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32 OVERVIEW OF RIVER. FLOODPLAIN ECOLOGY IN THE UPPER MISSISSIPPI RIVER BASIN A Natural '" '" .. j " " as " III " c: as .. .. E 03 ! , 31 $1 III 121115111121l~1271301331381 Oay I!! B Modified s ~ .s '" c: 0 .'" ~ w A'lerage high Average low 13111111211511812112412713013313(11 Day Average high .. . Sediment deposition " Figure 3-4. Natural and modified hydrologic patterns and floodplain diagrams at river mile 58 of the open-river reach of the Missis- sippi River. A. In the presettlement era, the natural river exhibited moderate annual flooding. B. Presently. both annual high and annual low flows are elevated. A Natural Average high '" } '" III :ij ~ 124 ! '" '" ". ". Ayerage low '" 1 31 8t 11112115111121124'271301331381 Day I!! * B Modified .s c: o .. ~ Average high W '" '" '" ". ". ". '" , " " Average low . Sediment deposition "121151181211241271301331381 Day Figure 3-5. Natural and modified hydrologic patterns and floodplain diagrams at river mile 218 of an impounded river reach of the Mississippi River. A. In the presenlement era, the natural river exhibited moderate spring floods foHowed in the autumn by a second flood of even lesser magnitude. B. Presently. the flood regime ha.~ been altered by dam operations that maintain target water levels throughout the year.