American River Common
The American River Common Features Project
includes levee work on the Lower American River, on the east bank
of the Sacramento River upstream of its confluence with the American
River, and in the Natomas Basin. It also includes additional upstream
river flow gages.
The work includes the strengthening of levees (impervious barriers
placed inside the levees, landside berms, and drainage ditches),
and raising levees at select locations. When completed, these improvements
will reduce the flood risk to about a 1 in 100 chance in any given
Folsom Dam’s eight low-level outlets are
too small to make full use of the downstream river capacity in the
early stages of a flood event. Releases through the existing eight
outlets combined with releases through the powerhouse are limited
to approximately 35,000 cubic feet per second (cfs) until the reservoir
level reaches the spillway. At this point, the reservoir’s flood
space is 40% full. The solution is to enlarge the existing outlets
and add two new outlets. This allows for releases to reach 115,000
cfs earlier during a flood event, creates more floodwater storage
space behind the dam during the peak of a flood event, and reduces
the peak amounts of floodwater sent down the American River.
These improvements, when combined with the Common Features work, will
reduce the risk of downstream flood damage to a 1 in 130 chance in
any one year.
Another significant part of the Folsom Dam Modification authorization
is the development of an updated flood management plan for the Folsom
Reservoir. The plan uses the new operation capabilities created by
the dam modifications and adopts new operations criteria, based on
improved long-range weather forecasting.
The proposed update will include the potential phased releases of
existing reservoir water storage in advance of predicted major storm
events. These advance releases will be based on the Advanced Hydrologic
Prediction System of the National Weather Service, which can forecast
reservoir inflows up to 3 to 4 days prior to storm events.
If adopted, the anticipated benefit of these advance releases would
be a more adaptable flood management plan that allows the creation
of additional flood storage area behind the dam. This additional flood
storage would enable the facility to control larger flood events without
downstream flooding while allowing for more flexible dam operations.
Adding this component reduces the flood risk to a 1 in 164 chance
in any given year.
Folsom Dam Raise
When combined with the Common Features Project
and modifications to Folsom Dam’s outlet works, raising Folsom
Dam 7 feet will reduce Sacramento’s flood risk to a 1 in 213
chance in any given year.
The work involves raising the concrete section
of the dam and the associated earthen dikes 7 feet and adding larger
spillway gates. These improvements will add 95,000 acre-feet of
floodwater storage capacity to the lake’s current 977,000
The plan to raise Folsom Dam includes ecosystem restoration and
habitat improvements. Ecosystem restoration involves planting native
riparian, upland, wetland, and woodland vegetation; terracing riverbanks;
and controlling non-native plants at two sites totaling about 620
acres along the lower American River. Habitat improvement involves
mechanization of the temperature control shutters at Folsom Dam
to better control the water temperature in the river for salmon
The Folsom Dam Raise project includes the design and construction
of a bridge over the American River near Folsom Dam.
Folsom Dam Bridge
The original plan to
raise Folsom Dam included the construction of a $36 million temporary
vehicle bridge to provide a detour for traffic that normally uses
the Folsom Dam Road to cross the American River. (For national security
reasons, the road over the dam is currently closed indefinitely.)
When Congress authorized the Corps to raise Folsom Dam in 2004,
it directed that the bridge be permanent and authorized the additional
$30 million needed for a permanent span.
The location of the bridge and its connecting road alignments are
currently under study. It will take a number of years to complete
these studies as well as the actual design and construction of the
bridge and connecting roadways.