CHICAGO AND VICINITT. 25 



away in the winter and spring, so that in reality the vegetation is almost 

 exclusively annual. The first woody plants to get a more or less per- 

 manent foothold here are willows {Salix nigra and .S. longifolia). 



While islands of the above type gain more soil than they lose, a 

 comparison of figs, g and 10 shows that the river erodes above and 

 deposits below. As a consequence, these islands migrate down the 

 river, as well as grow in area year by year. Hence the upper part of 

 the island is the oldest, as the vegetation well shows. Figs. 8 and 10 



Fig, 10. — Same island as shown i^fig. 9, but seen from below, and showing the constructive 

 action' of the river. Naked sand bar recently formed at the lower end of the island (left hand). 

 Ambrosia farther toward the right, willows on the older part of the Island (extreme right) . 



show at the lower end the sand bar, which comes to a point, and is so 

 young or so exposed to submergence as to be barren of vegetation. 

 Next comes the Ambrosia, then the willows, and finally a characteristic 

 flood plain forest (background of fig. S). The asymmetry of the 

 river island vegetation is in striking contrast with the zonal symmetry 

 of pond islands, as will be shown later (see fig. ig). The cause is 

 evident, viz., the relative lack of symmetry in river currents as com- 

 pared with pond currents. 



The gradual encroachment of the land upon a stream through con- 

 tinuous deposition is well shown along the Desplaines river, and to a 

 less complete degree along the Chicago river and Thorn creek. In 

 the Desplaines bottoms the sand bar and island formations of the Illi- 

 nois 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 (Arrowhead), Rumex verticillatus (Swamp dock), etc. 



