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

photogrammetric maps were made based on aerial photography taken between 1952
(San Juan Quad) and 1955 (Hollister Quad.) Tres Pinos Quad photos were made in
1953. The date of the published map is not material to the reference elevations, nor
are the dates of photorevisions. The topography is based on the original aerial data
except where specifically noted in purple overprint. For all of the upper Pájaro USGS
quadrangles, published revisions in the 1970’s, 1980’s and 1990’s all specifically note
no topographic revisions after the original 7.5 minute quadrangle aerial base surveys.
Field surveys in 2003 augmented a very detailed photogrammetric and 2-foot contourinterval
ground-based survey made privately for the San Benito River area by
Graniterock Company. Those December 2000 data with very detailed aerial
photography at a scale of 1-inch = 500 feet were provided to us digitally by
Graniterock. The earliest topographic surveys of 1917-1919, as published in the 1921
USGS topographic map, were made on site by plane table and alidade. Although the
contour interval on those maps was only 50 feet, the surveyors clearly and definitively
noted the heights of the stream banks with a “step” in the contour at the break in
slope. By measuring the stream gradient on the map and the length of the step at
map scale, the heights of the banks can be estimated. For the upper San Benito
River above Hollister, those banks were 7 to 8 feet high at the time of the early
surveys.
San Benito County staff cooperated to provide access to their mining operation files,
and to the plat maps and property records so that we could tabulate and attempt to
contact all property owners bordering the San Benito River below Tres Pinos. These
records are tabulated in Appendix 3. Those property owners were contacted where
possible and state, federal, and local agencies were polled to try to learn of their
concerns and interests in upper Pájaro watershed watercourses (Appendix 4). Field
investigations were conducted on the Llagas, Uvas, and San Benito tributaries as well
as portions of the main stems of the Pájaro. We investigated evidences of active
channel modifications, gauging station status, riverbed and bank conditions,
evidences of bed-form and plan-form erosion or change, and high-flow markers or
field evidence. These observational data are integrated into our findings and
opinions.
Findings:
Channel Incision:
We verified earlier reports (Goldner Associates, 1997, ) that the San Benito River had
been incising. Our findings for the thalweg elevations along the uppermost San
Benito above Hollister are as shown in this table:
DATE
Blossom
Rd
Hospital Rd Union Road Nash Road
1917-1919 350 ft elev 320-25 ft.elev 298-99 ft.elev ~275 ft elev
1955 330 ft elev ~308 ft elev ~295 ft elev 269 ft. elev
2000-2002 314.4 ft. elev 280.7 ft. elev 259.1 ft. elev
DRAFT 7/22/03
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Pajaro Watershed Flood Management