THE INFLUENCE OF FLOODS UPON ANIMALS 161 



which it promises to be established within a very few years. 

 The actual course of the stream in 1915 is represented by con- 

 tinuous lines. The cutting of the new^ channel and the perma- 

 nent pools established there (shown also on the map) come 

 properly under the discussion of the flood-plain and will be 

 considered there. The stippled sections represent gravel bars 

 and directly opposite each the stream flows against a relatively 

 high soil bank into which it is rapidly cutting at these points. 



In the changing of the course of a stream there are other effects 

 which incidently influence the animal life beside the important 

 one noted. Of these the features perhaps the most deserving of 

 notice are the oxbow lakes or ponds which are left isolated from 

 the stream, except in the flood season, as cut-off depressions in 

 the old channel. Many of these contain water throughout the 

 year, their bottoms being below the level of the water in the 

 stream, and may persist for a number of years before they are 

 finally filled by the deposition of material and the growth and 

 decay of aquatic and amphibious plants. In such formations are 

 found persisting those forms of animal life from the original 

 stream which are adapted to existence in quiet water. Later 

 purely pond forms may be introduced and become established. 



Changes within the stream itself may be many. Deep, quiet 

 pools may be created where there were none by the scooping out 

 of material from the bottom or by partial damming by the 

 piling up of stones and gravel to form a new rapids, increasing 

 the depth of water above that point. Sand bars, shoals, or mud 

 flats may be built up. Rapids may be cut down through the 

 action of successive floods. The sorting of the soil materials may 

 decidedly alter the character of the bottom leaving sand where 

 there was mud or flat stones where there was formerly sand, 

 etc. Each of these changes results in a change in the kinds of 

 animals which are to be found there. The deep, quiet pools will 

 provide a home for larger fish and different species of fish than 

 could exist permanently in the shallow water or in the rapids. 

 The stony bottom and stony rapids provide shelter for many 

 crustaceans and insect larvae such as May flies (Ephemerida) 

 and Stone flies (Plecoptera) while the mud bottoms abound in 

 worms and molluscs, e. g. the common species of fresh water 

 mussel, all being forms that do not thrive in other conditions. 

 The stony rapids, shown in figure 21, have been a favored habi- 



11 



