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SJ River flows may triple in 45 years due to climate shift
dry levee
This dry levee south of Woodward Avenue is part of the plan to enhance 200-year-flood protection. The levee is expected to be extended and made more robust.

Climate modeling by the Department of Water Resources that assumes that within 45 years water flow may triple in the San Joaquin River.

If that is the case plans and designs for state-mandated protection against a 200-year flood — a reference to a 1 in 200 chance of an event of such a magnitude in a given year and not the frequency — could be woefully inadequate.

 It also would mean the envisioned $180 million project now being pursued protect all of Lathrop outside of River Islands, southwest Manteca, the Airport Way corridor north to French Camp, and Weston Ranch may cost significantly more.

In addition to the 200-year flood protection complication the new river flow projections on the San Joaquin River will have on efforts to protect urban areas, it also means flooding frequency could increase significantly in rural South Manteca in the 5,000-acre River Junction Reclamation District. The area at the confluence of the Stanislaus and San Joaquin rivers has flooded 11 times in the 93 years since 11 miles of levees were built in 1927 to protect the farm area. A 12th major flood was barely averted two years ago when an alert farmer noticed a boil growing and was able to rally nears to stop a breach before state re-enforcement arrived.

The Manteca City Council when they meet Tuesday at 7 p.m. with the public being able to attend for the first time since the pandemic started in early March is being asked to join the cities of Lathrop and Stockton as well as San Joaquin County to ask the state for an extension for a 2025 mandate that construction start on upgraded flood protection.

Senate Bill 5 that put the mandate in place allows for one justified 5-year extension to 2030.

If work is not started on actual levee improvements as things sit now by 2025, no new construction will be allowed in the identified 200-year floodplain. That runs the gamut from new commercial, residential, and industrial to improvements that increase square footage such as home additions as well as new outbuildings such as barns.

While the extension could be justified simply based on having to re-adjust the project to take into account by new Department of Water Resources projected river water flows, the San Joaquin Area Flood Control Agency’s (SJAFCA) is also arguing the COVID-19 pandemic will create economic impacts making it difficult to raise the needed funds to do the work.

New construction taking place in the 200-year flood plain is already paying fees toward the work. The fear is construction may slow down and reduce the funds flowing to the agency to perform the work. In addition a property assessment of some type on all new and existing development is needed.

The SJAFCA project would also protects a portion of Stockton, French Camp, and the rural area between Weston Ranch and Lathrop.

River Islands at Lathrop — with 300-foot wide super levees — isn’t expected to have issues if water flows in the San Joaquin River triple by 2065.

Ironically a project River Islands has been seeking federal and state approval for — widening the Paradise Cut that bypasses the problematic elbows on the San Joaquin River at Mossdale and connects with the Old River between Tracy and Lathrop — has been tied up by federal agencies for more than 15 years. When plans for the project that will take pressure off levees protecting Lathrop and parts of Manteca was first submitted, federal officials said it would be an 18-month approval process.

SJAFCA officials estimate the five-year time extension will enable construction of more than 7,000 housing units, thousands of square feet of commercial and industrial space, and create almost 22,000 jobs.  Most importantly, it will ensure residents and properties in the Mossdale Tract area are fully protected from a 200-year flood event.   

That construction will not only generate funds to build better flood protection for growth but also existing homes, businesses, and schools.


   What would impacts

of 200-year flood be

Should a 200-year flood occur with multiple levee failures along the Stanislaus and San Joaquin rivers south of the Interstate 5 bridge before the merger with the 120 Bypass, engineers have indicated it would:

*flood 5,200 existing homes with 3 feet or more of water.

*endanger and force the overall evacuation of 50,000  residents in Lathrop outside of River islands, Weston Ranch in Stockton, southwest Manteca, and rural areas

*force the evacuation of San Joaquin Hospital — the county’s major trauma center — as well as the county jail.

*force first responders at five fire stations, the Lathrop Police Department and the county sheriff to abandon their stations and key communication centers in the middle of a major emergency.

*Lathrop High and Weston Ranch High would have water flowing through their campuses as would six other Manteca Unified elementary schools.

*force the closure of portion of Interstate 5 — the major West Coast freeway running from Mexico to Canada — and the 120 Bypass.

*water would swamp the wastewater treatment plant serving 84,500 existing Manteca residents and more than 13,000 of Lathrop’s nearly 26,000 residents.

*disrupt Union Pacific Railroad train movements as well as damage tracks that Altamont Corridor Express relies on.

*182 commercial and industrial properties from Costco to the Lathrop Target and Tesla Motors to Simplot would be flooded.

And that’s just for starters. Modeling shows a number of existing homes would likely suffer water damage in fringe areas that could receive upwards of three feet of flood water.

Manteca, Lathrop, and Stockton aren’t the only communities impacted by the Senate Bill 5 mandate. There are 85 cities in 33 Central Valley counties that have to comply.


To contact Dennis Wyatt, email