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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 (Pl. 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 (Pl. XLVIT.). 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 XI. and Pl. XLVII.). On 
August 30 the amounts of albuminoid ammonia (.382) 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 (Pl. XLVII.), 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 [linois 
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 
