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HABITAT REHABILITATION 235 upstream of perpendicular to divert flow, in progression, towards the inner bank. The result is hydraulically controlled point bar development and reduced downcutting throughout the bend. The channel bottom affected by the dikes is reduced in depth (3 versus 10 m) and has increased structure and hydraulic variation. Physical and computer model evaluations showed the design to be successful. Physical evaluations of the first structures constructed in 1989 are very encouraging. Ecological monitoring has been hindered by harsh sampling conditions in the high current velocity of the main channel. LIMITATIONS OF CURRENT APPROACHES Considering the rigorous criteria used to define true ecological restoration (Cairns, 1991) or ecological integrity (Angermeier and Karr, 1994), the projects described above fall far short of achieving Upper Mississippi River restoration or ecological integrity; but they were not intended to. They do, however, illustrate the two prevalent natural resource management philosophies in the Upper Mississippi River. The first philosophy embraces the concept of rehabilitation (Gore and Shields, 1995), which strives to protect or improve Upper Mississippi River aquatic habitats from adverse affects associated with navigation (i.e. high sedimentation rates in dammed reaches). The second philosophy embraces the concept of redesigning navigation structures in a more environmentally sympathetic manner (Petts, 1989). I accept the premise that some site-specific rehabilitation is necessary to protect and improve degraded habitats, but encourage techniques that work with the river's energy to produce self-sustaining results, as is the goal of the channel maintenance projects. Unpublished experiments show that water level regulation in the dammed portions of the system can be conducted in a manner to promote desired ecological conditions. In 1994, experiments were conducted to test the possibility that reducing water levels in the lower half of three navigation reaches (drawdowns) for a six-week period in the spring would allow emergent plant community development on exposed mud flats. Initial observations revealed substantial plant colonization of mud flats after three weeks of exposure. The plants survived partial inundation when water levels were returned to their normal higher level maintained by the dam (K. Dalrymple, Missouri Department of Natural Resources, pers. comm,). The possibility of manipulating water levels is encouraging for the potential to produce emergent plants, but presents some unique problems for fish. Moderate flows necessary to allow water level management flexibility (D. Busse, US Army Corps of Engineers, pers. comm.) usually occur during spring floods when floodplains should be inundated, whereas drawdowns used to prevent flooding present the opposite effect (Lubinski et al., 1991; Sparks, 1995). Drawdowns therefore prevent fish access to the most valuable spawning, rearing and feeding habitats (Sparks, 1995). A potential solution to be tested in 1995 is to operate two navigation reaches to produce plants and a third to produce lentic fish. Water levels in the pools manipulated to produce plants will be held more than O' 5 m below normal controlled water elevations, thus exposing mud flats. The third pool will be held constant near the normal water elevation to ensure that fish eggs are not exposed or young of the year fishes are not flushed from backwaters (Dalrymple, Missouri Department of Natural Resources, pers. commun.). The major drawback of these experiments is the lack of ecological evaluation except for plants. System-wide drawdowns Navigation reach scale water level manipulations are a step towards improved natural resource manage- ment on the Upper Mississippi River, but they will not restore the loss of abiotic controls (i.e. low river stages; Figure 2) imposed by navigation dams. I believe sediment-related problems associated with the Upper Mississippi River Navigation System need to be treated on a scale comparable with the perturbation caused by navigation dams. I propose for consideration by Upper Mississippi River System natural resource professionals, navigation system engineers, navigation industry representatives and recreational users a plan that would allow for infrequent (e.g. 10 year interval) system-wide drawdowns to expose and consolidate backwater and channel border sediments. The proposal is based on four considerations: (I) large river-floodplain ecosystems are adapted to, and indeed dependent on, hydrological disturbance (i.e. flood and droughts) (Welcomme, 1979; Junk et al., 1989;