J. L. Greenleaf — The Hydrology of the Mississijypi. 45 



it is the ruling tributary of the Mississippi system. Although 

 it has less than half as large a drainage area as the Missouri, 

 yet its low, high, and average flows all exceed in total amounts 

 the like volumes from the Missouri. In rates per square mile, 

 these flows are only exceeded in the cases of certain southern 

 rivers. In fact, the Ohio is rightly classed with the southern 

 rivers, even granting that the annual melting of snow and ice is 

 a prominent feature upon its basin. It has been pointed out 

 that upon most extended drainage areas, the rate of high flow 

 is pulled down by the averaging of districts where the rains 

 may be falling, with regions that happen at that time to be 

 comparatively dry. E'ot so for the Ohio. There is a strong- 

 tendency for moisture-laden air to be whirled in broad sheets 

 over the watershed clear to the Alleghanies, and deposit heavy 

 rainfalls during the same month over nearly the entire length of 

 the basin. When this is taken into account with the relatively 

 large average rainfall of 43 inches, and with the fact that the 

 winter rains are only slightly less in amount than those of 

 spring, the greatest of the year, and occur at the time when 

 the snow and ice are going out, it is easy to understand why 

 the Ohio should tower far above the other large tributaries of 

 the Mississippi in its rate of high flow. That the Ohio is 

 above even the upper Mississippi in its rate of low flow, is diie 

 primarily to its higher annual rainfall, and also to its important 

 southern branches extending into a region of considerable 

 autumn rainfall. 



The lower Mississippi, the main channel into which all the 

 tributaries flow, claims final attention. It is the resultant of 

 these tributaries, but a resultant in which the element of time 

 is a potent factor. If all the tributaries were high and low 

 together, it would be easy to compute the high flow or low flow 

 of the main stream by a simple process of summation. Owing, 

 however, to a lack of unanimity among the streams in this 

 respect, it is an exceedingly complicated and, in fact, impossible 

 task to reason out conclusions concerning the flow of the main 

 river simply from the general data for the tributaries. The 

 tributaries do conspire to a considerable degree in producing the 

 low stages of the main river, for the area of light rains in the 

 fall is verj^ widely extended over the Mississippi watershed. All 

 the rivers are not at their lowest during the same time, however, 

 for if they were the aggregate flow would be only 110,000 

 cubic feet per second, whereas the low water discharge of the 

 Mississippi may be taken at 175,000 cubic feet per second, 

 although lower gaugings have been recorded. 



Fortunately for the well-being of the dwellers within the 

 Mississippi valley, the tributary streams differ very widely in 

 their times of flood. A simple calculation shows that if high 



