J. L. Greenleaf — The Hydrology of the Mississippi. 33 



five to thirty-seven, and in this respect they are only equalled 

 by some of the tributaries in the far south. The actual volume 

 of flow, however, is not as great as the figures might lead one 

 to infer, for the annual rainfall is slight, being over a portion 

 of the tributary region only twenty-seven inches in depth. In 

 consequence these streams yield a yearly flow of 0*625 cubic 

 feet per square mile per second, rising to 0*928 cubic feet in 

 the case of the Wisconsin, while the Ohio, for example, 

 although only a thirty per cent stream, gives 0*953 cubic feet 

 of average flow, because of its greater rainfall. 



Why is it that the percentage of flow to rainfall for the 

 rivers mentioned, is sufficiently large to raise them into the list 

 of high flow streams in spite of their light rainfall? It may 

 be due, in part, to the spring rainfall being slightly greater 

 relatively to the summer rainfall for the first five hundred 

 miles or so of the Mississippi than for the second, and to the 

 later advance of warm weather at the upper waters causing a 

 delay in evaporation, and in the demands of vegetation. But 

 the principal reason must be sought in the character of the 

 country drained. A sandy soil that absorbs the rain and yields 

 it to the streams with comparatively little loss from evapora- 

 tion is a feature of a large part of this region. Pine forests 

 that check the dry winds and moisture-searching sun of sum- 

 mer also abound at the sources of the Mississippi, Saint Croix, 

 Chippewa, and Wisconsin. 



In contrast to the high flow tributaries is the Minnesota. 

 This river has nearly the drainage area of the Mississippi above 

 where the Minnesota enters it, but in the volume of flow does 

 not approach to the importance of the latter. It is a true 

 "prairie stream," running high at one season, and then dwind- 

 ling with dry weather, or a cold winter, to almost nothing. Its 

 percentage of average flow to rainfall is only twenty -three, and 

 its average discharge is 0*474 cubic feet per square mile per 

 second. The Iowa and Des Moines likewise flow from an open 

 prairie country and are no better in ratios of flow, yielding only 

 twenty-four and twenty per cent respectively, but in them the 

 increased rainfall shows its effect. Upon these watersheds 

 there is a rainfall of thirty-eight inches annually as against 

 twenty-eight for the Minnesota, and the ten extra inches raise 

 the rectangles considerably above that for the latter river. 



The Illinois drains 29,000 square miles to the east of the 



Mississippi, and is the most important tributary met with thus 



far in passing down the river. Its rainfall, percentage of flow, 



and discharge per square mile happen to be almost precisely 



the same as for the Iowa. The drainage basin also is similar 



in many respects. The land is level, or gently undulating, but 



not quite so pronounced a prairie region as the country lying 



Am. Jour. Sol— Fourth Series, Yol. II, No. 1. — July, 1896. 

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