Reservoirs - artificial water bodies created normally by construction of a dam across a stream, sometimes in combination with excavation of a depression - have been built in North America for hundreds and perhaps thousands of years. The Aztecs who built their city in the Valley of Mexico constructed dams to regulate water flows to the endorheic lakes of the valley. When Europeans occupied the continent in the 17th century they quickly began to build small dams that created ponds supplying power to mills for various purposes. Tens of thousands of such ponds were built in the eastern United States in the 17th-19th centuries. In addition, larger reservoirs were built in many areas in the 19th century to provide water to navigational canal systems.
In the early 20th century the scale of reservoir construction increased dramatically (Figure 4). In this period reservoirs were built principally for public water supply, flood control, irrigation, and hydroelectric power generation. Later in the 20th century recreation became a major justification for reservoir construction. By the 1970s, large-scale reservoir construction in North America began to meet obstacles. Foremost among these was public concern about loss of land and free-flowing aquatic habitat. This opposition arose wherever dams were being built: on land used by Native Americans in central Quebec or within easy reach of metropolitan regions in the United States. Other factors contributing to the decline of reservoir construction include increasing costs and the development of alternative approaches to problems of flooding or water supply.
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