Rivers, lakes, aquifers, and wetlands provide a myriad of benefits to human
economies. They provide water for drinking and hygiene, irrigation, manufacturing, and such goods as fish and waterfowl, as well as a host of "in stream" non extractive benefits including recreation, transportation. flood control, bird and wildlife habitats, and the dilution of pollutants. Such instream benefits are particularly difficult to quantify, since many are public goods that are not priced by the market economy. Thus the total global value of all services and benefits provided by freshwater systems is impossible to measure accurately but would certainly measure in the several trillions of dollars (Postel and Carpenter, 1997: 210).