Sedimentation is also a major concern on the UMR; rates of 1 to 3 cm/yr (0.4-1.2 in/yr) have been measured (McHenry et al. 1984). Erosion and sedimentation were both detected in comparisons between present elevation data and surveys before dam construction. Erosion was more prevalent in shallow areas and sedimentation more prevalent at greater depths. Erosion and sedimentation converge at depths of between 0.9 and 1.5 m (3 to 5 ft). This has resulted in a more homogeneous distribution of depth, which is dominated by areas 0.9 to 1.5 m (3 to 5 ft) in depth. Similar frequency distributions of water depth were observed for lower portions of Pools 8 and 13. Comparison of historical and present bottom geometry revealed the loss of elevational diversity. | ||
In areas of the UMR unaffected by navigation dams (the 40-mi stretch of river near Cape Girardeau), there was a 28% reduction in open water and a 38% reduction in woody and terrestrial habitat between 1891 and 1989 (Figs. 2c, d). Agricultural areas increased by 6,360 ha (15,700 acres). The 1,900-ha (4,710-acre) reduction of open water can be explained by the construction of levees and wing dams (also known as pile dikes). One large side channel that existed in 1891 was cut off by construction of a levee, reducing the area of water by 550 ha (1,350 acres). In all, nearly 2,000 km (1,240 mi) of levees now isolate more than 400,000 ha (988,000 acres) from the river during all but the highest discharge rates. | ||
Wing dams and levees, along with other changes to the watershed, have also had a major effect on habitats by changing the relationship between discharge and water-surface elevations. Wing dams have narrowed and deepened the main channel so that water elevations at low discharges are now lower than they were historically. Levees restrict flows and result in higher water elevations during high discharges. Water-surface elevations at relatively low discharges (60,000 cfs) have dropped about 2.4 m (8.0 ft) over the record 133-year period at St. Louis, Missouri, 0.5 m (1.5 ft) over the 52-year record at Chester, Illinois, and 1.5 m (5.0 ft) over the 60-year record at Thebes, Illinois. Water-surface elevations at relatively high discharges (780,000 cfs), however, have risen about 2.7 m (9 ft) over the record period at St. Louis, 1.5 m (5.0 ft) at Chester, and 1.1 m (3.6 ft) at Thebes. | ||
National Biological Service Environmental Management Technical Center 575 Lester Ave. Onalaska, WI 54650 |
References |
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McHenry, J.R., J.C. Ritchie, C.M. Cooper, and J. Verdon. 1984. Recent rates of sedimentation in the Upper Mississippi River. Pages 99-118 in J.G. Wiener, R.V. Anderson, and D.R. McConville, eds. Contaminants in the Upper Mississippi River. Butterworth Publishers, Boston, MA. |