204 
Ohio Biological Survey 
ciations of the flood plain formation, this has the most open undergrowth. 
Each year floods add their deposit of mud and debris, seemingly in great¬ 
est amount at this horizon. 
On very broad flood plains this association is usually wide. Else¬ 
where, it is narrow. Traced up or down stream, this is found to be the 
Fig. 57. Within the fourth zone of this flood plain formation. 
marginal zone. It is the most persistent flood plain association, border¬ 
ing all rivers, except along under-cut banks, and extending up smaller 
streams to their upper limit of flood plain formation (ci. fig. 26). It 
occupies the area reached by all high water. This zone is flooded several 
times during the late winter and spring, but rarely at other seasons. 
The flood plain does not rise evenly from the river to its highest 
parts, but consists of a number of levels separated by short abrupt slopes. 
The rise is usually gradual through the first three zones. Then there is 
an abrupt rise of five or ten feet to a higher flood plain. This, though 
not often flooded during the growing season, is always covered by all 
ordinarily high water. 
