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

Fig. 13 Two views of the Llagas Creek Channel showing roughness
elements. Left image is just above Soap Lake, right in central urban
area
Channel Diversion
For the Lower San Benito River, the Graniterock aerial photos and contour
maps permitted us to establish that a local mining strategy has been to
isolate various portions of the channels and to protect mining areas and
channel banks with berms. Some of these berms are built to the same height
as the natural Lake San Benito lakebed land surface. That elevation assures
that the berms are above any historic level of the river. The berms and dikes
reduce access by floodwaters to the full channel width and the incision
reduces access to adjacent floodplains so that the river is greatly constrained
and downstream flooding is increased. Figs 18-21 (Appendix 5 – Historical
Change in the San Benito River) show an example of this kind of
manipulation in the lowermost reaches of the San Benito River just upstream
from San Juan Road. Fig 18 is from the December 2000 Graniterock survey
and shows Highway 101 at its junction with Highway 129, and San Juan
Highway. The “A”s are placed on abandoned floodplain remnants. A road is
seen going from the sand mining operation area upstream (right) along the
crest of a constructed berm that is the same height at the Lake San Benito
agricultural lands. This berm thus isolates the present river from its floodplain
remnants, some of which are used for mining equipment storage and some
for agriculture as was the case in the earlier photographs (Fig 19, taken in
1939). Topographic detail can be seen in Appendix 6. Modifications from
1950’s through the 70’s are shown on the 7.5 minute USGS Quadrangles
shown in Fig. 20
Channel diversions are found throughout the Pájaro River basin north of Tres
Pinos. Because the natural channels in both Santa Clara and San Benito
counties were braided or wide and changing from year to year, early property
owners confined the channels widely. Llagas Creek is now confined by
berms over much of its length. Uvas is confined by major levees through
Gilroy. San Benito River is confined to protect the City of Hollister, to protect
various sewage treatment facilities, and to protect agricultural uses.
Agriculture and development do not exist on most of the natural floodplain
except above the City of Hollister where most of the floodplain is developed
and where gravel mining and public works has resulted in many training and
confining dikes. Below (downstream) of Hollister the natural floodplain is
used for cattle grazing and for a single sod farm. The Pacific Sod Farm
(Tom Galdos, personal communication, 2003) has cooperated to protect its
primary growing area with a low berm that was overtopped in 1998.
Overbank silts are needed for the operation of this farm, where each sod crop
excavates a portion of the soil resource, and we were told that production is
becoming marginal without further sediment accumulation.
RESTORATION OF CHANNEL FUNCTIONS
We estimate that an average one-fourth mile width of the 6.5-mile long lower
San Benito River below Highway 156 could be restored to provide an
DRAFT 7/22/03
40
Pajaro Watershed Flood Management