and state refuges have built low levees adjacent to the pools in which 

 water levels can be artifically controlled. These areas are not affected 

 by manipulation of pool levels unless the low levees are topped. 



In non-leveed areas, the manipulation of pool levels, particularly 

 on Pools 25 and 26, has an important bearing on their value for waterfowl. 

 Levels that eliminate or reduce the growth of moist soil plants in mid- 

 summer or do not flood them in the fall, reduce the use of these pools by 

 waterfowl. Pool fluctuations resulting from floods are understandably 

 beyond the control of pool management. However, other changes in pool 

 levels appear to be within the ability of management to control. 



Since 1939 the Army Corps of Engineers has manipulated water levels 

 in the study area to maintain a nine-foot navigation channel. This mani- 

 pulation resulted for the most part in stabilizing low water levels, which 

 benefited waterfowl populations. However, when the water levels are 

 dropped in the fall as a result of pool operations, moist soil and (under 

 extreme drawdowns) submerged aquatic plants are left stranded on mud 

 flats. This makes these food plants inaccessible to waterfowl. Narra- 

 tive reports from the Calhoun and Batchtown divisions of Mark Twain 

 National Wildlife Refuge indicate that drawdowns occurred during the 

 summers of 1952, 1953, 1956, and 1958 at the Calhoun unit (Pool 26); and 

 1951, 1954, 1961, and 1972 at the Batchtown unit (Pool 25). Moreover, 

 during the fall of 1956 and 1958 at the Calhoun unit and 1951 and 1961 at 

 the Batchtown unit, drawdowns ranged from 3 feet to over 5 feet making it 

 difficult or impossible for hunters to reach duck blinds. Waterfowl data 

 for the 1950' s were not complete but the effect of fall drawdown on water- 

 fowl can be seen in Figure 10. The pintail and wigeon show a decline 

 during the fall drawdowns of 1961 and 1972 on Pool 25. Narrative reports 

 from the Calhoun and Batchtown units indicate that during the fall 

 drawdowns of the 1950' s, duck use of the refuges was reduced. 



Increased sedimentation and concomitant turbidity have had a detri- 

 mental effect on waterfowl populations. As discussed previously in the 

 wetland plant section, by 1970 increased sedimentation and turbidity 

 resulted in a great reduction in submerged aquatic plants in the 

 Calhoun Refuge. With this loss of plants, the wigeon and green-wing 

 teal abundance plummeted on Pool 26 of the Illinois River. Figure 11 



152 



