J. L. Greenleaf'—The Hydrology of the Mississippi. 39 



fall, minimum and maximum flows, and the time variations in 

 the amount of flow for each of the principal tributaries.^ 



A glance over the diagram makes wide differences apparent. 

 The Ohio, Yazoo, Saint Francis and White have comparatively 

 large flood flows per square mile per second. The Arkansas, 

 Red, Missouri and Minnesota have especially small flows per 

 square mile per second. The determinating causes for these 

 and other differences are the amounts of rainfall on the water- 

 sheds, the distribution of these amounts through the seasons, 

 the degree of storage of winter precipitation to be liberated in 

 the melting of the snow and ice at spring time, the nature of 

 the country and vegetation, and the amount of natural storage 

 in the form of lakes and swamps. It should be borne in mind 

 that the southern rivers of limited drainage area have a heavy 

 rainfall, and a large amount of this occurs in the winter and 

 spring, when evaporation and the demands of vegetation are 

 slight. The same is true of the Ohio watershed, because of 

 the marked tendency for the moisture laden air-currents to 

 flow over it to the northeast in the early months of the year. 

 For these reasons one would expect the southern rivers to 

 carry high rates of flood-volume as compared with streams dif- 

 ferently conditioned. The tributaries farther north, on the 

 contrary, and particularly the Arkansas and Missouri, which 

 extend their lines of drainage far westward, have average 

 amounts of annual rainfall greatly below that of the Ohio, for 

 example, and of this the greatest activity occurs in summer, 

 when the tendencies are strongest to absorb and evaporate it 



* Adverse criticism is almost inevitable when one puts in concrete form con- 

 clusions from a series of data which are so liable to various interpretations as are 

 gaugings of the flow of rivers. It is very largely a matter of judgment as to 

 what shall be taken for the characteristic minimum flow or maximum flow, and 

 one's impression of the time of occurrence of the annual greatest flood, or of low 

 water, is apt to be influenced by some special case which has fixed itself on the 

 mind, rather than by impartial averages. It is the latter that I have tried to fol- 

 low. I hope to disarm adverse criticism by frankly holding myself open to con- 

 viction regarding the conclusions offered, and by stating that they are the result 

 of a somewhat extensive study of all the available gaugings by the United States 

 Engineer Corps, checked and compared with the opinions so kindly given by a 

 number of the officers stationed along the rivers. The aim has been to give the 

 discharges at decidedly low and high water, but at the same time to avoid using 

 any special and extreme case as a criterion. It is proper to state that discharge 

 data concerning the Yazoo and, above all, .the Saint Francis, are especially meagre 

 and more or less uncertain. Capt. Willard's gaugings upon the Yazoo are about 

 the only reliable figures to start with. The Saint Francis is so liable to overflow 

 from the main river as to make gaugings of it well nigh impossible of interpreta- 

 tion. Fortunately the climatic conditions are quite uniform over the region of 

 country cancerned, giving some authority to a comparison between neighboring 

 watersheds of limited extent. Therefore, by comparing flrst the Yazoo and 

 White, giving due value to the proportions of upland and bottom land, and then 

 applying the results to the Saint Francis, and checking by a proper balance be- 

 tween the three rivers, results have been obtained which I believe to fairly repre- 

 sent the truth. 



