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months in August or in the early part of September. It again increases in
the latter part of September and October, so that there is generally sufficient
water for a “fall run.” The hydraulic season in some years begins in the
latter part of April, but usually begins in May and lasts until the latter part
of October. The average length is about 170 days. About 20 days of this
time on an average have to be allowed for “clean-ups.”’

The amounts of water available for hydraulicking at different places in
the area, or which can be made available by the construction of ditches, can
be determined if the drainage areas and the average run-offs for these areas
are known. The drainage areas, except in the case of those which are
partly outside the area covered by topographical maps, can be determined
from the topographical map by tracing on cross-section paper the boundaries
of the drainage basins as shown by the contours and streams, and thus
determining the areas within the boundaries, or a planimeter may be used.
The average run-offs of the streams are not known, because measurements
are not available except for short periods. They may be approximately
determined by means of the relationship which exists between precipitation
and run-off in the area. The average annual precipitation at Barkerville
for thirty-six years is 36-47 inches. This amount is probably nearly the
true average for the whole area, for the meteorological station is situated
near the central part of the area, at an altitude of 4,200 feet. For the
higher levels, however, the average is probably slightly greater. The
surface run-off of streams in any region is derived from the precipitation,
but, as a general rule, forms only about one-third of the total precipitation,
for a large part of the precipitation is returned to the atmosphere by evapor-
ation and by transpiration from vegetation, and a part passes into the
ground water and does not reappear as springs in the drainage basin.

The mean annual run-off of Fraser river at Hope from 1912 to 1922, as
determined by the Dominion Water Power Branch of the Department of
the Interior, was equivalent to a depth of 15-61 inches of water over the
entire drainage basin of the Fraser. The average annual precipitation at
twelve meteorological stations in the Fraser drainage basin was 18-72
inches. All the stations, however, with the exception of Barkerville where
the average precipitation was 40-89 inches, are situated in the valleys at
comparatively low elevations where the precipitation is much less than in
the higher parts of the basin, from which the great part of the drainage
waters come. The precipitation in these higher parts probably averages
about the same as at Barkerville, but may be somewhat higher. It is
known that, as a rule, no run-off occurs in the United States in drainage
basins where the average precipitation is less than 18 inches.! In central
British Columbia the proportion of run-off to precipitation is higher because
of the higher latitude and less evaporation, but the run-off from a consider-
able part of the drainage basin of the Fraser in south-central British
Columbia, where the average precipitation is only 7 to 21 inches, must be
very small. The proportional run-off from the higher parts of the basin,
must, therefore, be fairly high and is probably about one-half the precipita-
tion. Unfortunately no records of discharge of the Fraser at Quesnel or at
points higher up stream are available. Discharge records of the North

1Hoyt and Groner: ‘‘River Discharge,’’ 1920, p. 156.
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