364 IOWA ACADEMY OF SCIENCE 
homes which might indirectly contribute sewage contamination 
to its waters. It forms the north fork of what was formerly 
known as Butcher’s Eun,' due to the location of a slaughter house 
in. it^ -valley years ago, and it is virtually a tributary of that 
Silightl.jr larger stream. It joins Butcher’s Eun a few rods from 
the juncture of that stream with the Iowa river. The smaller 
north fork was chosen rather than Butcher ’s Eun for the greater 
part of these observations because the latter has ceased to be 
normal owing to the encroachments of dwelling houses along 
the upper part of its course. 
North Fork has a sinuous course for about one-third of a mile 
through partly wooded bluegrass pasture. Its valley forms the 
characteristic Y of young streams. This valley secures its sup- 
ply of ground water near its head in the form of seeping springs. 
For some distance below this point the stream is scattered 
through a semi-boggy, grassy area with a definite channel only at 
intervals. The upper part of its course runs through soil and 
has less well defined alternating rapids and pools than would 
be the case if the material were more heterogenous. The lower 
portion of its course shows in sharp definition the alternation 
of pools and rapids. Some of these pools were perhaps a rod 
in extent and showed a depth of over a foot (Plate XI, figure 1). 
The rapids show little of the loose stone, gravelly character so 
often encountered in small streams but are formed by the out- 
cropping of solid shelves of stratified rock over which the water 
passes from one pool to the next (Plate XL, figure 2). Such 
rapids provide scant shelter and places of attachment for many 
of the common forms of rapid water life. 
The bed of the stream was filled throughout with quantities 
of the decaying leaves which had fallen from the trees in for- 
ested portions of the basin and had been concentrated by the 
rains in the main stream beds. These, with the algae covering 
the rocks and generously distributed throughout the .streams, 
afforded an unlimited food supply to all plant feeding forms. 
On April 10 the stream was dry for a few rods at its mouth but 
showed running water elsewhere in its course. It is said to go 
completely dry for weeks at a time during the summer months. 
ANIMAL ASSOCIATIONS OF THE BROOK. 
The animal life of such a brook as that described lends itself 
to ready, classification into special associations depending upon 
