102 BOTANICAL GAZETTE [ FEBRUARY 
>. creek. In the Desplaines bottoms the sand bar and island 
s { formations of the Illinois are largely absent, the currents being 
much less rapid. In the shallow water near the margin of the 
river are various hydrophytes, such as Sagittaria, Rumex verticil- 
latus, etc. The outermost fringe of land at ordinary low water 
is often almost as barren of vegetation as are the islands, but the 



Fic. 14.—Flood plain of the Calumet river near Chesterton, showing the begin- 
nings of terrace formation, indicated more by the falling elm than by the topography. 
soil is fine and hence makes a mud flat instead of a sand bar. 
Immediately after the spring freshets have gone, an alga vege- 
tation is frequently found on these flats, consisting especially 
of Botrydium and Vaucheria. Later in the season annuals, or 
even scattered perennials, may occur here, though the winter 
and spring floods uproot or bury most of this vegetation. The 
Ambrosia and willow vegetation soon appear as described 
above. The river maple (Acer dasycarpum) usually appears with 
or soon after the willows. After the willows the cottonwood 
(Populus monilifera) and the ash (Fraxinus Americana) soon 
come in. fig. rz shows an advancing flood plain of this type; 
willows are seen on the margin and cottonwoods farther back. 


