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30 OVERVIEW OF RIVER-FLOODPLAIN ECOLooy IN THE UPPER MISSISSIPPI RIVER BASIN C "iii..... c..~ >/- oS u: ;; > i'C '", 0. '~ '~ " A -g ~: . c, 1?i. ca. ,ji, :.g: :00: Presettlement Floodplain Mississippi river mile 58 C "- c..~ ~.;; u: C 'iii_ ~B .. " .;; -g . 0. ::> ;; .. . C !i 0. " ~ ~ .\. '~""~11.> . , -~:U-tiJ . Missouri Bluff Swale Ridge o:~ Aoodplain (distance = 4-9 miles) Illinois Bluff .. " C ,~ e 5 "iii_ S ~ 0.- ex: ~ ~~ " '~ 'C ~ " 'il '~ " B Missouri Bluff Area of detail in Ii ure3-4 B I I Agricultural levee Agricultural S I R'd levee wa e I ge Floodplain {distance _ 4-9 miles} Present Floodplain Mississippi river mile 58 c "iij- c..:g ~j; u: e , ~ ~ " 'C ~ " c "m_ c.:g >/- oS u: e " '5 " 'C ~ " .. " .;; -g . 0. ::> c , . 'iii...... . 'Q.<Il. , >/"' : 0';;: 'u: ;; .. . c !i 0. " ~ o ~~ Oxbow lake Illinois Bluff Figure 3-1. Cross-section diagrams of the Mississippi floodplain at river mile 58 of the open-river reach. A.ln the presettlement era, the floodplain was covered with bottomland hardwood forests. 8, Presently. levees and dikes restrict river meandering, and most of the bottomland hardwood forests have been cleared for agriculture. between bluffs, Four river categories were identified by the Upper Mississippi River Basin Commission (UMRBC) (1982): river valley bounded by bluffs. river bounded by floodplains. river bounded by lerraces. and river wilh ter- races located outside of a tloodplain (fig. 3-3), Terraces are high landfonn fealures Ihal are rarely or no longer flooded and floodplains lhat are periodically inundated by overbank flows (Maddock. 1976). Similar to other large alluvial riv- ers. the natural UMR not only interacted with its tloodplain Ihrough periodical overbank floodwaters but also through changes in river and tributary positions, Older woodlands and marsh areas were eroded. and old backwater lakes were filled in with sediments. New islands and marsh communi- ties were fomled, and new backwater lakes created. The sometimes dramatic scenario of the natural river-floodplain landscape was captured in the notes of early explorer Henry Lewis (1854), ...The islands do not originate in the middle of the river, but since its bed is constantly changing;. the current hurls itself against the islands and drags them into the flood along with their trees and brush. The power of the current is so irresistible that we can cite an example in which an island of twenty-five acres was swept away in less than three weeks. Thousands of trees Ihrown into the stream in Ihis way form snags and sawyers. From the Fall of 51. Anthony to New Orleans there are more than three thousand islands that were fonned thus.... While the above description illustrates the sometimes powerful nature of Ihe flooding Mississippi and the dra- matic changes that could occur to the landscape, the average spring flood of Ihe time was much more subtle. Natural annual hydrologic patterns from the 1800's (figs, 3-4A and 3-5A) exhibit moderate annual flooding, In lhe presettle- ment era. spring floodwalers gradually rose out of the river- banks and tlowed into the adjacent floodplain, While the entire floodplain was flooded during exlreme events in some years, most often the flooding occurred in the lowest lying floodplain habitats adjacent to the Mississippi and its tributaries. Later in the summer, the floodwaters retreated to well within the riverbanks. At times, the river became so shallow in some reaches Ihat il was possible 10 walk across. The forests along (he river corridors consisted of many species, including cottonwood. pecan, willow, silver maple. sycamore. elm. sweelgum. and hackberry (lab Ie 3-1).