382 
Lake Slough and along the west bank of the river, no access of 
river water is possible from the north or east until bank height 
is exceeded by the flood. At all levels below this, water enters 
the lake by the slough, which forms its outlet, or backs in from 
Seeb’s Lake (Pl. IT.). Another line of access is the low margin 
to the northwest between it and Thompson’s Lake. The rank 
- growth of living or dead vegetation which at all times fills this 
region, effectually blocks any localized current here, and no 
channel has opened in this region. Probably much of the water, 
as indicated by the distribution of drift, enters the lake from 
its southern end. The same reasons which prevent access of 
water from the north also tend to restrict the flow through this 
area at times of general overflow, and the fact that Thompson’s 
Lake (Pl. II.) affords for backwaters impounded to the north a 
channel where resistance is much less than in the shoal, forest- 
begirt, and rush-filled Flag Lake, tends also to divert the mov- 
ing backwaters to that region. Consequently, Flag Lake is in 
the main an impounding area whence the impounded water 
is drawn off as levels decline, but which is not generally trav- 
ersed by the waters of general overflow as are Quiver and 
Thompson’s lakes. It is thus one of the most strongly localized 
of all plankton stations, and the unity of its environment is 
more continuously maintained than that of any of the localities 
thus far examined. 
Its vegetation has been described on pages 249-250, and it 
will suffice in this connection to call attention to the predom- 
inance of the emergent and succulent types in its waters, and 
to the fact that little, if any, of it is ever carried away by flood 
or currents as it is from Quiver and Dogfish Lakes. This is a 
large factor in maintaining the local fertility of this area. 
This is a favorite haunt of migrating water-fowl in fall 
and spring, and contains breeding grounds of the few summer 
residents. It isalso much resorted to by the German carp, now 
one of the most abundant fish in the Illinois. Fish enter the 
lake in numbers when levels rise, but leave again before low 
water in the slough (Pl. I,) prevents their departure. Evi- 
