34 IOWA STUDIES IN NATURAL HISTORY 
river valley within the county. As the river passes through its 
rocky bluffs, these will remain more constant and can be located 
in general as lying on the short side of any loop of the stream; 
in other words, these materials are thrown down as the river 
dashes its force against the opposite bank. Both the sand bar 
ana the mud fiat, of course, lose their moist character as the 
season advances. Hach has its distinct group of animals. Ex- 
amples for the sand-bar would be the ant lion larva, and the 
tiger beetle, and for the mud flat would be the Elapharus beetle. 
The sand bars also provide a place for the snapping turtle and 
soft-shelled turtle to deposit their eggs. 
THE STANDING WATER SERIES 
Bodies of standing water undergo a succession of changes 
comparable to those in the establishment of streams as the 
process of attaining base level is carried out. In the case of 
standing water, however, this is a retrogressive action, as lakes, 
which represent the original condition, are, by cutting off of 
their outlets and filling in of their basins, reduced to mere ponds, 
and these in turn to swamps and bogs, and eventually to dry 
land. As was indicated in the paragraph on lake basins, lakes 
are only nominally present in Johnson County at this time. 
The plant and animal life of any lake can be clearly understood 
only by an understanding as to the cause of the lake. For ex- 
ample, a lake of purely glacial origin such as Swan Lake would 
not be expected to have identical flora and fauna with an ox-bow 
lake formed by the river and fed by occasional overflows from 
the stream which gave it existence. Swan Lake, the most exten- 
sive lake in the county, has been so reduced within the last fifty 
years that it is little more than a shallow swampy pond. The 
pasturing of its banks and the watering of stock from it have 
done much to alter its original associations. It still provides a 
nesting place for the coot and the gallinule, and forms a resting 
place for many of our migratory water birds during their spring 
and autumn travels. It has become so shallow and so completely 
clogged with vegetation that muskrats build their homes in the 
deepest parts. ; 
(1) Ponds. As with the lakes of the county, the ponds may 
