From
past experience in the use of cedar timber it is safe to assume that
the life of this structure will be from twenty-five to thirty years,
and possibly longer. Its cost was $15,000.*
Debris Dams.—Debris
dams are obstructions placed across the beds of streams for the purpose
of holding back the sand and gravel coming from the mines, to prevent
their entering into the navigable streams and damaging the land in the
valleys below. They may be placed either in the mountain canons or in
the valleys where storage room can be conveniently obtained. These dams
or barriers may be composed of stone, wood, or brush, as
circumstances require. The structures are not designed to impound
water, but simply to check the velocity of the current carrying the
mining and other debris and to allow the deposit of the material behind
them, and therefore they partake more of the character of retaining
walls than of water dams.
"The
deposits in the streams consist of stones several cubic feet in
volume—cobble, gravel in all sizes, sand in various degrees of.
fineness, and a mixture of extremely fine sand and clay, popularly
known as ' slick-ens.' This latter material, being easily transported,
is constantly in motion, even in the low stages of the stream. The same
is probably also true of the finer sands, and in particular streams is
true of the gravel, at least in the upper portions, where the beds are
confined and where the slopes are steep.
"
When the high stages of water come they find the beds of the streams
dotted at the ends of the mining sluices with mounds of detritus, which
sometimes form dams across the beds of the stream.
"The
effect of the flood-water is to sweep these deposits, excepting perhaps
the largest pieces of stone, and to carry them away to lower parts of
the river. The fall of some of the principal streams serving as outlets
to the mines is in places 50 and even 75 feet to the mile. A rise of 20
feet more or less in a narrow bed with such a fall is sufficient to
move material with great effect.
"The
periods and stages of high water vary very much here as elsewhere ;
but the rainfall, be it large or small—and there is great variation in
this respect—comes mainly in two or three months, so that there is,
except
*
The above description of the Bowman dams is essentially the same as
that written for the author by Hamilton Smith, Jr., who planned and
constructed the dams.
