Dams and Reservoirs

Currently, the United States has more than 75,000 dams and reservoirs with a total storage capacity of about 860 million acre-feet. While these bodies of water also provide recreational and power-generation uses, many are primarily intended as water storage devices. By accommodating seasonal variations in streamflow, reservoirs allow water resource administrators to plan for the average downstream flow rate. Despite their general utility, there are a number of points to consider about dams and reservoirs:

• Many of the best areas to build high capacity reservoirs have already been used. (OTA, 1983)
• Sedimentation, the deposition of eroded rock, is dropping reservoir capacity by about 1.5 million acre-feet each year (OTA, 1983).
• Dams require maintainance to prevent safety hazards to downstream populations and habitats.
• There is growing environmental resistance to dams, focused on their negative effects to animal habitats and migration patterns.
• Surface evaporation rates put practical limits on the size of reservoirs. A study of U.S river basins suggests that the Lower Colorado, the Upper Colorado, and the Rio Grande, have already reached this point (OTA, 1983).

Although reservoirs and dams have helped solve national water needs in the past, these points help explain why they may not be the best solution for the future (Frederick, 1995).