r918| ROBBINS—BOULDER PARK 503 
and this by a willow thicket of Salix chlorophylla and S. padophylla, 
or in certain instances the mud flat along streams may be invaded 
directly by Salix species, and still in other cases, especially where 
the soil is sandy rather than muddy, Agrostis is initial and is fol- 
lowed by a mixed community similar to that on more gravelly 
. 6.—Along shore of oxbow lake 9: at shore edge i is an almost pure 
siete of Carex utriculata; bordered on water side by Eleocharis palustris- 
Ranunculus association; tall willows on farther side are —S of willow thicket stage. 
stream banks. This is replaced by willow thicket, which in turn 
gives way toa mesophytic grassland or meadow scrub. 
Oxbow 20 (fig. 7) represents an oxbow in an early stage of forma- 
tion. Some water is still flowing through the old channel (egh). 
The cut-off is clearly marked. From ¢ to d a sand bar is being con- 
structed and now almost reaches the water surface and extends 
from shore to shore. The outlet end of the oxbow will, of course, 
be the first to close. Then will follow the filling of the inlet, thus 
completing the formation of a closed body of water having the 
