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of the main stream. Not all of the floods which flush this 

 tributary appear with corresponding prominence in the hydro- 

 graph of the main river, which is the one plotted upon all the 

 diagrams pertaining to Spoon River. In many instances they 

 coincide. All instances in the chemical diagrams (PI. XLVI. 

 and XLVII. ) of abrupt, steeple-like eminences in the curves of 

 albuminoid ammonia and organic nitrogen (and also of oxygen 

 consumed) are due to sudden floods, and appear most promi- 

 nently when the date of collection of the water sample coin- 

 cides with the initial stages of the flood. This is well shown 

 in September, 1898 (PI. XLVII.). Not all of the samples from 

 flood waters were collected at times which afford evidence for 

 the enriching effect of the initial stages of these tributary 

 flushes. The relative amount of these and other forms of 

 nitrogen which floods bring to the river is well shown in this 

 flood of September, 1898 (Table XL and PI. XLVIL). On 

 August 30 the amounts of albuminoid ammonia (.32) and 

 organic nitrogen (.6) are normal for that season of the year. 

 With the flood of the first week of September these amounts 

 increase more than tenfold (being 3.6 and 8.32 respectively), 

 falling again a week later to the normal (.2 and .48). A large 

 part of this matter is in suspension. For example, in the flood 

 of May, 1898 (PI. XLVIL), about 86 per cent, of the albuminoid 

 ammonia (2.32) and 90 per cent, of the organic nitrogen (5.46) 

 was in suspension. 



It is not plankton, neither is it to any large extent sewage, 

 which the tributary floods of Spoon River bring to the Illinois 

 as organic nitrogen, but largely organic debris not yet decayed. 

 The sewage-laden river habitually carries much less of these 

 substances than these tributary flood waters laden with this 

 organic debris from fertile prairies. The latter thus become 

 very important agents in maintaining the fertility of the 

 river water. The effect of these periodic additions of nitroge- 

 nous substances by tributary floods upon the plankton of the 

 river will be discussed in another connection. 



A decrease in these nitrogenous substances attends the two 



