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

Figure 7 -- 1938 Lower Pájaro Valley flood. Photo point appears to be near present
Highway One.
The lack of gauge record for 100 years at Chittenden does not limit our analyses back
only to its start in 1940. Study of precipitation records for the periods for which we
have gauge record permit comparison with those same records for the 40 or more
years before stream gauge record. Where we have 100-years of record for daily
rainfall, such as at Hollister, we see that the 1955 event was by far the largest
cumulative net storm rainfall. Although Hollister recorded 1.0 to 1.38 inches in single
days in 1935, 1936, and 1937, and although Watsonville and Hollister recorded more
than 1 inch per day for three consecutive days in 1937, it was the Christmas storm of
1955 that set the standard for the Pájaro watershed. Beginning December 20 th , Santa
Cruz mountain summit areas recorded more than 10-inches a day through the 23 rd .
Hollister recorded 1.93 inches on the 22 nd , 3.75 on the 23 rd , and 1.01 on the 24 th . In
the southern Santa Clara County area the February 2-4, 1945 storm, with over 10
inches in a day at Morgan Hill, provided the maximum historical rainfall period, and
that overlaps with Chittenden discharge record where flow was significantly less than
in 1955, 1998, and 1995. It is thus reasonable to postulate that the Chittenden gage
has recorded the largest Pájaro River events of the past 100 years, and that sustained
high flows must have been greatest in 1955, followed by 1998.
There is the anomaly of the 1998 flow record at on the San Benito River that merits
further discussion. The official USGS gauge record indicates that the Pájaro tributary
peak flow was greater than the downstream flow at Chittenden in 1998. According to
the U.S. Geological Survey Field Office Supervisor, Larry Freeman, this may reflect a
real difference where flood storage in the lower San Benito River below the Hollister
gauging sites retains flow and diminishes the peak at Chittenden. However, the
gauging station on the San Benito River was washed out in 1998 and the flow had to
be estimated based on water backed up at the Highway 156 bridge, rendering the
estimate good only to ± 25 percent (see Appendix 1). Indirect evidence, presented in
the next chapter on flood storage, supports Freeman’s hypothesis that there is a large
flood storage volume still available in the Lower San Benito River.
DRAFT 7/22/03
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Pajaro Watershed Flood Management