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CHAPTER 2: ECOLOGICAL OVERVIEW-POSTFLOOD RECOVERY AND ECOSYSTEM MANAGEMENT II PRESENT RIVER E!>;GINEERI!>;G Early navigation improvements, including the 4- and 6-foot channel projects, proved inadequate for many com- mercial shippers, In 1913 the first UMR lock and dam was put into operation at Keokuk. Iowa, It was constructed by a private firm and was one of the largest hydroelectric devel- opments of its day. A minority group of waterway interests fought unsuccessfully between then and the Roosevelt era to create a reliable 9-fool navigation channel from 51. Louis, Missouri, to St. Paul. Minnesota. There was significant opposition to the project. but following the Great Depres- sion, public works projects were passed by the U.S, Con- gress with assistance from the White House (Hoops, 1993), Thirty-five locks and dams were ultimately constructed dur- ing the 1930's to help "revive" UMR navigation and create jobs during Roosevelt.s New Deal era (figs, 2-1. 2-5), Sig- nificant levee construction had occurred before this time and continued concurrent with navigation improvements. Floodplain habitats were being "reclaimed" at an alarming rate (fig. 2-4). The dams were designed to impound water to create a 9-foot navigable channel during low to moderate flow peri- ods (fig, 2-5), Dams have some control over moderate flooding but do not affect large floods, They are designed so that the gates are either raised completely or lowered on the lIlinois River during periods of high discharge. Mainte- nance of artificially high water elevations. reduced current velocity, and floodplain constriction all contributed to the ecological degradation of the UMR (UMRBC. 1982; Bell- rose and others. 1983; Grubaugh and Anderson, 1988; Bhowmik and Adams, 1989), Operation of the dams increased water-surface eleva- tions in the lower and middle portions of the river reaches commonly called "navigation pools," The tenn "pool" is misleading because it implies that the river is impounded like a reservoir, when in fact it is still riverlike in fonn and function. Navigation dams did, however, increase low-flow elevations and created three hydrologically distinguishable regions within the reach between two dams. Three regions are loosely defined as "upper pool" (upstream one-third to one-half of a reach). "midpool" (middle half to two-thirds of a reach), and "lower pool" (downstream one-third to one- half) (Fremling and others. 1989), Upper pool reaches retain most of their predam hydrologic characteristics and contain narrow channels. The midpool reach shows effects from downstream impoundment (i.e.. increased low-flow water- surface elevations), but the upstream end responds more naturally to hydrologic events, acting as a transitional zone between the upper and lower pool reaches, The lower pool reach is the most hydrologically disturbed, Dams have cre- ated large open-water areas ("impoundments") over what was once productive floodplain habitat (fig. 2...{j). Pooled reaches of the Mississippi River exhibit hydro- logic zonation in relation to their proximity to the impound- ing dam and floodplain gradient. The relations under three levels of control are shown in figure 2-7. In unregulated river systems. lateral expansion is determined by floodplain topography and relief, In general. dam point control creates the hydrologic condition described above, The same low-flow pool shape is maintained on pools controlled at a midpool control point. but an additional hydrologic/ecological perturbation occurs during moderate flows. Water releases are increased with flow to moderate flooding at midpool reaches, The result is reduced flooding in midpool reaches and lower pool drawdowns that leave shallow backwater and channel border habitats exposed (fig, 2-8), The timing of drawdowns is especially detrimen- tal because riverine organisms are adapted to flooding dur- ing seasonal periods of high discharge. Drawdowns reduce habitat availability during spring floods. when floodplain resources are most critical to successful reproduction and growth of river fauna. At some level of high discharge. all dams go to "open river" (fig, 2-7). where they have no influence on the river at all. There is some leeway between open river and severe flooding because of dam placement on the floodplain and structural flood protection measures (levees). Most dams. except Lock and Dam 19. operate under open-river condi- tions for some part of each year because they are intended to augment navigation at low to moderate flow only. Navigation dams have a highly visible impact on the river. but other, less obvious. impacts have also occurred. In the middle Mississippi River. huge levees constricted the floodplain. and channel training structures eroded the river- bed. In the Illinois River. water diversions from Lake Mich- igan transported huge volumes of water. sewage, and contaminants. Each of the four reaches is discussed in more detail below. UPPER FLOODPLAIN REACH (POOLS 1-13) RIVER ENGINEERtNG Fourteen navigation dams were constructed in the upper floodplain reach between Minneapolis, Minnesota, and Clinton. Iowa, Pools 2-8 and 10 use midpool control points or a combination of mid pool and dam point control. A unique feature of the upper floodplain reach was the pur- chase of approximately 50 percent of the floodplain envi- ronment to create the Upper Mississippi Fish and Wildlife Refuge (UMRBC. 1981), Only 8.000 acres have been sequestered from the river behind levees. High spots on the floodplain created many islands when water levels were raised by the dams (fig, 2...{j). Their number declined as they were eroded by wind- and boat- generated waves rolling across the open area (impound- ment) of the lower pool reach, Lower pool reaches have also