that
as the width is increased the suspensory power is diminished, so that
the degree of protection becomes greater as the system is developed. We
can imagine a condition of a river when comparatively little is carried
suspended, and nearly the whole of the material transported is rolled
in waves on the bottom.
"
This condition is more and more approached as the dams are raised. It
seems, therefore, to be good policy to give the first dam in the
cafions considerable height.
"
It will be understood that permanent protection can be attained only by
building dams in proportion to the amount of detritus turned out by the
mines. The system must be continued at least as long as the mines are
worked.
" If this system of restraint had proceeded pari passu with
mining during the past thirty years it can hardly be doubted that the
condition of the country affected would to-day have been much better
than it is." *
The
height of floods in the Yuba is only twelve feet at the Narrows, and
the water is fully loaded with all the material it is capable of
transporting. To insure protection permanent structures are therefore
required. On sand or gravel bottoms mattresses of trees or brush may be
used to prevent settling; but where the supply of rock is abundant,
convenient, and cheap, masses of stone can be blasted from the side
hills, and, by means of derricks or otherwise, be easily arranged as
required. The larger the rocks are the better; the largest being put on
the down-stream side, so placed as to permit the draining through of
the water; the smaller rocks on the up-stream side. The slopes on both
sides should conform to the requirements of the structure. As the dam
is built the material will gradually deposit itself against it on the
upstream face; the water draining through the rocks leaves behind in
the dam the sand, which gradually fills up the spaces as the bed of the
river is raised. Waste-ways may be readily provided on one or both
sides of the dam, which would have the practical effect of lengthening
the crest of the dam and of thereby reducing the depth of water passing
over it in freshets, in the proportion already
* Col. Mendell's Report on Mining Debris in California Rivers, p. 41.
