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

When the balance between the volumes of water and sediment that are supplied to a
river from its tributaries and from its bed and banks is changed, the river system
attempts to rebalance itself. Rivers cannot store energy. They must use it as they
gain it, dropping 300 feet in elevation, for example, from Hollister to the ocean. If
water flow volumes exceed sediment volumes, the river will attempt to erode sediment
from its bed and banks to rebalance itself and equilibrate its rate of work with the
potential and kinetic energy available to it. The river that drains a watershed is
adjusted to carry the range of floods and sediment inputs that occur naturally in that
watershed. If a period of major landslide activity occurs, for example along the fault
zones in the Upper San Benito River, that sediment is stored in the channel awaiting
sequential years of flood flows to move it downstream. This leads to natural channel
aggradation, or build-up of sediment in the bed. After this occurs, flood flows
redistribute that sediment year after year, parceling it out for transport through lowgradient
reaches downstream. The steep gradients on the depositional areas of the
San Benito River near Hollister (18-20 ft per mile) are the result of the great natural
instability of the watershed hillslopes upstream.
When a river is deprived of sediment or when flood flows exceed the volumes
necessary to carry the sediment entrained in that flood flow, the river erodes its bed
and/or banks. Gravel and sand mining in the natural riverbed act to “starve” the river
of sediment, and lead to channel incision (downcutting) and/or bank cutting. When
downcutting is severe, the river can no longer store floodwater in its floodplain
because it cannot access its floodplain as it would naturally do every 2 to 3 years (see
Fig, 11). If riverbed mining exceeds the long-term natural sediment supply, the
watershed system is said to be in disequilibrium. That is, the natural form of the
watercourse and its watershed are no longer balanced with the water and sediment
that are moving through it. One of the most extreme examples of this imbalance is on
the Lower Russian River in California where the sediment-starved middle reach
around Healdsburg has incised as much as 20 feet and is now completely separated
from its floodplain. As a consequence of this loss of flood storage, flooding
downstream has increased in frequency and severity to the point that the area around
the town of Guerneville has become the Nation’s focus for the federal flood insurance
debacle where people repeatedly claim flood losses that cumulatively far exceed the
values of the properties (James Witt, personal communication, 1997). At the Monte
Rio gauge on the lower Russian River, 5 or more “100-year floods” have occurred
since 1986.
DRAFT 7/22/03
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Pajaro Watershed Flood Management