Pájaro River Watershed Flood Protection Plan - The Pajaro River ...

Shown in Table 1 are annual instantaneous maximum peak discharges from
two longterm stream gages – one on the San Benito River near the City of Hollister
and one on the Pájaro River at Chittenden just upstream of the end of the Corps of
Engineers Flood Control project.
The San Benito River near Hollister gage had a drainage area of 586 square
miles, while
the current gage located at Highway 156 has a drainage area of 607 square
miles. The drainage areas at the two gage locations are within 3.5 percent of one
another and the combined record can be considered as one continuous record since
1950. The drainage area at the San Benito stream gage is approximately half of that at
the Pájaro River at Chittenden gage. Data has been collected on the Pájaro River
continuously since 1940. The four largest instantaneous peak events shown on the
following table are in the 1956, 1958, 1995 and 1998 water years.
The ratios for the peak discharges at the Chittenden gage divided by the peak
discharges at the San Benito River gage for the four major flood years are:
Water Year
Ratio
1956 3.217
1958 2.026
1995 1.287
1998 0.728
Because the ratio of the drainage areas at the gages is approximately 2.0, one
might expect that the peak discharges maintain about that same ratio. However, the
1956 event, the Christmas 1955 flood, shows much more of the peak discharge
attributable to the Soap Lake portion of the Chittenden gage’s drainage area. The April
1958 flood was fairly evenly distributed. The two most recent floods, the March 1995
flood and the February 1998 flood, had much more of their peak discharge coming
from the San Benito River portion of the overall watershed at the Chittenden gage site.
The following table shows the average daily discharges on the two rivers for
the four largest flood recorded at the Chittenden gage. The ratios of the sum of the
average flows for the maximum three consecutive days are shown below:
Date Chittenden San Benito Ratio
12/1955 45,300 cfs-days 10,040 cfs-days 4.512
4/1958 44,480 cfs-days 12,580 cfs-days 3.536
3/1995 41,120 cfs-days 19,170 cfs-days 2.145
2/1998 45,800 cfs-days 25,790 cfs-days 1.776
DRAFT 7/22/03
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Pajaro Watershed Flood Management