393 



Spoon River, while its connection with Flag Lake is early es- 

 tablished over the low sandy bank which lies between them. 

 At stages above six feet the "cut road" (PL II.) and the marshy 

 sw^ale above it fill, and connection with Flag Lake at its lower 

 end and with the river is established. Its area is about 

 doubled by the time the river reaches bank height and general 

 overflow ensues. 



The lake is of somewhat uniform depth in the middle half, 

 but shoals toward either end. In prolonged low water, when 

 the slough at the northern end is practically cut off from the 

 lake, extensive mud-flats are exposed in the northern area, and 

 a portion of the southern end for about half a mile is also left 

 bare except when prolonged and heavy winds drive the water 

 towards one or the other end of the lake. The depth at lowest 

 river levels in the central region, which includes about two 

 thirds of the total area, is 3 ft. Laterally the w^ater does not 

 shoal until within 10 rods of the shore. There is thus a large 

 area (about two square miles) of water with uniforui condi- 

 tions in this particular. 



With slight exception the bottom is of the softest alluvial 

 mud, several feet in depth, overlying a sandy blue clay. The 

 shores along the southern, western, northern, and northeastern 

 margins are also of soft alluvium and of a marshy character. 

 The eastern shore, for most of its extent, and limited stretches 

 along the western one, together with the spit which makes out 

 into the lake on that side, are of sand and of a firmer consist- 

 ency. 



All of the shores are bordered by a belt of vegetation, 

 which has been described on page 246 . 



This lake is the largest of the reservoir backwaters exam- 

 ined by us, and is one of the permanent type, resembling in all 

 important particulars except that of submergence in times of 

 general overflow and its reservoir relation to the river a typ- 

 ical lake of an alluvial prairie country. Its position in the 

 bottom-lands brings it into intimate connection with the river, 

 the source of most of its water supply, while at times of flood 



