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<br /> <br />CHAPTER 2: ECOLOGICAL OVERVIEW-POSTFLOOD RECOVERY AND ECOSYSTEM MANAGEMENT 19 <br /> <br />considerable portion to the local economy. but it has dwin- <br />dled to a few hardy individuals who struggle to make a liv- <br />ing, The decline has been a result of perturbed river- <br />floodplain environmental controls (Sparks. 1992). including <br />hydrology and access to productive floodplain habitats, <br /> <br />ILLINOIS RIVER REACH <br /> <br />RIVER ENGINEERING <br /> <br />The Illinois River (fig, 2-1) is divided into the upper. <br />middle, and lower sections on the basis of geomorphic and <br />ecological criteria (Sparks and Lerczak. 1993), The upper <br />minois is the reach above Starved Rock Lock and Dam; the <br />middle and lower Illinois reaches extend downstream to the <br />confluence with the Mississippi River. The upper river has <br />been modified by canal construction that linked it with Lake <br />Michigan via the Des Plaines River and Cal-Sag Canal. It is <br />a steep-gradient stream, much different in character from <br />the lower river because of its narrow floodplain and exten- <br />sive urbanization. The upper reach was greatly influenced <br />by industrial and sewage pollutants following the diversion <br />of water from Lake Michigan, <br />The middle and lower river are much more characteris- <br />tic of a river-floodplain ecosystem than is the upper reach, <br />When water was diverted from Lake Michigan in 1900. <br />low-flow water elevations were increased by more than 1.0 <br />meter. Sustained high-water elevations created many large <br />backwater lakes in what had previously been smaller lakes <br />and ponds of various shapes and sizes (fig, 2-4), Permanent <br />water-surface area doubled after the diversion. <br />Five locks and dams were constructed bel ween 1933 <br />and 1939 (fig, 2-1), The dams stabilized low-discharge <br />river stage above the increase from water diversions, and <br />the amount of wetland habitat declined as the shallow <br />marshes were transformed to large, permanent lakes. Today, <br />120.000 acres of floodplain habitat have been sequestered <br />behind levees (fig. 2-10), A unimodal hydrograph has per- <br />sisted through all the hydrologic manipulations (fig, 2-2D), <br />The conversion of floodplain for agriculture has been <br />extensive (fig. 2-10). Levee construction has sequesrered <br />more than 50 percent of the floodplain habitat (Thompson. <br />1989), Less than 5 percent of the floodplain is in public <br />ownership. <br />Sedimentation rates have been extremely high due to <br />intensive row crop production. field dminage, and tributary <br />stream channelization throughout the basin, Because of <br />levee construction and high rates of sedimentation, Illinois <br />River backwater lakes have lost considerable water depth. <br />They may revert to terrestrial habitat in the next 50- I 00 <br />years (Bellrose and others, 1983; Bhowmik and Adams. <br />1989), <br /> <br />WETLAND HABITATS <br /> <br />Aquatic plants flourished following the expansion of <br />aquatic habitat (Starrett and Fritz. 1965); emergent marshes <br />were inundated. Excessive sedimentation and wastewater <br />pollution eventually reduced aquatic plant abundance. and <br />the backwater lakes were converted to broad, open-water <br />habitats after the 1950's. Because there are few wind and <br />wave breaks in the large lakes, wind-generated waves resus- <br />pend sediments, which decrease light penetration through <br />the water. <br /> <br />INVERTEBRATES <br /> <br />Declines in invertebrate abundance are well docu- <br />mented (Richardson. 1921; Paloumpis and Starrett. 1960; <br />Sparks and Sandusky. 1983), Sewage transport from Chi- <br />cago resulted in a biological wasteland in which only the <br />most tolerant worms and midge larvae were found. Finger- <br />nail clams that supported many bottom feeders almost com- <br />pletely disappeared from the system during the mid-1950's. <br />Epiphytic invertebrates declined along with aqualic plants, <br /> <br />FISHES <br /> <br />Fish populations were once among the most productive <br />in the world. but they too suffered from pollulion. sedimen- <br />tation. and loss of the floodplain. A study of hoopnet <br />catches from the 1930's. 1940's. 1950's. and 1970's shows a <br />tmnsition from a lacustrine to a riverine community <br />(Atwood. 1984) following the loss of backwater lakes when <br />they were converted to agricultural levee districts. High <br />concentrations of pollutants from urban areas contaminated <br />sediments and fish foods, causing morphological abnormal- <br />ities in fish (Sparks and Lerczak. 1993), <br />While the above description of the Illinois River <br />sounds dismal, there are signs of recovery. Popular sportfish <br />populations are increasing, and "rough" fish populations are <br />on the decline. The occurrence of abnormalities in fish is <br />also reduced (Tom Lerczak. personal commun,. 1993). <br />Mechanisms responsible for this recovery are improved <br />water quality attributable to extensive sewage treatment <br />facilities serving large urban areas (i,e.. Chicago and Peoria) <br />and controls on point source pollutants. <br /> <br />MIDDLE MISSISSIPPI REACH <br /> <br />RIVER ENGINEERING <br /> <br />The middle Mississippi River (see fig. 3-7 in Chapter <br />3) has a character much different from the reaches discussed <br />earlier, Floodplain development in the middle Mississippi <br />River was extensive by the late 1800's; agriculture occupied <br />about one-third of the floodplain habitat. River regulation <br />was limited to maintaining a 6-foot channel, and river <br />