Now these two distinct harvests
of flood waters were gathered simultaneously and poured out on the plain
in one magnificent avalanche. The basins of the Yuba and Feather, like
many others of the Sierra, are admirably adapted to the growth of floods
of this kind. Their many tributaries radiate far and wide, comprehending
extensive areas, and the tributaries are steeply inclined, while the
trunks are comparatively level. While the flood-storm was in progress
the thermometer at Knoxville ranged between 44 deg. and 50 deg.; and when warm
wind and warm rain fall simultaneously on snow contained in basins like
these, both the rain and that portion of the snow which the rain and
wind melt are at first sponged up and held back until the combined mass
becomes sludge, which at length, suddenly dissolving, slips and descends
all together to the trunk channel; and since the deeper the stream the
faster it flows, the flooded portion of the current above overtakes the
slower foot-hill portion below it, and all sweeping forward together
with a high, overcurling front, debouches on the open plain with a
violence and suddenness that at first seem wholly unaccountable.
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