88 



Types of Aquatic Environment 



connecting or cutting off back waters, by shifting cur- 

 rents, by disturbing the adjustment of the vegetation, 

 and by causing the migration of the larger animals. At 

 low water the Illinois River above Havana has a width 

 of some 500 feet; in flood times it spreads across the 

 valley floor in a sheet of water four miles wide. 



The rise of a river flood is often sudden ; the decline 

 is always much more gradual, for impounding barriers 

 and impeding vegetation tend to hold the water upon 

 the lowlands. The period of inundation markedly 

 affects the life of the land overflowed. Cycles of seasons 

 with short periods of annual submergence favor the 

 establishment of upland plants and trees. Cycles of 

 years of more abundant rainfall favor the growth of 

 swamp vegetation. Certain plants like the flood-plain 

 bulrush shown in the preceding figure seem to thrive 

 best under inconstancy of flood conditions. 



