THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS xli 



is little, at least in the La Grange pool, to vary this monotony 

 of mud banks and fringing willows. The backwaters have been 

 reduced to the lakes, sloughs, bayous, and marshes which abound 

 everywhere in the bottom-lands. Many of these, as, for example, 

 Phelps and Flag lakes, have ceased in their reduced condition to 

 contribute to the river. Others, like Thompson's Lake, maintain 

 a connection with the river by means of a long and tortuous bayou 

 or slough through which the current flows in or out as the rela- 

 ti\-e levels of the two fluctuate. This lake receives but little water 

 from a few springs and creeks along the bluft's, and like many others 

 in the bottom-lands serves only as a reservoir from which the water 

 is slowly drawn off as the river falls, but when once the lower stages 

 are reached its contributions cease. Still others, like Quiver and 

 Matanzas, maintain direct and open connection with the river, 

 and since they receive tributary streams they continue to feed the 

 river, but in reduced volume. Though the number of tributary 

 areas is thus much reduced at low-water stages, the individual 

 peculiarities of the tributary waters in the bottom-lands become 

 more pronounced. As each one loses its connection with the 

 general flood it becomes a separate unit of environment, with its 

 local differences in those factors which determine the character of 

 the plankton developing in its waters. The resulting contributions 

 may thus differ greatly in amount and component organisms, and 

 accordingly tend to diversify the river plankton of low water to a 

 degree even more marked than that of high water. 



" With the confinement of the river waters to the channel goes a 

 marked condensation of the sewage, which, under conditions of 

 uninterrupted low water, leads at times to an excessive development 

 of the plankton, or, if the river is closed by ice, to stagnation con- 

 ditions. But few years, however, offer such opportunities; for, as 

 a rule, in most low-water periods sudden and heavy rains are wont 

 to occur, which flush the stream, wash away the sewage and plank- 

 ton-laden waters, and store anew the reservoir lakes without caus- 

 ing any considerable overflow. After each catastrophe of this sort 

 the decline of the flood affords a new and favorable opportunity for 

 the development of the plankton."* 



The effects of change of temperature, of differences of turbidity, 

 of chemical conditions of the waters of the stream, and the like, are 

 discussed at length in Dr. Kofoid's report. t 



*Bull. 111. State Lab. Nat. Hist., Vol. VI., Art. II., pp. 151-156. 

 tLoc. cit., pp. 168-252. 



