Our lJ\'ini; Rcxoiin I'S — Aqudtic Ecoxystcms 



235 



from gitnind surveys conducted by (he 

 Mississippi River Commission. Higii-iesoiulion 

 land-cover/land-use data were created from 

 1:15.000 (scale) color infrared aerial pho- 

 tographs taken in 1989. Data for 1891 and 1989 

 were compared for Pools 4. 5. 8, 13. 26. and for 

 a 64-km (40-mi) stretch of river, near Cape 

 Girardeau. Missouri, which is not affected by 

 navigation dams. Historical aerial photographs 

 from 1939. 1954, 1967. and 1989 were used to 

 measure island loss in an area just upriver of the 

 dam in Pool 8. 



Long-term daily data at three stations on the 

 open-liver portion of the UMR were analyzed 

 to evaluate changes in the relationship between 

 discharges and water-surface elevations. 



Status and Trends 



Comparison of the land-cover/land-use data 

 between 1891 and 1989 in the dammed poilion 

 of the UMR showed that open water and marsh 

 habitats generally increased, mostly at the 

 expense of grass/forb. woody ten"estrial, and 

 agricultural classes. For example, the combined 

 classes of open water and marsh in Pool 8 have 

 increased from 3.600 ha (8.900 acres) in 1891 

 to 9.500 ha (23.430 acres) in 1989 (Figs. 2a. b). 

 Similar increases in these two classes were 

 found at Pools 5 and 13. In Pools 4 and 26 

 increases were less significant. 



In many pools inundation created an 

 impounded area with a mosaic of islands, open 

 water, and marsh, which, in general, increased 

 aquatic habitat for fish and wildlife. Although 

 dam construction has benefited aquatic habitat 

 in many pools, the reservoir aging process has 

 reduced these benefits, especially in areas just 

 upriver of dams. For example, island areas have 

 been steadily eroding upriver of the dam in Pool 

 8 (Fig. 3). The dam that forms Pool 8 began 

 operating in 1937. and photographs taken 2 

 years later showed 253 ha (624 acres) of 

 islands. By 1989 the island area in the same 

 location was reduced by 79% to 52 ha (129 

 acres). 



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 com- 

 parisons between present elevation data and 

 surveys before dam construction. Erosion was 

 more prevalent in shallow areas and sedimenta- 

 fion 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 distribu- 

 tions of water depth were observed for lower 

 portions of Pools 8 and 13. Comparison of his- 





A 



torical and present bottom geometry revealed 

 the loss of elevational diversity. 



In areas of the UMR unaffected by naviga- 

 tion 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 terres- 

 trial habitat between 1891 and 1989 (Figs. 2c. 

 d). Agricultural areas increased by 6.360 ha 

 (15.7(M) acres). The 1.900-ha (4.71()-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 histor- 

 ically. Levees restrict flows and result in higher 



{sf>k:^^r^! vile -r / 







I I Open water 



I I Marsh 



I I Grasses/lorbs 



I I Woody terrestrial 



H Sand/mud 



I I Agriculture 



I I Urban/developed 



Fig. 2. Land-cover/land-use com- 

 parisons for a portion of Pool 8 in 

 (a) 1891 and(b) 1989. and the 

 open river near Cape Girardeau, 

 Missouri, in (c) 1891 and (d) 

 1989. 



I»~.-.^'/ 



i t ' 



"^./■r, 



^-1 





!• \:i ^1- 







1939 ^ 



I- y ^ 



1954 



li 



1967 



1989 



U 



u 



Fig. .V Island loss that has occurred in Pool 8, in the area just upriver of the dani. since construc- 

 tion of the lock and dam system. 



