STREAMS AND DAMS - 2
A layman who is not used to digging
around in law books usually derives his
knowledge of the common law from his
lawyer and his neighbors. Actually in
most jurisdictions, the legal aspects
of dam building are not so complex as
to hinder the building of small private
dams. We have pointed them out only in
order to emphasize the advisability of
finding out about them before rather
than after building a dam. Another matter
to be investigated is whether the water
in the stream is safe for bathing. The
way to find this out is to have sam-
ples analyzed for B. coli. Such analyses
are made without cost in many states
by the state health department. A letter
to the state health officer will usually
bring some sterile bottles to be filled
with water samples and returned for the
analysis. Although the direc- tions which
are supplied for handling the samples
should be followed strictly, it is usually
better to take stream samples when the
water is at a high rather than at a low
level because it is at high water levels
that most of the contamination occurs.
Of course, if samples taken when the
water level is intermediate or low show
the presence of dangerous numbers of
B. coli, there is no point in having
another analysis made when the water
is high. On the other hand if the water
samples taken under flood or near flood
conditions prove to be impure, it is
sometimes advisable to take samples again
when the level is more normal.
Types of dams
Dams can be made of any strong material
which is watertight or can be laid
up in structures which are watertight.
The dam at the traditional old swimming
hole was usually made of logs held
in place with stones and stakes and
plastered up with mud. It was taken
for granted that frequent repairs would
be needed. In fact, filling up the
chinks every day or so to stop the
larger leaks was part of the fun of
using the place. Furthermore, no one
worried very much if a new dam had
to be built every year. In a few locations
it is still permissible to build such
temporary dams, but in some states
they are prohibited because of their
potential dangers. Such dams can cause
considerable damage if they should
break when the stream is in flood and
release all the impounded water in
one mass. Small stone masonry dam.
Another method of dam construction
is to build a massive bank of soil
in such a way as to make what is called
an earth dam. Small earth dams are
rather widely used in the construction
of ponds, par- ticularly ponds built
on farms for fish propagation or water
storage. The features of a good earth
dam are as follows: The base is very
broad in relation to its height. In
fact, the slope of the earth in the
exposed portions of the dam should
not exceed the natural angle of repose
which, for most soils, is between 35° and
37° with the horizontal. Either
all of the soil used will be of a kind
which is not permeable to water when
compacted, or the core of the dam will
be built with such soil. The dam is
protected from elements that would
erode its surface. For example, its
top is not used for a walk, and cattle
are not allowed near it. Most important
from this standpoint is the provision
of an outlet which will carry off all
the discharge of the stream even at
flood times, for an earth dam is quickly
destroyed by water flowing over its
crest. Earth dams are not well adapted
to swimming holes because of the protection
that they require and also because
their long, underwater slope takes
up a con- siderable portion of the
pond area.
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