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 Jarge number of species 
of mussels found at some stations above this dam is due largely to the 
presence of sections of the stream where riffles 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 
débris 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 
