San Joaquin River Flood Risk Management System

this is California’s Central Valley a
large flat region stretching approximately 450 miles through the
center of the state it’s home to industry thriving cities
and some of the nation’s most productive agricultural land. the region depends on
a system of levees bypasses and reservoirs to reduce the probability of
floods damaging millions of dollars of property and causing loss of life. the
northern part of the Central Valley is part of the Sacramento River watershed
and the southern part of the valley is part of the San Joaquin River watershed.
flood waters from both watersheds flow into the Sacramento and San Joaquin
Delta before flowing into the San Francisco Bay and ultimately the Pacific
Ocean. as a result flood risk in the Delta is affected by flood runoff
originating in both watersheds historically as seasonal runoff from
nearby mountain ranges flowed into the valley it would spill over its
riverbanks and pool for miles across the valley floor essentially making it an
inland sea. this was a natural occurrence for thousands of years. when settlers
moved into the rich and fertile valley in the mid-1800s they immediately began
efforts to manage flooding. early efforts were focused on building levees to
confine floods to defined channels this resulted in areas being flooded less
often but also increased stages and depths elsewhere. these early efforts
lack coordination and often led to disputes over induced damages between
landowners. in the early 1900’s the US Army Corps of Engineers began working
with state and local agencies to reduce flood risk in the Central Valley. close
coordination throughout the decades since has resulted in a combination of
federal local and state owned components that reduce flood risk within the San
Joaquin Valley. flood risk management starts in the upper watershed of the San
Joaquin River where 12 reservoirs provide dedicated flood storage space
during the flood season. these reservoirs act like shock absorbers
the downstream rivers and channels. when possible floodwaters are temporarily
stored by the reservoirs and slowly released at the downstream channel
capacity. additionally these multi-purpose reservoirs transition from
dedicated flood storage space to water conservation space to store spring
snowmelt for irrigation and hydropower at the upstream end of the San Joaquin
Valley floodwaters leave the mountains and flow into the San Joaquin River
where they are diverted into Chowchilla and Eastside bypass system. this levee
bypass system was constructed parallel to the San Joaquin River to a point
upstream from the Merced River. reaches downstream of the Merced River our part
of the federal lower San Joaquin River and tributaries project. the project was
authorized in 1944 and extends to the Sacramento-San Joaquin Delta. in addition
to the bypasses the project consists of levees channelization and bank
stabilization. downstream of the Stanislaus River the San Joaquin River
becomes a distributary stream with flows splitting off from the main channel at
Paradise cut and Middle River the remaining portion of the San Joaquin
River flows past the City of Stockton the reach below Stockton is maintained
as part of the federal Stockton deepwater Ship Channel project. multiple
streams that drain the western slopes of the Sierra Mountains within this reach
result in a complex region of commingled sources of potential flooding. many of
the San Joaquin River flood risk management features were built nearly a
century ago using typical construction methods available at that time. these
methods have significantly improved due to advancements in technology and
experience during past flood events. over the last 25 years engineers have gained
a better understanding of how levees fail. under seepage through seepage
instability and erosion are the primary failure modes that can often occur with
little to no warning. the vulnerability of levees to these modes of failure was
very apparent during the 1955 1986 and 1997 floods and the most recent 2017
high water year where portions of the city of Stockton came dangerously close
to catastrophic flooding. the San Joaquin area
flood control agency or SJAFCA is a joint powers authority and local agency
created in 1995 to manage flood risk for the City of Stockton and surrounding
County area they have been working hand-in-hand with the Corps of Engineers
through a series of cost share agreements and projects to improve the
levees in the region and reduce residual flood risk. the Corps, SJAFCA and the
State of California’s Central Valley flood protection board are seeking
federal authorization for additional levee improvements within the Lower San
Joaquin River feasibility study area. if approved and authorized construction
will address the geotechnical and erosion problems affecting the levees
within the urbanized region and help reduce flood risk for over a hundred and
sixty thousand residents

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