Although water in rivers, lakes, and wetlands contains only 0.01 percent of the world's freshwater and occupies only 1 percent of the earth's surface, the global value of freshwater services is estimated in the trillions of U.S. dollars. Dams have had the greatest impact on freshwater ecosystems. Large dams have increased sevenfold in number since the 1950s and now impound 14 percent of the world's runoff. Almost 60 percent of the world's largest 237 rivers are strongly or moderately fragmented by dams, diversions, or canals. In 1997, 7.7 million metric tons of fish were caught from lakes, rivers, and wetlands, a production level estimated to be at or above maximum sustainable yield for these systems. Freshwater aquaculture contributed 17 million metric tons of fish in 1997. Since 1990, freshwater aquaculture has more than doubled its yield and now accounts for 60 percent of global aquaculture production. Half the world's wetlands are estimated to have been lost in the 20th century, as land was converted to agriculture and urban areas, or filled to combat diseases such as malaria. At least 1.5 billion people depend on groundwater as their sole source of drinking water. Overexploitation and pollution in many regions of the world are threatening groundwater supplies, but comprehensive data on the quality and quantity of this resource are not available at the global level.