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



west of the Mississippi. An extensive swamp drains into its 

 Lead waters from Indiana. 



Jnst below the Illinois is the mouth of the Missouri. Its 

 drainage area is three times as great as that of the entire Mis- 

 sissippi above the junction. It rises in the heart of the Rocky 

 Mountains and flows for nearly three thousand miles through 

 mountain land and prairie. Grreatly varying conditions of 

 hydrology prevail within the limits of the watershed, but one 

 fact stands pre-eminent : of all the branches of the Mississippi 

 it makes the poorest record for the area it drains. If its flow 

 was proportionately as great as from the Ohio, its discharge 

 alone would equal the entire volume that passes ISTew Orleans 

 each year. The diagram illustrates the peculiar weakness of 

 the Missouri, due primarily to its low average rainfall, and to 

 the extremely small percentage of flow. Twelve per cent of 

 less than twenty inches rainfall give only 0*178 cubic feet per 

 square mile per second for the average discharge. It is well 

 to remember, however, that these are averages, and that in 

 flood the Missouri is a mighty torrent of muddy water. 



A short distance below the mouth of the Missouri, the 

 Ohio enters, with less than half the drainage area of the former, 

 and more than the total yearly discharge. It reaches so far 

 eastward that dwellers on the Atlantic coast look upon its 

 upper waters as neighboring streams. Mountains and wood- 

 land cover a large portion of its basin. The rich, undulating 

 farm lands of Ohio, Indiana and Illinois are tributary to it. 

 The large average discharge is not due especially to the per- 

 centage of flow, for its thirty per cent is a moderate 

 amount, but to the forty-three inches of annual rainfall. The 

 warm moist air currents which flow from the Gulf region up 

 the Mississippi valley have a decided tendency to the north 

 and east rather than westerly, and this is chiefly the cause of 

 the preponderance of the Ohio over the Missouri. The area 

 of its rectangle in the diagram equals seven-ninths of the com- 

 bined areas of all the rectangles preceding it, including the 

 Missouri. 



The Arkansas is another large river entering from the west, 

 draining 189,000 square miles, and is in many respects a small 

 imitation of the Missouri. Its average ratio of flow to rainfall 

 is only sixteen per cent, and all that tends to raise its standing 

 is the larger average rainfall of twenty-eight inches.* 



The Hed River is the last important tributary of the Missis- 



* The "White River, draining 28,000 square miles, has been included with the 

 Arkansas in Figure 2. It is true that they enter the Mississippi together, but 

 strictly speaking they are distinct rivers. It will be noticed that where it is 

 more necessary, as in Figure 4, the Arkansas and White are given separate treat- 

 ment. 



