12 ILLINOIS BIOLOGICAL MONOGRAPHS [110 



has fallen across the stream. At Homer Park, an artificial dam about five 

 feet in height causes slack water for several miles up the stream. This dam 

 markedly affects the mollusk fauna, the tumbling of the water over the 

 dam mixing air with the polluted water and providing the dissolved oxygen 

 so necessary to naiad life. It is probable that the large number of species 

 of mussels found at some stations above this dam is due largely to the 

 presence of sections of the stream where rififles provide the oxygenating 

 agent. Mussels were usually found in or near such habitats. The mussel 

 fauna below the Homer Park dam numbers 28 species while above the dam, 

 as far up stream as Sidney, only 17 species occur, 10 species not passing 

 the barrier, although the environment does not differ essentially. The 

 current in the river is rapid over the riffles but rather sluggish in the deeper 

 places. The difference between high and low water (spring and fall) is 

 about six feet. The streams usually vary in width from ten to thirty feet. 



In the late summer and fall the small tributary streams (creeks and 

 rivulets) flowing into Salt Fork and other branches of the Big Vermilion 

 are usually either dry or contain scattered pools of water throughout their 

 length. They do not contribute any water, therefore, to the larger stream 

 at this time of the year. The mollusks living in these tributaries bury 

 themselves in the mud during this period of dry bottom and hibernate. 

 Many die at this time. 



The banks of the stream valleys, exclusive of the small tributaries, are 

 for the most part high and well wooded especially where the valley floor 

 is wide enough to permit meandering, in which cases the flat floodplains 

 are abundantly wooded. These flat areas vary from a few hundred feet 

 to a half mile in width. About two miles above Sidney an island has been 

 formed by the forking of the stream, the area embraced being about 650 

 by 1200 feet. At this place the right bank is 20 feet high and the left bank 

 quite low. The presence of fossil shells indicates that the island was prob- 

 ably the result of silt accumulation during a long period of time. The 

 wooded banks of the stream alternate with farm lands, some in pasture 

 and others in crops. Many of the crop lands have a fringe of timber border- 

 ing the stream. That the stream is high and powerful during the spring 

 when it is in flood is evidenced by the tangled mass of logs and other woody 

 debris which thickly cover the flood plain areas along the valley. Such 

 conditions were especially noted between Sidney and Homer Park. 



The current varies somewhat, being relatively sluggish in the backwater 

 above dams and riffles, but quite swift over the shallow places. During the 

 flood periods of spring and early summer the current is quite swift and in 

 places becomes torrential. This condition is indicated by the large number 

 of trees which have been thrown on the flood plains far above the margin 

 of low water. As measured during the month of October the current in 

 the Salt Fork at Urbana and a few miles down the stream had a velocity 



