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



flow from all the smaller streams in the northwestern part of 

 the Mississippi watershed. 



Passing now to the extreme upper waters of the river, the 

 upper Mississippi above the mouth of the Minnesota, and the 

 Minnesota itself show certain interesting and instructive dif- 

 ferences. The total amount and the distribution of the rain- 

 fall is nearly the same for both, the Minnesota having a slightly 

 more pronounced concentration of rainfall in the summer 

 months. The Minnesota is preeminently a prairie stream, 

 whereas the main river is characterized, to an unusual degree, 

 by lakes and swamps. It is not strange, therefore, that the 

 iiood-flow per square mile is nearly fifty per cent larger from 

 the Minnesota than from the main stream. IsTeither is it to be 

 wondered at that the Minnesota dwindles away to 0*031 cubic 

 feet per square mile per second in low water, exhibiting the 

 true character of a prairie stream, while the natural lakes and 

 swamps of the main river hold it, in spite of the low rainfall, 

 up to 0*154 cubic feet per square mile per second. 

 ' The upper Mississippi, at the point where it unites with the 

 Missouri to form the lower river, takes high rank in the matter 

 of rates of flow, in spite of its average climatic conditions 

 being essentially those already described as pertaining to the 

 northern and western tributaries. Its high rate of low flow is 

 largely the result of the natural storage in the immense num- 

 ber of lakes and swamps in the States of Minnesota and Wis- 

 consin. This storage affects to a marked degree the main 

 river above St. Paul, the Saint Croix, Chippewa, Wisconsin, 

 and other tributary streams. As a result, these rivers with- 

 stand very successfully the depressing effect of the prairie 

 streams like the Minnesota, which enter from the west, and 

 they maintain the low flow at 0*144 cubic feet per second per 

 square mile.* But not alone in its low flow as compared with 

 the climatic conditions, is the upper Mississippi noticeable. 

 Although its average annual rainfall is slight, yet it has heavy 

 summer rains widely enough distributed over its 173,000 

 square miles of watershed to hold it well up toward the ranks 

 of the high flood-flow tributaries. Also, its spring melting is 

 sufficiently concentrated in point of time to have a very 

 important influence upon the records of flood volume. 



The Ohio demands special mention because of pronounced 

 activity among its fellow tributaries. In a hydrological sense 



* Attention is here called to the fact that this paper is written throughout with 

 reference to the natural conditions of the Mississippi drainage ; the conditions 

 which would pertain if the artificial storage works installed by the United States 

 Government at the sources of the river were not existing. These works, uncom- 

 pleted as they are, have resulted in maintaining a stage at St. Paul during the low 

 season considerably more than a foot above the natural conditions of the river. 



