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that varies to yellowish or blackish tints with silt of clay or 

 loam origin. When diatoms are abundant a brownish tinge is 

 very evident, and with OscilJaria rising in quantity, as it does 

 in some semi-stagnant waters in late summer, a blackish tint 

 becomes pronounced. In midsummer and early fall, when 

 water-blooms rise, we find varying tints of green according to 

 the kind and quantity of chlorophyll-bearing organisms present. 



The turbidity, as above suggested, is due to a great 

 variety of factors, one of the most important of which is the 

 plankton itself. Indeed, under some conditions turbidity be- 

 comes a token by which the relative abundance of the plank- 

 ton may be estimated. The presence in our plankton of vast 

 numbers of the most minute planktonts, such as the flagel- 

 lates, renders this relation of plankton and turbidity more 

 prominent in our waters than it is in waters where such organ- 

 isms are less abundant. 



The turbidity otherwise is due to non-living solid matter 

 in suspension. This is brought in by tributary streams, and is 

 torn loose from the shores and bottom by the current of the 

 river, the movements of fish, the wash from steamboats, and 

 the constant sweeping of the river channel by fishermen's 

 seines during the open season at stages when seining is possible 

 in this place. The dust from prairies and forests brought by 

 winds; the waste from factories, distilleries, glucose-works, and 

 cattle-yards; and the sewage of a score of cities along the banks, 

 — all make additions to the burden of the water. 



Microscopical examination of the plankton has revealed 

 the diverse character and origin of the silt which accompanies 

 it. Fine fragments of quartz, bits of mollusk shells, small 

 pieces of coal or ashes, minute particles of loam or clay, and 

 the fecal pellets of aquatic organisms — especially of mollusks 

 and of insect larvae — constitute the heavier element of the silt. 

 To this is added a variable but ever considerable quantity of 

 exceedingly fine sediment of earthy or clayey origin, some of 

 which remains long in suspension. The coarser and lighter 

 silt consists largely of comminuted vegetation, both terrestrial 



