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\095 The'II'lfJ!J REGULATED RIVERS RESEARCH & MANAGEMENT. VOL. 11,227-238 (1995) '612V HABITAT REHABILITATION ON THE UPPER MISSISSIPPI RIVER CHARLES H. THElLING Illinois Natural History Survey-Long Term Resource Monitoring Program, Pool 26 Field Station, 4134 Alby Street, Alton, IL 62002, USA ABSTRACT Upper Mississippi River ecological integrity has been severely compromised by human activity during the last 50 years. In response to the continuing decline of natural resource values, two approaches for protecting and improving the Upper Mississippi River-floodplain ecosystem have been used. Habitat rehabilitation and enhancement projects are being con- structed at 54 locations to provide site-specific rehabilitation. The projects are designed to counteract the adverse eco- logical effects of sedimentation through (I) flow introductions, (2) the isolation of backwaters; and (3) flow diversions and wave breaks. Channel maintenance projects are being re-evaluated in an attempt to construct or modify existing river training structures that are environmentally sympathetic. The latter approach works with the river's energy, whereas the former attempts to overcome riverine processes. Both approaches have significant limitations because they affect limited areas. A proposal is presented that restores some ecosystem integrity by re-establishing occasional low river stages that occurred before the implementation of the Upper Mississippi River Navigation System. KEY wows: Mississippi River; restoration; rehabilitation; river engineering; navigation INTRODUCTION The Upper Mississippi River-floodplain ecosystem is a dynamic system that responds to abiotic controls now largely defined by humans (Starrett, 1972; Bhowmik and Adams, 1986; Chen and Simons, 1986; Grubaugh and Anderson, 1988; Lubinski et al., 1991). Above the confluence with the Missouri River, the Upper Mississippi River Navigation System is controlled with 32 low head dams (Figure 1) to augment low discharge water surface elevations to maintain a 2.7 m deep navigation channel (Figure 2). Below the Missouri River confluence to the Ohio River the channel is maintained with channel training structures (dikes) and dredging only (Chen and Simons, 1986). High river stages (floods) are unaffected by navigation dams and dikes (Chen and Simons, 1986; Lubinski et al., 1991; Sparks, 1992; 1995), but levee districts have sequestered much of the floodplain. More than 50% of the Illinois River floodplain (Starrett, 1972), about 50% of the Upper Mississippi River floodplain below navigation reach 19 to the Missouri River (Peck and Smart, 1986) and nearly the entire floodplain below the Missouri River confluence (Peck and Smart, 1986) is protected by levees. High sediment delivery from a largely agricultural basin (Starrett, 1972; Nielsen et al., 1984) is gradually filling slow-flowing non-channel aquatic habitats (backwaters and side-channels) created and maintained by the dams (Bhowmik and Adams, 1986). Ecological succession is evident from dynamic aquatic and wetland plant communities that have developed in response to reduced water depths and new terrestrial habitats (Peck and Smart, 1986; Sparks et al., 1990). Forests, disturbed originally by the direct effects of logging and inundation, continue to be affected by water-table disturbances imposed by navigation dams (Nelson et al., 1994). Initial increases in fish populations after dam construction (especially lentic species; Fremling and Claflin, 1984) were followed by declines in commercial fisheries (Sparks, 1984; Fremling et al., 1989). Some river reaches are severely compromised with respect to the ecological integrity of the river- floodplain ecosystem, whereas other reaches maintain much of their original ecological form and function (Sparks, 1984; Chen and Simons, 1986; Peck and Smart, 1986; Sparks et al., 1990; Sparks, 1995). Some issues of river-floodplain development are regionalized in the northern (dams and backwater sediment resuspen- sion) or southern (levees and dikes) regions of the system, whereas others are system-wide concerns CCC 0886-9375/95/060227-12 ~ 1995 by John Wiley & Sons, Ltd. Received 15 July 1994 Accepted 7 February 1995