158 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



usually overflows its banks at least yearly. In such a flood as 

 that of 1913 it attains a width of nearly a half mile, altering its 

 course, sculpturing its flood-plain, and sweeping aAvay fences, 

 crops and even buildings during its brief period of devastation. 

 Figure 15 shows a mapping of its course for five miles with the 

 area covered at high water indicated in shading. 



The most careful study was given to that portion near the 

 mouth of Wilson's creek and this has been given in an enlarged 

 mapping (fig-ure 16) to show greater detail and a careful ap- 

 proximation of the changes in the course of the stream over this 

 area. The se])arate letters indicate the places from whicli were 

 taken the photographs that serve to illustrate the physiographical 

 and vegetational features. 



WAYS IN WHICH THE FLOODS INFLUENCE ANIMAL ASSOCIA- 

 TIONS. 



In analyzing the ways in which floods affect the local distri- 

 bution of animals, that is, animal associations, it is at once rec- 

 ognizable that all of the efforts are carried out in one of two gen- 

 eral ways: viz., first, by abruptly changing the habitats of the 

 animals toi>ogTaphically ; and second, by directly changing the 

 composition of the associations themselves without necessarily 

 affecting the physical habitat. 



Of the foregoing, topographical changes may be brought aliout 

 in several ways: (1) the course of the stream may be directly 

 altered, (2) the character of the stream may be altered, that is 

 the local habitats within the stream itself may be subject to 

 physical disarrangement, and (3) changes may be brought about 

 in the physiography of the flood-plain. 



It is too obvious to need elaboration that in the alteration of 

 the course of a stream areas which were once the abode of wholly 

 terrestrial forms come to harbor only those which are aquatic, 

 and likewise aquatic habitats are transformed into those which 

 can support only terrestrial forms of life. The map comprising 

 figure 16 is an enlarged sketch of the aforementioned ])ortion of 

 the stream near the mouth cf "Wilson's creek, on which an 

 attempt has been made to show the changes in the course of the 

 river in recent years. Dotted lines indicate the approximate bed 

 of the river in 1870 (from data gathered from men who have 

 spent upwards of fifty years in tlie immediate vicinity), in 1900 

 and a new channel which the stream is cutting for itself and in 



