QUATERNARY LAKES IN THE MISSISSIPPI BASIN 487 



Apparently the river valleys were once filled to a position as high 

 as the surface of the filling on the tributaries, but have now been 

 partly cleared out, the surface of the fill being lowered about 30 

 feet. The part remaining is about 150 feet thick and extends 

 about 120 feet below low water, the range between high- and low- 

 water stages being about 30 feet (see Fig. 3). 



In this connection it seems worth while to note that when the 

 discharge of a stream is increased, the vertical distance between 

 the bottom of the channel and the flood-plain is also increased, 

 and this comes about not alone by scouring out the channel, but 

 also by building up the alluvium. Thus, without any change 

 in size of load, it is possible to produce thick alluvium by simply 

 increasing the volume of water. 



To return to the lakes themselves: they differed from most 

 bodies of quiet water in that the position of the surface varied 

 greatly every year, for it was controlled by the various stages of 

 the rivers. If the range between high and low water had been the 

 same that it is now the surfaces of the lakes would have fluctuated 

 between limits about 10 to 40 feet apart. But the lakes formed a 

 huge reservoir so that with the same discharge as at present the 

 rivers would not have risen nearly so much in times of flood. 



Indeed, to raise the surface of the lakes and rivers one foot, it 

 took over one hundred billion cubic feet or nearly a cubic mile of 

 water; moreover, every rise of 5 or 10 feet would double the dis- 

 charge of the rivers, so that tremendous floods could be taken care 

 of without great increase in depth of water. 



Terminology. — It seems probable that the rather extensive 

 development of deposits and resulting topographic features such 

 as are described in this paper will lead to the introduction of some 

 new descriptive terms. Perhaps it will be found convenient to 

 use "contragradation" or "dam gradation" for that kind of stream 

 aggradation which is caused by an obstruction, or, more broadly, 

 decrease in velocity, and perhaps to invent still other terms for 

 the aggradation due to increase in load and decrease in volume. 

 In case the obstruction develops so rapidly as to produce ponded 

 water, such as is described in the present paper, the deposit is on 

 the whole very fine-grained and the top nearly horizontal though 



