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 

 JSeeb's Lake (PI. 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 (PI. 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 is also 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 (PI. II,) prevents their departure. Evi- 



