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

Figure11 Cartoon showing how incision reduces flood storage and riparian habitat
Methods:
The Pájaro Watershed above the Chittenden gauge was inspected carefully to
determine where natural floodplain areas were no longer being inundated in major
floods. The 1998 flood was of a magnitude such that it should have covered most of
the natural floodplains that function in balance with the Pájaro River and its tributaries.
If, as the probability plot for Chittenden suggests, the 1998 event was a 30-100 year
magnitude flood at various places throughout the watershed, then the floodplain
should have carried water with sufficient depth and velocity to leave a record in the
surface soils. Surface soil characteristics on a floodplain reflect the depositional, and
occasionally erosional, passages of flood waters where they are vegetated and cause
slowing of flood flows. Along the San Benito River, low-lying riverside bench lands
below the level of the agricultural Lake San Benito land surface have very young
poorly developed soils that are characteristic of flood deposits formed in the last few
hundred years (Fig 8). These deposits are very different from the moderately
developed soils on the higher Lake San Benito and Lake San Juan surfaces (Fig 9).
The current active stream channels do not have any silt-size organic-rich soil
development at all. Thus, it is possible to differentiate unambiguously where
floodplains have been abandoned in the last century.
Surveying of channel cross-sections and elevations was done at several places to
compare with historic data. A good plane table topographic map was made in 1917-
1919 in the Hollister area (Hollister, USGS, 30-minute quadrangle) and
DRAFT 7/22/03
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Pajaro Watershed Flood Management