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Salix nigra Nanthium canadense 
salix longifolia Rudbeckia hirta 
Populus deltoides Bidens frondosa 
Salix cordata Polygonum virginianum 
Ambrosia trifida Helianthus strumosus 
Impatiens biflora Apocynum cannibinum 
Impatiens pallida Lobelia syphilitica 
Rumex crispus Cicuta maculata 
Later stages of the succession occur at other points. The succession 
is rapid, the tlood-plains of this kind quickly become mesophytic. The 
large flood-plain at the bend of the river is thoroughly mesophytic, as 
shown by the vegetation of the undisturbed portions of it. It is to be 
noted, however, that the very narrow flood-plains of this type do not 
become mesophytic quickly, because of their submergence at every flood. 
By reason of the high gradient of the stream, the current is swift at 
flood time, and very little material is deposited. In this respect, these 
areas differ from the usual hydrophytie flood-plains. 
A third feature of the gorge floor is the presence of numerous and 
well-defined flood-plains of a distinctly xerophytiec nature. These occur 
commonly just below the fall lines, in distinction from those of the sides 
of the ponded stretches. The manner of their formation is easily seen. 
As already mentioned, the gradient of the stream is high, and at flood 
times the swift current carries considerable masses of rock and coarse 
gravel over the lower land immediately below the various fall lines. As 
au result, these flood-plains below the fall lines and at the curves of the 
stream are composed of gravel and rock fragments. The islands at the 
fall line near the lower end of the gorge are subject to the same con- 
ditions. 
The vegetation of these areas is quite distinctive. On the upstream 
side, no plants occur for a considerable distance from the water's edge. 
This corresponds roughly to the bare lower beach of a lake or the ocean, 
and is the region that is covered by the slight rises of the river during 
the summer. The pioneer plants are usually Xanthium canadense and 
Bidens connata. Back of these occur Salix longifolia, Platanus occidentalis 
and Populus deltoides. Of these, Platanus is the typical species, and 
from the bluffs the xerophytie flood-plains may be picked out by the 
