194 
Ohio Biological Survey 
Factors limiting associations of cobble shores .—The plant associations 
of the cobble or gravel shore, whether of mainland or island, are not con¬ 
trolled by newness of land, but by water height and topography. The 
lower shore association occupies the area which is exposed during low 
water. The second zone, or upper shore, is submerged in the summer 
only for very short periods, if at all, during unusually high water. The 
lower shore is not being built up or extended outward by deposition; 
neither are the islands increasing in size by gradual accession of deposited 
material at their lower ends. The succession is not related to slow con¬ 
structional changes, as is the succession on depositing shores or deposi- 
tional islands; nor is it accompanied by gradual encroachment of one 
association upon the next younger. The streams along which the cobble 
shores, and in which the residual islands are found, are essentially eroding 
streams. The lower shore is cut below the upper shore; the upper shore, 
cut below the general level of the flood plain. The materials of which 
the islands are composed are too coarse to be moved during low water, 
even by the swift current assumed by these rivers in passing through the 
narrow channels around the islands. Changes are effected only during 
high water, when the cutting and carrying power of the stream is enor¬ 
mously increased. Nevertheless the beach-like shores and the residual 
islands of the two Miamis are remarkably stable. Changes of sufficient 
magnitude to destroy the upper shore vegetation rarely occur. 
C. THE FLOOD PLAIN 
1 . Pond-swamp associations. 
Swampy conditions or ponded waters are found in a number of places 
on the flood plain. Here are included natural and artificial depressions, 
and sloughs. 
Low places which are filled with water for at least a part of the year, 
are very commonly found at the foot of a pronounced rise in the flood 
plain. They owe their existence to the levee nature of the alluvial de¬ 
posits. Other depressions are found whose origin is not as easily ex¬ 
plained. Artificial conditions also have contributed more or less in the 
formation of a number of flood plain ponds. And a number also have 
been destroyed by draining or filling. Most of those that remain are 
small, and do not contain the wealth of aquatic vegetation which local 
check lists credit to larger ponds of the region which were drained 
years ago. 
