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<br />6 OVERVIEW OF RIVER-FLOODPLAIN ECOLOGY IN THE UPPER MISSISSIPPI RIVER BASIN <br /> Upper Floodplain Reach - RM726 Lower Floodplain Reach - RM218 <br /> ". '" <br />~ " <br /> > Posldam <br />.. .. <br />m m '" <br />m m <br />~ ~ <br />~ ~ <br />.8 '" 0 <br /> .Q <br />m m '" <br />l!! l!! <br />m m <br />" " <br />.s .s '" <br />c c <br />,Q ". 0 <br />~ .~ <br />m m <br />" " '" <br />~ ~ <br />ii: ii: <br /> '" '" <br /> " " " '" '" '" '" '" '" ,,, ,,, ,,, " " " '" '" '" '" '" '" "" ,,, ,,, <br /> A Day of year B Day of year <br /> Middle Mississippi. RM46 illinois River Reach. RM61 <br /> '00 '" <br />" " <br />> .!; <br />.. " <br />m m <br />m m <br />~ ~ '" <br />~ " ~ <br />0 0 <br />.Q .Q <br />m m <br />~ Posldam ~ <br />~ " ~ '" <br />1i> " <br />.s .s <br />c " c <br />,Q ,Q <br />~ ~ '" <br /> m <br />" " " <br />~ Predam ~ <br />ii: " ii: '" <br /> " " " '" '" '" '" '" '" "" '" ,,, " " " '" '" '" '" '" '" "" '" '" <br /> C Day of year D Day of year <br /> <br /> <br />Figure 2-2. Hydrographs at long-term gage stations in each of <br />fourriver reaches (Lubinski, 1993). RM, river mile. The areas <br />between the Mississippi River dams shown in figure 2-1 are <br />called pools and are named after the downstream dam. For exam- <br />ple. Poo126 is located between Dams 25 and 26. A. The upper <br />floodplain reach (headwaters to Pool 14) in the predam era was <br />characterized by an average pattern of high river stage during <br />snowmelt and spring rains that tapered to summer low.flow river <br />stages, rose with fall rains, and froze at a moderate river stage in <br />the winter. Navigation dams increased average water-surface ele- <br />vations approximately 1 meter and eliminated natural low-flow <br />river stages. B, The lower floodplain reach (Pools 15-26) had a <br />similar hydrograph except that flooding started earlier and lasted <br />longer, given its more southerly location and large watershed. Ice- <br />~ver was not as pronounced as in the upper floodplain reach. Nav- <br />igation dams raised low~stage water-surface elevations almost 4 <br />meters in some areas, and flood flow stages are reduced by 1.5 <br /> <br />During summer low-flow periods. both habitats are likely to <br />develop plant communities that are submergent in deeper <br />water, emergent in shallow and fluctuating waters, semi- <br />aquatic above the "average" low-water level, and tolerant <br />to various degrees of inundation above the "average" <br /> <br /> <br /> <br /> <br />meters at this location near Grafton, Illinois. C. The middle Mis- <br />sissippi River is hydrologically distinct from the upper Mississippi <br />River because of the strong influence of Missouri River flows. <br />The predam hydrograph is unimodal, with a peak flow depression <br />that may correspond to a transition from snowmelt to rain in the <br />basin. River stage ranges are wide, and fluctuations can be rapid. <br />River flow is highly regulated by channel training structures to <br />increase scour in the main channel. The postdam average hydro- <br />graph is elevated, and peak flow distribution is modified from the <br />unregulated hydrograph. Storage dams on the Missouri River reg- <br />ulate discharge to maintain navigation (see Chapter 6). D. In the <br />Illinois River, the combination of water diversion from Lake <br />Michigan and navigation dams has increased water-surface eleva- <br />tions about 1.5 meters. Seasonal patterns appear unaffected. which <br />provides hope for resource management on the lIlinois River. Low <br />river stages are necessary throughout the system for the mainte- <br />nance of a diverse, healthy river-floodplain ecosystem. <br /> <br />high-water level (see figs. 3-1 and 3-2 in Chapter 3), Island <br />vegetation varies on the basis of degree of inundation. Char- <br />acteristic patterns. are barren or grassy islands in the lower <br />reaches of the UMR and forested islands in the upper <br />reaches and Illinois. River. <br />