Igor] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 99 
support a pond vegetation, they have become essentially deposit- 
ing rather than eroding streams, and we find there the develop- 
ment of a flood plain. While the river is still confined within 
narrow walls and may thus be called young, there may be 
embryonic patches of flood plain, representing alternations of 
erosion and deposition in the stream. ig. 7 shows such a con- 


IG. 11.—Flood plain of the Desplaines river at Glendon park, showing 
€ncroachment on the river. Willows in the foreground, cottonwoods farther back. 
dition of affairs ; though the stream is young and more destruc- 
tive than constructive at that point, there are to be seen small 
flood plain areas with their typical tree inhabitants. 
There is no place where flood plain development can be bet- 
ter studied than on growing islands in relatively rapid and yet 
essentially depositing streams, such as the Illinois river at 
Starved rock. Fig. 8 gives a general view of the Illinois islands 
and flood plain. In figs. g and zo the lower island (foreground 
of fig. 8) is seen close at hand. Any obstacle, such as a par- 
tially submerged tree trunk, serves to check the river current 
and cause a deposition of sand or silt, and before long a sand bar 
