses
under highly uncertain conditions. Conflicts between traditional uses like hydropower,
and domestic and irrigation water supply have been resolved at least partially in the past.
The situation changes drastically when no common performance measure can be
established for conflicting objectives. This is the case of environmental impacts
associated with reservoirs built to support traditional water uses like irrigation, urban and
industrial water supply, and hydropower. Dams are a physical barrier in the river system,
whose effects, besides the obvious change from a stream to a lake environment in the
reservoir itself, are related to the change in the flow regime (at different time scales),
water diversion from the river channel, blockage of downstream flows of nutrients and
sediments, change in water temperatures and oxygen levels, and impeding fish and
wildlife migration