The
coarse detritus which gets into the streams and is subjected to the
action of floods is moved along when the grades are over 40 feet to the
mile, and is deposited mostly when the grade is lessened to between 30
and 20 feet. " The sands predominate greatly " when the grade is
reduced to 10 feet and less.*
The
finest and lightest material is held in suspension until the velocity
of the water carrying it is greatly reduced. The amount of material
suspended in the California rivers has been estimated from tests made
of these waters, but these tests have not been continued for a
sufficient length of time to afford an}' reliable results.
The
deposition of this material on lands overflowed during high water was
one of the original causes of the disputes mentioned above.
Up
to the year 1880, the total area in the Sacramento Basin thus affected
is estimated by the State Engineer at 43,546 acres, a large portion of
which was of little value and had always been subject to overflow.
The
catchment area on the east side of the Sacramento Valley is very
large, and the descent from the high sierra to the valley is very
abrupt and precipitous. During the stormy seasons immense quantities of
water, caused by rainfall and melting snows, are rapidly discharged
into the lowlands, where the river channels, having but small areas f
and light grades, are unable to carry them off, and floods invariably
follow.
The
reservoirs which have been constructed by the hydraulic mining
companies in the mountains partially mitigate the evils arising from
this source.
THE DUMP.
It
is impossible to lay too much stress on the importance of the dump, as
without it hydraulic mining could not be carried on. Where thousands of
cubic yards of
* Report of Lieutenant-Colonel Mendell, pp 33 and 34. + See vol. ii. p. 7 Trans. Tech. Soc. of the Pacific Coast.
