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10 OVERVIEW OF RIVER-FLOODPLAIN ECOLOGY IN THE UPPER MISSISSIPPI RIVER BASIN 1842, in Simons and others. 1975), The channel was deeper and wider than upstream, and many sand islands and side channels were created and destroyed with fluctuating water levels. The channel was much more dynamic than upstream because flows were greater (Simons and others, 1975). Pre- dam hydrographs show a unimodal hydrograph (fig, 2-2) but with a different character than the Illinois River because of the influence of the Missouri River. The Illinois River differs from the Mississippi and Missouri Rivers because it was a slow flowing river with a very low gradient (Mills and others. 1966; Talkington. 1991), Floodplain depressions behind natural levees. braided channels. and side channels formed many deep. per- manent as well as shallow. ephemeral wetlands (fig, 2-4), Average floodplain width in the lower Illinois River is 4.1 miles (Simons and others, 1975). and prior to 1903. the reach below Starved Rock supponed 56.000 acres of back- water lakes and wetlands (Bell rose and others, 1983), A unique feature of the Illinois River floodplain was an abun- dance of sand prairie habitats (William and Frye. 1970) created by glacial processes. The Illinois River exhibits a unimodal hydrograph (fig, 2-2D), PLA"TS Basinwide land cover/land use may influence nutrient and energy transport to the river. Historical plant communi- ties throughout the basin of the upper floodplain reach of the UMR consisted of a mix of maple/basswood forests. oak savannas, and northern floodplain forests (Populus, Sa/ix. Ulmus) (Klichler. 1964), In the lower floodplain reach the basin expanded into an oakjbluestem savanna that contin- ued southward into the middle Mississippi reach (Klichler. 1964). where the great prairies oflhe Missouri River basin affected the Mississippi River, The Illinois basin was largely oakjbluestem savanna (Klichler. 1964). Floodplain vegetation was much more diverse than basinwide generalizations can account for. Because of the many microhabitats crealed by topographic and hydrologic variation. tloodplains supponed high diversity of plant spe- cies, Nelson and olhers (1994) (also see Chapter 3) used original land survey records to reconstruct historical forest composition and plant community distribution on the flood- plain, The method lends itself to forestry studies. but gener- alizations of broad habitat types can be made, A detailed description of the systemwide presettlement herbaceous flora may be impossible to reconstruct, but a thorough description of the vegetation near St. Louis, Missouri, was completed in 1908 (Hus, 1908). and similar effons may exist for other locations. Descriptions of historical plant communities can help reconstruct past faunal communities because of distinct habitat associations of many species. Plant species and wetland plant community structure were most likely similar to the highly diverse river-f1ood- plain plant communities found today except that herbaceous wetland and prairie plants were more widely distributed in numerous small floodplain wetland habitats (Nelson and others. 1994), Algal and aquatic production was probably higher in the UMR and Illinois River because sediment delivery from the unpenurbed basin was lower than present day, Densely vegetated riverbanks and backwater lake shorelines served to trap suspended sediments and improve water clarity. INVERTEBRATES Invertebrate populations in the predam era are largely undocumented but were probably very similar in species composition to what is found today, Richardson (1921) conducted extensive studies in the Illinois River and found high abundances of mollusks. caddisflies, and maytlies, Zooplankton, invertebrates indicative of good water quality, and epiphytic invertebrates were abundant because of greater algal availability. better waler quality. and abun- dance of aquatic plants. Production of invertebrates was likely very high because of the presence of a natural hydro- graph that exposed aquatic invenebrates to high habitat diversity and food resources on the floodplain. Mussel fauna was diverse throughout the basin, and most of the 297 North American species occurred in the Mississippi River basin. In the UMR. 46 species were once common, Substrate diversity (gravel. sand. and mud) sup- ported many species with differing habitat requirements. Mussels inhabited a variety of channel and backwater habi- tats that were previously more common in the UMR. FISHES Fish species composition was similar to what is found today because few fish species have been extirpated from the syslem completely, Repons from the Crawford County Weekly Courier. Prairie Du Chien. Wisconsin (June 16. 1852), suggest that every kind of fish could be taken from the river "from a catfish of fony pounds to a tadpole" (Car- lander. 1954), Other repons indicate that fish abundance was high and that the river could be counted on as food sup- ply for the soldiers at Fon Snelling, Minnesota (Carlander. 1954), On the Illinois River, travelers with LaSalle in 1687 reported fish so dense that the travelers did not even need a net to catch the fish, Paddlefish and sturgeon were strange new fishes to the European explorers; there were reports of lake sturgeon as big as a canoe, and catfish weighing 100 pounds or more were commonly caught (Carlander. 1954),