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36 OVERVIEW OF RIVER-FLOODPLAIN ECOLOGY [N THE UPPER MISSISSIPP[ R[VER BAS[N Yeager (1949) documented the effects of river impoundment on floodplain forests following the comple- tion of Lock and Dam 26 at Alton, Illinois, in 1938. After 6 years, trees on the lowest pennanently inundated flood- plains were nearly completely eliminated. Only the most flood-tolerant species remained in areas where the ground- water table was raised to near-surface level. On higher floodplain elevations less affected by inundation, trees showed beller survival, but less flood-tolerant species like pin oak suffered heavy mortality. A comparison of presettlement and presenl forests within a portion of Pool 26 revealed significant changes in composition and structure (Nelson and others, 1994). The presettlement forest was dominated by several species. including hackberry, pecan, elm, willow, and cottonwood, whereas the present floodplain forest is dominated by one species, the silver maple (table 3-1). Sediments are rapidly accumulating in the artificial backwaters. which is creating new mudflats usually invaded by willow and soon over- taken by the more shade-tolerant silver maple. Similar results were revealed (Moore, 1988) in southeastern Minne- sota and northern Iowa. where silver maple replaced ash as the single dominant species in the present floodplain forests, while oak and hickory species were reduced. DIMINISHING OF FLOODPLAIN FORESTS Agriculture and urban development have been two major causes for rapidly diminishing forests throughout most of the UMR floodplain. According to data presented by Peck and Smart (1986), by 1929, fannland and urban areas had expanded to about 22 percent of the total area in the UMR floodplain, while forests were reduced to approxi- mately 29 percent of the total area. Construction of navigation dams increased the water-surface area of the Mississippi River and eliminated forests from pennanently inundated areas (Green, 1947; Yeager, 1949). However, the net loss of forests between 1929 and 1973 was slight, about 2 percent of the total area (Peck and Smart, 1986). A recent study revealed that by 1989, forests occupied only about 14 percent of the total area from bluff to bluff in the UMR floodplain (Laustrup and Lowenberg, 1994). The percent of forested areas is highest in Navigation Pools 2-13 (18.2 per- cent), intennediate in Pools 14-27 (13.6 percent), and low- est in the open-river reach (12.4 percent). Two sets of geographic infonnation system (GIS) maps are provided to illustrate changes in forest acreage along with changes in other land-cover/land-use types between 1891 and 1989. One set of the GIS maps displays an impounded reach at Pool 26 in Alton, Illinois (fig. 3-6). This map does not depict the entire floodplain, but the trend of change is well represented. Field notes ofGLO office surveyors in 1817 and plat maps based on the GLO surveys indicate that the Pool 26 floodplain was about 63 percent prairie wetlands with forests bordering the riverbank and tributary streams. Agriculture had nearly eliminated the prairies by [891, while forests were less affected. The second set of the GIS maps are from the open river near Cape Girardeau, Missouri (fig. 3-7). Field notes of the GLO surveyors in 1809 and related plat maps indicate that the floodplain at this location was completely forested prior to settlement. Agriculture had eliminated much of the forests by [891, and by 1989, agri- culture became the predominant land-cover type. The remaining forests are primarily limited to areas immediately adjacent to the river channel and to State preserves, conser- vation areas, and private hunting clubs. REGENERATION OF PIONEER FORESTS Little information is available on qualitative changes of forests in the UMR. Some assessments have to be made on the basis of infonnation from studies of other large river systems. According to these studies, flooding and lateral movement of the river create and maintain a constant influx of new alluvial soils, which are quickly colonized by early pioneer forests (dominant species may be willow and cot- tonwood) and then develop into old pioneer forests (domi- nant species may be cottonwood. sycamore. willow, and others). As old pioneer forests develop into transitional for- ests (dominant species may be silver maple, boxelder (Acer negundo L.), hackberry, elm, ash, and others), they will be eroded away by the river and then develop into early pio- neer forests again. Only a small portion of the forest reaches late-successional or climax stages. Survival of early pioneer forests is comparable to a reversed "]" curve (Everitt, 1968; Johnson, 1992). That is, the acreage of stands decreases with the increase of stand age with more pioneer forests and less climax (oak-hickory) forests. On the Missouri River the influx of new alluvial soils has been greatly reduced since the construction of large res- ervoir dams. Between the Oahe Reservoir and the Garrison Dam, the erosion rate has decreased from 133 hectares per year in the late 1800's (predam) to a present rate of21 hect- ares per year (postdam), while deposition decreased from 165 to 1.3 hectares per year between the same two periods (Johnson, 1992). Because of the reduced fonnation of new alluvial soils required for cottonwood-willow regeneration, the present forests of the Missouri River system are made up of fewer pioneer forests and more transitional forests (fig. 3-8) (Johnson, 1992). On the basis of the likely changes predicted by Johnson and others (1974) and Simons and others (1974), the acreage of floodplain forests in the open-river reach of the UMR is expected to increase in the near future. Fields between wing dams and side channels have been rapidly filling with sediments. All natural side channels may disap- pear, even in the absence of further human-induced changes in river hydrology or geomorphology. While some of the