v_^ Types of Aquatic Environment 



with greater serenity than on land. Diatoms grow 

 and caddis-worms forage and community life is actively 

 maintained. 



Silt — Part of the substance of the land is carried 

 seaward in solution. It is ordinarily dissolved at or 

 near the surface of the ground, but may be dissolved 

 from underlying strata, as in the region of the Mam- 

 moth Cave in Kentucky, where great streams run far 

 under ground. But the greater part is carried in 

 suspension. Materials thus carried vary in size from 

 the finest particles of clay to great trees dropped whole 

 into the stream by an undercutting flood. The lighter 

 solids float, and are apt to be heaped on shore by wave 

 and wind. The heavier are carried and rolled along, 

 more or less intermittently, hastened with floods and 

 slackened with low water, but ever reaching lower 

 levels. The rate of their settling in relation to size 

 and to velocity of stream has been discussed in the 

 preceding chapter. 



Silt is most abundant at flood because of the greater 

 velocity of the water at such times. Kofoid ('03) has 

 studied the amount of silt carried by the Illinois River 

 at Havana. Observations at one of his stations 

 extending over an entire year show a minimum amount 

 of 140 cc. per cubic meter of river water; a maximum 

 of 4,284 cc, and an average for the year (28 samples) 

 of 1 .572 cc. Silt in a stream affects its population in a 

 number of ways. It excludes light and so interferes 

 with the growth of green plants, and thus indirectly with 

 the food supply of animals. It interferes with the free 

 locomotion of the microscopic animals by becoming 

 entangled in their swimming appendages. It clogs the 

 respiratory apparatus of other animals. It falls in 

 dejx >sits that smother and bury both plants and animals 

 living on the bottom. Thus the best foraging grounds 

 of some of our valuable fishes are ruined. 



