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34 OVERVIEW OF RIVER-FLOODPLAIN ECOLOGY IN TIlE UPPER MISSISSIPPI RIVER BASIN migrating waterfowl found an abundance of acorns and other food during their /light south. THE MODIFIED RIVER Human impacts on the UMR were minimal before the nineteenth century. In the early nineteenth century. Euro. American settlement within the UMR valley increased steadily. During the 1830's, snags and other local ohstruc- tions such as shoals, sandbars, and rocks were removed from the main.stem Mississippi River to ensure a safer pas- sage for steamhoats (UMRBC, 1982). In the second half of the nineteenth century, steamboat traffic increased sharply. In 1878, in response to the increasing navigation demands, Congress authorized the U.S. Anny Corps of Engineers to develop and maintain a 4.5-foot-deep navigation channel between St. Paul, Minnesota, and SI. louis, Missouri. To divert river flows into the main channel, wing dams were constructed perpendicular to riverbanks. Side channels were cut off with closing dams and many riverbanks were stabi- lized by revetments. In 1907, Congress authorized a deeper 6-fool channel project (UMRBC, 1982). Subsequent river modifications consisted of further river contraction and bank protection and the construction of the first lock and dam at Keokuk, Iowa, in 1913. In 1927, Congress authorized the development of a navigation channel 9 feet deep and 300 feet wide between the mouth of the Missouri River near SI. Louis to the mouth of the Ohio River, near Cairo, Illinois. The 9-foot channel project resulted in much more extensive flow constriction and many more bank stabilization structures. This portion of the UMR is approximately 201 miles long and is referred to as the "open river" because locks and dams are not used along this stretch of river to maintain the navigation channel. In 1930, Congress authorized the extension of the 9-foot channel between St. louis, Missouri, and SI. Paul, Minnesota. During the 1930's, a series of27 locks and dams was constructed. Each dam impounds water during low river flows to maintain a minimum 9-foot-deep navigation channel. This portion of the UMR is approximately 652 miles long and is referred to as the "impounded river." Each river reach is named after the lock and dam. For example, the reach downstream of Lock and Dam 25 and upstream of lock and Dam 26 is referred to as Navigation Pool 26. Prior to or concurrent with navigation projects, private and Federal levee systems were built to manage floodwater. At the open-river reach, construction of State and Federal levees started afler 1881 (Chen and Simons, 1986; Johnson and others, 1974), but was not intensive until after 1907 (Simons and others, 1974). Dikes and revetments lock the position of the river, and levees prevent overbank flow from spreading. By restricting channel meandering and by increasing flood intensities. humans have altered the condi~ tions to which the natural forest species were adapted (fig. 3-48). This has resulted in forests of less diversity because willow and silver maple have replaced most of the other species present in the early 1800's. The flood regime has been altered due to dam operations that maintain target water levels throughout the year (fig. 3-58). This stmtegy of water-level management eliminates the period of low flow that was part of the natural river's hydrologic pattern. Thus forest species now bordering the river in impounded reaches must be well adapted to high soil moisture content throughout the growing season. Silver maple is well adapted to the modified conditions and. as such, has become the most abundant species on the floodplain. It is quite possible that our future forests may be entirely dominated by silver maple because this species is best adapted to modified con- ditions, especially high soil moisture and increa<;ed flood disturbance. Although the entire modem-day UMR is highly restricted and regulated, events like the flood of 1993 are constant reminders of one very important characteristic of large floodplain rivers, namely, that the river and its flood- plain are closely linked due to processes beyond human control. PRESETTLEMENT FLOODPLAIN FORESTS Before Euro-American settlement, the floodplain of the UMR consisted of vast swamplands, prairies, marshes, and forests (Finiels, 1797; Turner, 1934; Nelson and others, 1994). At the southernmost portion of the UMR near Cape Gimrdeau, Missouri, the presenlement floodplain landscape was dominated by forests (fig. 3-1A). In 1809, Government Land Office (GlD) surveyors recorded 19 tree taxa along the Mississippi riverbanks as witness trees. Of these. cotton- wood (Popllllls del/aides Marshall) and sycamore (Pia/an liS occidentalis L.) were the two most dominant species. Fur- ther away from the river, in that portion of the floodplain that today is shielded by the mainline levee, GlO surveyors recorded a total of 3 I tree taxa (tahle 3-1). Of these taxa, elm (Vlmlls rllhra Muhl, V. spp.), hackberry (Cel/;s occi- del/lalis L.), sweetgum (Liqllidambar s/yracijllla L.), and ash (Fraxinlls pennsylmnica Marshall, F. spp.) were the most dominant species. Close associates are hickories (Carya spp.), white oak (Qllerclls alba L.), American beech (Faglls grandifalia Ehrh.), and yellow poplar (Liriodendron wlipifera L.). North of the open river near SI. Louis, prairies domi- nated the presenlement floodplain landscape. Forests were restricted to areas along the riverbanks, tributary streams. and isolated groves surrounded by floodplain prairies. GlO surveys at the confluence of the Illinois and Mississippi Rivers in 1817 recorded 18 taxa of witness trees (table 3- I).