156 



MISSOURI BOTANICAL GARDEN. 



occupy the land and formerly did so to a greater extent, 

 but at the present time they have largely been cleared away. 

 As is to be expected, sloughs everywhere encroach upon the 

 land-surface, and swamps, the result of slowly draining 

 lakes, are plentiful. Both in the lower part of the American 

 Bottom and close to the bluffs are a number of lakes which 

 receive the water from the numerous creeks descending 

 from the Illinois plateau.* The lakes of the American 

 Bottom (plate 20) may be readily interpreted as being parts 

 of the old bed of the river, isolated through a series of cut- 

 offs. The origin of such lakes, which are typical of the swamps 

 of all gulf rivers but especially of the Mississippi, hes in the 

 winding course of rivers through their flood plains. As 

 pointed out by Marbutf these windings or meanders arc 

 primarily caused by some obstruction which deflects the 



current 



banks 



tion of this bank and deposition on the other. This action 

 causes a slight bend of the river bed. Since the process con- 

 tinues there finally comes about, through deposition on the 

 inner curve and excavation of the outer curve, a decD bend. 



bow 



Tlie size and length of existence 



determined 



which the greater length 



of the stream, and consequent 

 lower gradient and decreased velocity, play important parts. 

 In a meander of great length the difference in water level on 

 opposite sides of the narrow neck will be appreciable and 



back 



lower side. 



becomes 



and soon forms a new channel for the entire river, forming 

 a cut-off. It may be noted that for each river there appears 

 to be an average width of meander before the cut-off is made. 

 This width, for the Mississippi, is about G miles, but is some- 

 times exceeded, as at New Madrid. However, it seems but 



For the hydrography see Bowman, I. 



Resources 



St. Louis District. 111. State Geol. Surv. Bull. 5. 1907; and Helm, E. G. 

 The Levee and Drainage Problems of the American Bottoms. Journ. 

 Assoc. Eng. Soc. 35 : 99. Sept. 1905. 



t Marbut, C. F. The Physical Features of Missouri. Missouri Geo- 

 logical Survey. 10 : 95. 1S9G. 



