450 



This is the largest production for this period of the year in our 

 records, exceeding the average, .24 cm. 3 , by 75 per cent, and ap- 

 proximating or exceeding the monthly average in each instance. 

 The cause is to be found in the relatively stable conditions at a 

 level sufficient to prevent sewage stagnation beneath the ice- 

 sheet which covered the stream prior to the March flood. 



Spoon River continued to discharge barren waters (av. .011 

 cm. 3 ), while the backwaters, with the exception of Quiver and 

 Thompson's lakes in March, produced a more abundant plank- 

 ton than the channel. Quiver Lake produces .66 cm. 3 , — an ex- 

 cess of 20 percent, above the usual production for this season, — 

 and in the first two months has 2- to 3- fold the usual plankton 

 content as a result of the moderate levels which make the lake 

 a reservoir without greatly increasing its current. When, how- 

 ever, the general current of overflow passes through it with the 

 March flood, production drops to one fifth of the mean for that 

 month. In Thompson's Lake the mean production is 1.15 cm. 3 , 

 56 per cent, below the mean production for these months. It 

 also falls below in January and March, when hydrographic con- 

 ditions are such (PI. XXXIX.) that channel water is diverted 

 through it, and rises above the mean by 25 per cent, in Febru- 

 ary, when the run-off is diminished by falling levels. In Phelps 

 Lake the mean production, 3.74 cm. 3 , is 23 per cent, below the 

 mean for these months. This lowered production, which also 

 falls below the mean in the last two months, is due, in part at 

 least, to the invasion of Spoon River water with the higher 

 levels. 



The various years of our operations may be briefly charac- 

 terized as follows. 



1894. A year of low water and fairly stable hydrographic 

 conditions, with nearly average production in channel and open 

 backwaters and deficiency in the vegetation-rich Quiver Lake 

 in the months of our records. 



1895. A year of continued and but slightly interrupted low 

 water, with stagnation destroying the winter plankton and 

 tending to abnormal production in early summer in channel 



