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<br />
<br />Histarit:all;', the rUvrlopment if the United Slllits has
<br />prom:ded along the principal waterway if the IUllion,
<br />where cities Iuu'l: been rUve/oped and redeveloped ot'l:r the
<br />decades.
<br />
<br />Pittsburgh, Penns)"!vania, tarry 1960s.
<br />(Compare this photograph with thou on page 24.)
<br />
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<br />16
<br />
<br />Fluctuating Lake Levels
<br />
<br />Closed-basin lakes are susceptible to dramatic (5- to 15- foot), long-term
<br />fluctuations in their water levels as a result of variations in precipitation, run-
<br />off, and evapotranspiration. Flooding associated with this situation can last
<br />for years; examples of such lakes are the Salton Sea, the Great Salt Lake, and
<br />the Great Lakes. Short-term fluctuations can be triggered by sustained strong
<br />winds and by sharp changes in barometric pressure. Human activities, such
<br />as dredging, diversions, water consumption, and regulation by structural
<br />works, can also affect lake levels.
<br />
<br />Changes in Water Levels In the Great Lakes, 1900-1986
<br />
<br />Lake
<br />
<br />(Lake Surface Elevation, in Feet)
<br />Monthly Mean
<br />1900-1986
<br />
<br />Range
<br />(winter low to summer high
<br />monthly means)
<br />
<br /> Average ~laximum Minimum Average IVlaximum Minimum
<br />Superior 600.61 602.24 598.23 1.2 2.1 0.4
<br />:\1ichigan- Huron 578.33 581.62 575.35 1.2 2.1 0.4
<br />St. Clair 573.40 576.69 569.86 1.7 3.3 0.4
<br />Erie 570.50 573.70 567.49 1.6 2.8 0.9
<br />Ontario 244.73 248.06 241.45 20 3.6 0.7
<br />
<br />Levels are referenced to International Great Lakes Datum 1955.
<br />
<br />Soor<<: u.s. Army Corps of Engineers
<br />
<br />Floodplain Losses
<br />
<br />Throughout the history of the United States, the prevailing view has been
<br />that humans should use and modify the natural environment, including flood-
<br />plains, to meet their needs, For centuries people have been settling on the banks
<br />of the country's rivers, streams, and oceans, taking advantage of the water
<br />supply, transportation, energy source, wildlife habitat, and other benefits flood-
<br />plains provide. Unfortunately, human development on floodplains usually
<br />results in flood damages, In the United States the result of this widespread
<br />damage was a second wave of activity, during \vhich individuals and govern-
<br />ments enthusiastically engaged in the construction of dams and reservoirs,
<br />levees, flood walls, and stream channelization projects in efforts to prevent
<br />or limit damages to development that was either knowingly or inadvertently
<br />placed within the floodplain. Thousands of water supply projects, particularly
<br />in the arid vVest, dramatically changed the natural resources of riparian areas.
<br />Millions of acres of inland and tidal wetlands were filled or drained, causing
<br />loss of natural flood storage areas, a lowered capacity for flitration of poilu.
<br />tants and groundwater recharge, and reduction or elimination of some
<br />\\rildlife species.
<br />By the late 1970s it was estimated that from 3.5 to 5.5 million acres of
<br />floodplain land had been developed for urban use, including more than 6,000
<br />communities with populations of 2,500 or more, Annual growth in these
<br />floodplain areas was behveen 1.5% and 2.5% during the 1970s, roughly twice
<br />that of the country as a whole. The coastlines of the United States have been
<br />attracting people and their accompanying property and infrastructure in ever-
<br />increasing numbers for several decades. The 1980 U.S, Census units within
<br />50 miles of the Atlantic and Gulf coastlines increased in population from 34.1
<br />million in 1940 to 63.3 million in 1980-an increase of 85%, compared with
<br />70% for the nation as a whole. The population of Gulf Coast counties
<br />increased by 200 %.
<br />In 1991 the floodplain lands in 17,466 examined communities occupied a
<br />total of 146,600 square miles (93.8 million acres), including about 9.6 million
<br />
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