SURFACE FEATURES. 39 



valleys out of a primitive general level. Thus our so-called 

 hills are only valley-sides; or, in other words, their apparent 

 elevation is dne to their having been left in their original 

 positions, when the surrounding or intervening material was 

 removed by the denuding action of water and the atmosphere; 

 which agencies are perhaps as active now as they have ever 

 been. Consequently the rivers and streams of Iowa, and 

 more especially their valleys, constitute its most conspicuous 

 physical features. 



All streams that rise in Iowa, rise upon the incoherent 

 surface deposits, occupying at first only slight depressions 

 in the surface, and scarcely perceptible. These successively 

 coalesce to form the streams. Those portions of the valleys 

 of the upper branches thus formed which run upon the 

 surface deposits alone have usually sloping sides, indistinct 

 flood-plains and muddy banks. In the following descrip- 

 tions of rivers, these portions so generally uniform in all, are 

 omitted, the depressions they occupy being included when 

 reference is made to undulations of the surface. 



As preliminary to the consideration of the rivers of Iowa, 

 in groups or as individual streams, the reader is referred to 

 the map of the State accompanying this report, to show its 

 geographical as well as geological features; and also to the 

 following table which is introduced to show the slope in feet 

 per mile of the more important of them. In the case of the 

 two great rivers, those portions of their courses which border 

 upon Iowa almost alone are considered; and in the case of 

 the smaller rivers, it will be seen that the slope of their prin- 

 cipal portions only are given in the table, the upper portions 

 of them and their branches being omitted. 



Physical geographers use definite terms to designate 

 different portions of the courses of rivers, and also of dif- 

 ferent parts of their valleys. Thus the river valley is divided 

 into sides, flood-plain* and channel or bed. There is hardly 



*The flood-plains of rivers are usually called "Bottoms" and "Bottom-lands" in 

 this part of the country. 



