CHICAGO AND VICINITT. 



23 



gonum sagittatum, and two or more species each of Eupatorium, 

 Lobelia. Mentha, Lycopus, and Bidens. The most characteristic 

 spring brook shrub is the alder (Alnus incand), though the extensive 

 northern development of alder thickets has no parallel here. 



As the energy of the developing stream is checked, the conditions 



Fig. 8. — General view of the Illinois valley near Starved Rock, showing islands and an exten- 

 sive flood plain with trees along the margin. Young islands in the foreground, older islands in 

 the background. 



for plant life become more favorable. In the quiet pond-like waters of 

 an older stream there may be found many of the aquatics that frequent 

 the ponds and lakes. In fact, the flora that is given later as character- 

 istic of half-drained ponds and lakes (such as Calumet lake) may be 

 transferred almost bodily to sluggish streams, such as the Calumet and 

 Desplaines rivers. 



When streams are old enough, and therefore slow enough, to sup- 

 port a pond vegetation, they have become essentially depositing rather 

 than eroding streams, and we find there the development 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. 

 Fig. 7 shows such a condition of affairs; though the stream is young 



