546 



Illinois. Under these circumstances the plankton indigenous 

 to the channel itself is of small volume as compared with that 

 contributed from backwaters. At low water, however, it may 

 take 23 days, or more, for the water to traverse this distance. 

 This frequently results in such a development of channel 

 plankton that it not only rises above that in many of the 

 backwaters, but also takes on a characteristic facies which 

 at once stamps it as largely indigenous in the channel itself in 

 its origin. Instances of this phenomenon appear with pro- 

 longed low water, as in June-July, 1895, when channel plankton 

 exceeded that in all the backwaters and was characterized by 

 the great abundance of Moina. In like manner in November- 

 December of every low-water year there has been an unusual 

 development of CUiafa, principally Carches'nim Jacliuuoni't and 

 predatory forms feeding upon it. This, at least as shown by 

 the catches of the silk net, does not often exceed the plankton 

 content elsewhere, but its dominant organisms form relatively 

 a small proportion of the backwater plankton at such times. 

 The -channel plankton is, then, largely indigenous. The autumn 

 of 1897 saw a similar indigenous devevelopment of Chlaini/- 

 dontonas in the river which was not equaled in the backwaters. 

 The plankton of the Illinois River is the result of the 

 mingling of small contributions by tributary streams, largely 

 of littoral organisms and the quickly growing algee and flagel- 

 lates, and of the rich and varied plankton of tributary back- 

 waters, present to an unusual degree in the Illinois because of 

 its slightly developed flood-plain, and from which it is never 

 entirely cut off even at lowest water. Data are lacking as to 

 the effect of the contributions of the Illinois and Michigan 

 Canal upon the plankton of the river. To these elements is 

 added such further development of the contributed or indigenous 

 organisms as time permits or the special conditions of nutri- 

 tion and sewage contamination facilitate. Though continually 

 discharging, the stream maintains the continuous supply of 

 plankton, largely by virtue of the reservoir backwaters — the 

 great seed-beds from which the plankton-poor but well-fertilized 



