126 
IOWA ACADEMY OF SCIENCE 
plateaus are nearly level, or slope back from the watercourses. 
There are numerous pockets, or sink holes, from which drainage 
is impossible. 
The subsoil is almost entirely a coarse gravel, the interstices of 
which are filled with finer material. The thickness of this gravel 
deposit of glacial drift is evidently, in most cases, several hundred 
feet. Very little clay was found. There are some boulders and out- 
cropping rock. Overlying the gravel in the timber areas is a sur- 
face covering a few inches to two feet in thickness of porous loam 
and humus, and the space between trees is almost filled by impen- 
etrable undergrowth. The gravel which underlies this region is the 
best possible natural filter for surface impurities, and is of such 
thickness and extent as to preclude the access of surface pollution 
to the watercourses by percolation. The percolation, of course, of 
water takes place, but the material is of such a nature that 
thorough filtration is effected before the water has sunk very far 
into the ground. 
Nature has made excellent provision for rendering innocuous 
objectionable material deposited on the natural ground surface of 
the watershed. . In the case of direct pollution of the stream by 
deposit of objectionable or dangerous matter therein, nature's re- 
sources for self-purification are somewhat restricted, and reliance 
must be principally on the effect of time and aeration of the con- 
taminated water from the point of such contamination to its point 
of consumption. In some cases the water is cold enough to be a 
factor, by reason of such low temperature, in self-purification. Ex- 
perience shows that this reliance is not always warranted, and 
sanitarians have placed a ban on direct pollution of streams which 
are the sources of supply of potable water." 
From the results of the investigation of the watershed which it 
was my privilege to examine, we found a considerable bed of clay 
on one of the tributaries of the Cedar River, on what is known as 
Taylor Creek; here a number of large springs issued and entered 
Taylor Creek where the present Miller logging camps are located. 
Another series of large springs emptied into the Cedar River near 
the present intake of the Seattle water supply. I need only say in 
this connection that the water coming from these springs contained 
an unusually small number of organisms ; it was bright and spark- 
ling. Bacteriological analyses were made of water from various 
sources from the Cedar River and its tributaries, with the following 
results : 
