of trappers 1 catches for Calhoun Point taken from Yeager and Rennels 

 (1943: 49). Harvested muskrats numbered 50 to 75 during the 1938-39 

 trapping season and jumped to 225 during the 1939-40 season. Other 

 marsh habitat created by the nine-foot channel would have had similar 

 beneficial effects on muskrat populations. 



Increased sedimentation caused by intensified agricultural tillage, 

 coupled with decreased water velocity and increased water levels as 

 a result of the navigation dams, have filled in shallow marsh areas. 

 The plants that occupied these areas, mainly river bulrush and smart- 

 weeds, are used by muskrats to build houses (Brown and Yeager , 1943: 456). As 

 a result of sedimentation, many of the narsh plants which were re- 

 sponsible for the increase of muskrat populations had been replaced by 

 moist soil species. Narrative reports for the Mark Twain National 

 Wildlife Refuge (Calhoun and Batchtown units) indicate that by 1960 

 muskrats had gradually changed from building houses to using bank dens. 

 The initial benefit derived from the nine-foot channel project has 

 retrogressed as a result of sedimentation. 



Information obtained from narrative reports also indicates that 

 extreme fluctuations as a result of manipulation of pool levels have 

 a detrimental effect on muskrat populations. Keenlyne (1974: 20), 

 Bellrose (1943: 175), and Steele (1946: 20) have found that extreme 

 fluctuations adversely affect muskrat populations in the upper Missis- 

 sippi and Illinois Rivers. Steele (1946: 20) indicated a drawdown of 

 six or more inches following a freeze-up after muskrats had established 

 winter quarters and gathered emergency food supplies would force ma'ny 

 of the animals to abandon their lodges during mid-winter due to inac- 

 cessibility of food in the vicinity of their lodge. The dams could be 

 beneficial for muskrats by stabilizing water levels. However, draw- 

 downs, especially in Pool 25 in 1951, 1954, 1955, and 1961, have been 

 deleterious to muskrats. 



Muskrat populations at the Calhoun and Batchtown units of Mark 

 Twain National Wildlife Refuge increased initially but declined in the 

 middle 1940' s as a result of pool level manipulation. The population 

 then began to increase in the 1950' s until the middle 1960's except 



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