J. L. Greenleaf — The Hydrology of the Mississippi. 43 



before it reaches the streams. Again, heavy rainfalls are liable 

 to be somewhat local in their extent, when the entire drainage 

 basin of a large river is considered. In the case of the Mis- 

 souri, for instance, while the early spring rains are falling upon 

 the regions near its mouth, the upper and elevated watershed 

 is supplying comparatively nothing to the flow. The natural 

 tendency of such a condition as this is to pull down the average 

 rate of the high flow per square mile for an extensive drainage 

 area. On account of the above reasons it is not surprising that 

 the northern and large western tributaries of the Mississippi 

 should carry comparatively small rates of maximum flood 

 volume per square mile. To be sure, they have an advantage 

 over the strictly southern rivers in drawing upon large quanti- 

 ties of melting ice and snow which may concentrate the winter 

 precipitation in a brief period of melting. The winter pre- 

 cipitation upon their basins, however, is far lower in rate than 

 for the other three seasons of the year, and the melting occurs 

 chiefly before the heaviest rainfalls, which tend toward the late 

 spring or summer. There result from this simply two freshets, 

 the first due to melting aided by rains, and the second due 

 principally, if not entirely, to rains alone. Between these two 

 freshets occurs a more or less pronounced tendency to a merely 

 fair or average stage of water. 



Upon turning to consider the rates of low flow and their 

 causes one is confronted at once with the much smaller annual 

 rainfall upon the entire northern and western portion of the 

 Mississippi watershed. Behind this prominent fact are a num- 

 ber of minor considerations. Thus, the proportionate, and 

 probably the absolute loss from evaporation and absorption by 

 vegetation is greater for regions with a light, than for those 

 with a large rainfall. The proportion of natural storage per 

 square mile in the form of lakes and swamps is smaller for 

 large than for small watersheds. Finally, the climatic condi- 

 tions are such over the major part of the Mississippi watershed, 

 that the period of light rainfall in the autumn dominates a 

 wide extent of country. So, while it is true that all portions 

 of any one of the large watersheds will not at any one time be 

 giving their maximum flood volume for the year, the converse 

 is not true, for they may uniformly concur in yielding their 

 minimum flows. The entire watershed of the longest tributary 

 of the Mississippi, for instance, may at one and the same time 

 be suffering from a greatly diminished rainfall, and one part 

 of the area cannot be counted upon to maintain the flow while 

 another part is subject to drouth. The above causes readily 

 account for the exceedingly low flow per square mile of 0*04:7 

 cubic feet per second from the Missouri, and 0*024 cubic feet 

 from the Arkansas, and for the uniformly low rate of minimum 



