138 
MALARIA 
stages and may retain water for a long time. 
Such situations frequently form important 
breeding places for A. quadrimaculatus. 
Large streams with low gradients usually 
build flood plains on either side of their 
channels. Flood plains are composed of 
materials which streams carry during peri¬ 
ods of flood and deposit on the sides of their 
channels when their velocities return to 
normal. Most of the rivers in the southeast¬ 
ern United States have formed flood plains 
which, in the case of the larger streams such 
as the Mississippi and Tennessee Rivers, 
may be several miles in width. During pe¬ 
riods of flood, often every spring, the rivers 
may overflow their flood plains and the 
rapidly moving water may cut shallow 
channels in them. These channels, or 
“chutes,” may hold water for long periods 
after the flood-stage has subsided. 
Even in years when no flood occurs these 
channels may be filled by rainfall to form 
temporary ponds; and often they may com¬ 
municate with a spring or a small stream 
whose course has been blocked so that a 
swamp is formed. The blockage of tribu¬ 
tary streams by the formation of the flood 
plains is a common occurrence. Even 
though the stream may find a new course 
into the river, its channel below the point 
of blockage may still communicate with the 
river and may be filled with stagnant water 
from it. Situations of this sort are called 
“sloughs.” 
As the principal streams approach the 
coast, the’ land adjacent to them becomes 
flatter and more poorly drained. As a con¬ 
sequence, there is a tendency toward the 
formation of extensive swamps and marshes, 
the latter tending to become brackish near 
the sea coast. 
A considerable portion of the south¬ 
eastern United States is underlaid by lime¬ 
stone. This mineral is subject to solution 
by percolating ground water, especially 
when the latter contains carbonic and 
humic acids. As a consequence changes 
take place in the limestone substrata which 
result in the formation of a characteristic 
topography. In the early stages of the solu¬ 
tion of the limestone caves are formed which 
become larger with the passage of time. If 
Fig. 2. Limesink and solution depression areas in the southeastern United States (from Boyd and 
Ponton 1933). 
