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CHAPTER 3: MODlFlCA TIONS OF UPPER MISSISSIPPI RIVER AND EFFECTS ON FLOODPLAIN FORESTS 35 These forests were dominated by hackberry, pecan (Carya illinoinensis IWangenl] K. Koch), elm, willow (Salix spp.), and close associates, including silver maple (Acer sacchari- num L), pin oak (Quercus pa/ustris Muenchh.), and ash. The GLO surveyors of both these study areas described some of the conditions they encountered on the floodplain. In the open-river reach near Cape Girardeau, Missouri, surveyors encountered numerous cypress swamps. In 1809 the swamps vatied greatly in size, the high water making the largest ateas inaccessible. In 1850, another survey was attempted in the Horseshoe Lake area. After measuring one section line. the deputy surveyor noted problems related to flooding: "The line cannot be surveyed at this time, in consequence of the overflow [that] occurred by the Ohio-river backing up the Cache-river [sic], which has inundated the country in many places along the section line, to a depth of three to five feet.. .:' (Government Land Office, 1850). Near St. Louis, a deputy surveyor noted at the Mississippi riverbank "level rich bottom and subject to inundation of from 2 to lOft deep as appears by the water marks on the trees. Timber cottonwood, sycamore, elm, red bud, and pin oaks, undergrowth vines and bushes of various sorts:' After measuring 57 chains (about 1150 meters) away from the riverbank, he also noted, "Top of bank 10 feet high (on) south side of a lake where (we) enter prairie. The prai- rie is good rich soil and fit for cultivation" (Government Land Office, 1844). Trees of very large size were not uncommon in the GLO notes; cottonwoods were recorded as 5 and 9 feet in diameter. According to GLO survey records, presetllement floodplain forests further north in southeastern Minnesota (Houston County) and northeastern Iowa (Allamakee and Clayton Counties) were dominated by ash (Fraxinu.. spp.) and silver maple (Moore, 1988). A total of 26 taxa was recorded among 950 trees in the GLO records. Other com- ponents recorded were hickory species (Carya spp.), two walnut species (Juglans cinerea, 1. nigra), and five oak spe- cies (Quercus a/ha, Q. him/or Willd., Q. macrocarpa Michx., Q. velutina Lam., Quercu.. spp.). PRESENT FLOODPLAIN FORESTS AT THE OPEN-RIVER REACH Federal levees and navigation structures have changed the character of the Mississippi River and its hydrologic regime at the open-river reach. Because floodwaters are restricted to a much narrower area between levees, intensity and duration of flooding are aggregated (fig. 3- IB; Belt, 1975). Elevated floodwaters are now more likely to overtop tree root crowns and remain this high for an extended period of time. As a result, tree growth may be adversely affected, and some tree species that are less flood tolerant may disap- pear (Johnson and others, 1974). Within levee districts, where flooding can no longer occur, the impact of levees on the forests is the opposite. In these districts, moisture and nutrients are no longer replenished by periodic overbank flows. Also, because the bed of the main channel has been lowered as much as 11 feet (Johnson and others, 1974), less moisture may be available from the underground water table. A 1993 survey (table 3-1) at the open-river reach near Cape Girardeau. Missouri, indicates that changes in forest composition and structure since presettlement are related to changes in hydrology resulting from navigation structures and the Federal levees. The number of species encountered has decreased on both sides of the levees. Adjacent to the Mississippi River and between levees, species such as oak (Quercu.. spp.), American beech, walnut (ful?/ans ',pp.), pecan, and hickory have disappeared, and the abundance of cottonwood and sycamore. two pioneer species that require newly fonned and somewhat sandy substrates for regenera- tion. have also decreased significantly. Willow and silver maple have replaced cottonwood and sycamore as the dominant species. Sediments that rap- idly accumulate in the fields between wing dams have nar- rowed the river channel. These newly fonned sites usually are quickly invaded by willow, which are soon replaced by the more shade-tolerant silver maple. Outside the mainline levee and within the levee districts, tree species typical of pioneer and transitional forests such as cottonwood. sycamore. elm, and hackberry have decreased since preset- tlement. Pin oak has become the most dominant species because the floodplain has been drained for agriculture (table 3-1), and flooding has been eliminated, and pin oak possibly prefers the resulting drier site conditions. PRESENT FLOODPLAIN FORESTS AT THE IMPOUNDED RIVER REACHES The effects of navigation dams have significantly changed natural hydrologic patterns within the impounded reaches of the UMR (Grubaugh and Anderson, 1988). How- ever, the degree of change varies with proximity to a navi- gation dam. Annual water-level patterns of the river immediately below each dam are most similar to the natural or predam hydrologic pattern, as displayed in fig. 3-2A. During high flows. the rapid current, reduced in sediment load, scours the channel. During low flows, water levels drop to well within the riverbanks, and the water table in the floodplain is similarly lowered. Immediately above each dam, water-level patterns are most dissimilar to natural or predam patterns. At these sites, water levels are most severely raised and most stable throughout the growing sea- son (fig. 3-2B). Floodplain forests immediately upstream from navigation dams are probably subjected to high soil saturation throughout much of the year due to elevated water tables.