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Chapter 3 MODIFICATIONS OF THE UPPER MISSISSIPPI RIVER AND THE EFFECTS ON FLOODPLAIN FORESTS By Yao Yin I and John C. Nelson2 INTRODUCTION Large floodplain rivers are among the most highly pro- ductive ecosystems worldwide. Their high productivity is believed by many biologists to be closely related to periodic interactions between the aquatic river environment and the terrestrial floodplain environment (Brown, 1985: Junk and others, 1989), Processes such as flooding. sedimentation, and erosion are powerful natural forces that shape and maintain the character of plant communities within large river-floodplain ecosystems. Species adaptations to water regimes still dictate plant a"emblages despite human alter- ations to the system, For this reason, it is likely that hydro- logic alterations are a key factor to understanding past and present characteristics of floodplain forests. It is no wonder that from the beginning of recorded human history. civilizations prospered near large floodplain rivers. Before Euro-American settlement, many portions of the floodplain along the upper Mississippi River (UMR) were made up of prairie and forested wetlands (Finiels. 1797), The presettlement bottomland forests were diverse in age structure and high in species richness because the Mis- sissippi River and its tributaries meandered freely within the floodplain environment (figs, 3-1A and 3-2A), These flood- plain communities and their abundant wildlife populations provided Native Americans with all the necessities of life, However, to survive in the floodplain. these inhabitants had to be aware of and adapt to the often dramatic but predict- able changes in river stage, Spring flooding could destroy villages, so Native Americans built far away from the river on the edge of the floodplain (Munson and Ham. 1971). I University of Tennessee and NalionaI Biological ServicelEnviron- mental Management Technical Center. 2111inois Natural History Survey, Long Tenn Resource Monitoring Program. When river stages were low, such as in late summer, tempo- rary camps were used along the river to gather fish, mussels, and other food, Today. the UMR and its floodplain are much different than during the presettlement period. Navigation structures, levee systems, and stream channelization within the flood- plain have had wide-ranging effects on the natural pro- cesses, particularly hydrology. that helped shape and maintain the character of the river-floodplain system, A large portion of the UMR floodplain is no longer periodi- cally inundated, and the hydrologic patterns have changed in the river as well as on the floodplain. In general. agricul- ture and urban development have greatly reduced floodplain forest acres. especially in river reaches where a mainline levee system has been established, Compared with preset- t1ement forest composition. the present floodplain forests are generally less diverse. Silver maple has dramatically increased in abundance. and pioneer forests have probably been greatly reduced throughout much of the UMR flood- plain. Our intention in this document is to summarize some of the changes from presettlementto the present, putting special emphasis on hydrology and floodplain forests. The discussion is based on information from Government Land Office surveys conducted during the early part of the nine- teenth century. as well as published literature, current field data. historical river stage data. and field observations by the authors. THE NATURAL RIVER Although major drainage patterns within the UMR basin were established prior to Pleistocene glaciation. the modem course of the UMR emerged following the Wiscon- sin epoch of glaciation approximately 10.000 years ago (Bray, 1985; Hoop. 1993), The Wisconsin period of glacia- tion consisted of a series of episodes and multiple ice sheets. As a result. the modem UMR corridor consists of a combi- nation of floodplain. terraces, and river channels located 29