Although humans have diverted water from its natural channels throughout history, the era of "megadams" was ushered in by the Bureau of Reclamation Act of 1902. Even the biggest and most prominent of the U.S. dams and their associated lakes (Grand Coulee, Hoover, Glen Canyon) are dwarfed by a number of other dams around the world (e.g., Owen Falls, Uganda; Kariba, Zimbabwe/Zambia; Bratsk, USSR; High Aswan, Egypt, Akosombo, Ghana; Three Gorges, China, and others.) The advantages of dams are: water storage, flood control, irrigation, electrical power generation, industrialization, increased croplands, and improved navigation conditions. However, the lakes behind dams often submerge beautiful and historic areas, cover ancient artifacts, and cause massive relocation of people who live along river banks. Often the displaced people are indigenous natives who are marginalized by the rapidly developing global economic culture. Dams and the associated lakes induce seismicity in previously aseismic regions. Seepage and evaporation from the big lakes can be significant (e.g., ~15% for the Nile system), and groundwater levels can be affected. On a longer term, the consequences to the geologic and biologic systems are severe: sediment is deposited behind the dams, and so erosion is increased downstream. Failure of a dam could cause massive flooding. Natural fertilizers are not replenished by annual floods, and the use of artificial fertilizers increases toxic minerals and salinity. These enter the food chain in potentially toxic levels. Waste of human and nonhuman origins is concentrated by lack of drainage, in places leading to increase disease threats. Downstream, where such contaminated water enters an ocean or sea, even the large bodies of water are affected, e.g., changes in the sardine and shrimp populations of the Mediterranean caused by the dams on the Nile. Thus, in an era where nearly every form of energy generation has pros and cons, the pros, cons, and consequences of big dams must be weighed against those of alternate energy sources.