108 ANIMAL COMMUNITIES OF STREAMS 
In the autumn of 1906 Professor Child found that the May-fly and 
stone-fly nymphs were not present in the riffles but were present in the 
moderately swift and more quiet parts below. The spring of 1906 was 
a dry spring and the females probably laid their eggs in the moderately 
swift instead of the preferred swift water. The distribution is deter- 
mined by the conditions at the time of egg laying. 
We note that even in the larger streams the weather conditions 
affect the presence and absence and abundance of animals. The mores, 
however, remain essentially the same. 
2. TRANSVERSE STUDIES 
Cross-section studies of streams are of interest as showing a hori- 
zontal arrangement of forms belonging properly to different formations. 
This is best illustrated in the cross-sections of curves where there is a 
horizontal gradation of current and in the size of material of the bed. 
Figs. 64 and 65 illustrate this. The burrowing May-fly nymph, belonging 
to the silt, is in the finest materials of the inside of the curve; passing 
toward the center of the stream we next encounter the sphaerid (Sphae- 
rium) and a little farther in the snail (Campeloma integrum), with it often 
mussels (Anodontoides ferussacianus); and still farther into the stream 
we find, clinging to the larger stones, the long snail (Pleurocera elevatum). 
While depth of water may be a factor here, the size of bottom material 
is of first importance. 
3. LONGITUDINAL STUDIES 
(Figs. 66, 67, 68, 69) 
If one passes from the headwaters of a stream to its mouth, he will 
usually find either the spring brook formation or the intermittent 
formation in the upper course, the swift-water formations in the middle 
course, and the sluggish stream or river formations in the lower course. 
There are very numerous variations of this and several of them deserve 
comment. Large streams with a large drainage area and much sedi- 
ment, and with much of the upper part in a young stage, are subjected 
to many changes in the lower courses, such as silting-up at the end of the 
flood periods and washing out later. This often prevents the development 
of the vegetation formation and favors the shifting sand and gravel formations. 
a) Rejuvenation, ponding, and retarding of erosion.—Streams are 
often dammed by some obstruction in their mid course, or erosion is 
checked at a point by a hard stratum, or the stream which has reached 
base-level is rejuvenated by a lowering of the water level at the mouth. 
