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<br /> <br />Modifications to River Valleys <br /> <br />CONSTRUCTION OF LEVEES <br /> <br />Levees are built along riverbanks to protect <br />adjacent lands from flooding. These structures <br />commonly are very effective in containing smaller <br />magnitude floods that are likely to occur regularly <br />from year to year. Large floods that occur much <br />less frequently, however, sometimes overtop or <br />breach the levees, resulting in widespread <br />flooding. Flooding of low-lying land is, in a sense, <br />the most visible and extreme example of the inter- <br />action of ground water and surface water. During <br />flooding, recharge to ground water is continuous; <br />given sufficient time, the water table may rise to <br />the land surface and completely saturate the <br />shallow aquifer (see Figure 12). Under these condi- <br />tions, an extended period of drainage from the <br />shallow aquifer takes place after the floodwaters <br />recede. The irony of levees as a flood protection <br />mechanism is that if levees fail during a major <br />flood, the area, depth, and duration of flooding in <br />some areas may be greater than if levees were not <br />present. <br /> <br />CONSTRUCTION OF RESERVOIRS <br /> <br />The primary purpose of reservoirs is to store <br />water for uses such as public water supply, irriga- <br />tion, flood attentuation, and generation of electric <br />power. Reservoirs also can provide opportunities <br />for recreation and wildlife habitat. Water needs <br />to be stored in reservoirs because streamflow is <br />highly variable, and the times when streamflow <br />is abundant do not necessarily coincide with the <br />times when the water is needed. Streamflow can <br />vary daily in response to individual storms and <br />seasonally in response to variation in weather <br />patterns. <br />The effects of reservoirs on the interaction <br />of ground water and surface water are greatest <br />near the reservoir and directly downstream from <br />it. Reservoirs can cause a permanent rise in the <br />water table that may extend a considerable <br />distance from the reservoir, because the base level <br />of the stream, to which the ground-water gradi- <br />ents had adjusted, is raised to the higher reservoir <br />levels. Near the dam, reservoirs commonly lose <br />water to shallow ground water, but this water <br /> <br />,.- .- <br /> <br /> <br />'"'-' <br />~"">.""! <br /> <br />',.,...... .~i' <br /> <br />.~ ,. <br />, .:"t#,;.; .." <br />";<:>;_', '__0;_ .....,.,. <br /> <br /> <br />Breached levee along the Mississippi River. <br />(Photograph courtesy of the St. Louis Post <br />Dispatch.) <br /> <br /> <br />Reservoir in California. (Photo- <br />graph by Michael Collier.) <br /> <br />68 <br />