341 



or two possible exceptions, were all made in tributary water, 

 though the chosen location could not always be reached. 

 These exceptions were at times of backwater from the Illinois. 



Spoon River has near its mouth a width of 75-100 ft., and 

 a depth below low-w^ater mark in the Illinois of 10-14 ft. It 

 runs between almost vertical banks of alluvium (PI. XIV.), and 

 has a hard gravelly bottom full of sunken logs which form 

 treacherous snags at low water. The current at the point of 

 collection at low water may be scarcely perceptible, while at 

 times of sudden flood, due to local storms in its water-shed, it is 

 so strong that a boat enters it with difficulty. At such times 

 its load of silt and drift is very great. During the heated term, 

 and especially when the heat pulses occur and there is little 

 wind to ruffle the surface, the green water-bloom on this stream 

 is remarkable, exceeding — possibly because of protection from 

 wind — that of the main stream in lividness and density. 



The turbidity of this stream (see Table IV. and p. 179) is 

 greater than in any other locality and serves as a general index 

 of its silt content. It is, for example, in 1898, 31 cm. (average 

 of disc readings), while in the Illinois River in that year the 

 average is 40 cm. 



COLLECTIONS. 



All collections were made with the plankton pump. Ex- 

 amination of the plankton of this stream was begun in August, 

 1896, and continued at a fortnightly interval until December of 

 that year, and thereafter until the close of operations in March, 

 1899, at approximately a monthly interval. From the charac- 

 ter of the curves of plankton production in the Illinois 

 River we may infer that collections at this long interval in 

 Spoon River will fail to give us any adequate or accurate delin- 

 eation of the movement in production in this stream. Further- 

 more, in the summer season at least, the plankton of Spoon River 

 is composed largely of those small planktonts — such, for exam- 

 ple, as En(/lena and Trachelomonas — which almost wholly escape 

 through the meshes of the silk net. A comparison of the plank- 

 ton of the two streams on volumetric data derived from the 



