32 IOWA STUDIES IN NATURAL HISTORY 
sometimes the case with those of the brooks. This classification 
most usually takes the form of grassy rapids, stony rapids, etc., 
each with its differing associations of animals. Those streams 
of the county sufficiently constant throughout the year to merit 
being called creeks are labelled upon the maps. They are as 
follows: Knapp, McAllister, Lingle’s, Mill, Jordan, Silver’s, 
Mud, Turkey, Saunders’, Rapid, Ralston, Snyder’s, Buck, Otter, 
Pardieu, Clear, Old Man’s, of which Picayune and Dirty Face 
are tributaries, and Deer. These have all been located and their 
extent given in the chapter on drainage. 
With the increased size of the creek and the increased depth 
of its valley, the processes of erosion have gone along far enough 
to have done considerable sorting of the soil materials. This is 
evidenced by deposits of sand and gravel at the rapids and on 
the opposite bank of the stream from that which is now being 
cut. The finer constituents of the soil go, for the most part, to 
make up loam and silt, which is carried considerable distances, 
and frequently a small mud bank will take the place of the sand 
bar at the inner side of the curve of the stream. Of almost all 
of these creeks the course is being altered from year to year 
except where they flow directly over well-defined rocky channels 
as in the case of Turkey Creek throughout part of its extent. 
There is, of course, every degree of intergradation between the 
brook and the creek and again between the creek and the river. 
Some of the Johnson County creeks, more notably Clear Creek 
and Old Man’s Creek, present certain of the characters of the 
larger and more mature stream; these characters are particu- 
larly a broad flat valley and a placid course rarely interrupted 
by rapids. Such fishes as the common bullhead, for example, are 
taken quite regularly as far up Clear Creek as Tiffin. Debris 
along the banks of the creeks forms a favorite haunt of the com- 
mon water snake. 
(3) River habitats. The course of the rivers of Johnson 
County, the Iowa River and the Cedar River, has been discussed 
in sufficient detail. The mature river represents the final stage 
in the development of a stream before the land has again 
reached base level. It is practically one long continuous pool, 
interrupted at rare intervals by rapids. The forms of life which 
