The San Joaquin River is now at flood stage at the Airport Way bridge 10 miles south of Manteca.
The river level at the Vernalis gauge just north of the bridge was at 29.44 feet as of 11 p.m. Friday. That reflects nearly a foot rise over 30 hours earlier.
Flood stage is at 29 feet.
Moderate flood stage is at 32 feet while major flood stage is at 37.3 feet
The concern now isn’t levees being topped as water would have to rise almost 8 feet.
The problem is as water rises and pushes against levees boils can develop that undermine the integrity of the earthen barriers shored up in places by rock.
Those boils can keep getting larger until a failure occurs if they are not addressed quickly and effectively.
The levees came under mandatory 24 hour flood watch south of Manteca when the water reached 24.5 feet.
The San Joaquin River has been closed to recreational boat use between the Stanislaus River 10 miles south of Manteca and the Deepwater Channel in Stockton.
The closure imposed Friday by the San Joaquín Sheriff’s office was made to take a proactive step in projecting levees between south Manteca and Weston Ranch area that protect more than 55,000 people in the two communities as well as Lathrop on the east side of the river.
The move was prompted by a need to avoid wakes from boats pounding rain-saturated levees with the arrival of high tides, increased water releases upriver and more heavy rain that is on the way.
The outflow at New Melones Reservoir that sends water down the Stanislaus River to join the San Joaquin was at 1,396 cubic feet per second on Thursday at midnight
Don Pedro Reservoir — on the Tuolumne River that joins the San Joaquin less that four miles south of where the Stanislaus River does — was releasing 10,255 cubic feet per second.
A cubic foot is equal to the water volume one basketball can hold passing through a given point in a second.
A week ago releases were significantly smaller when the river at Airport Way was at 20.48 feet. New Melones was releasing only 48 cubic feet per second while Don Pedro was releasing 6,799 cubic feet per second.
The San Joaquin River is also taking increased reservoir releases upstream from other rivers from the Merced River to the Fresno River that will flow beneath the Airport Way bridge.
The current storm system that arrived today and will pass through by the end of Sunday has the potential in the Manteca, Ripon, and Lathrop area to add 0.25 inches of rain with as much as 6 inches of snow possible at Tioga Pass at 9,943 feet on Highway 120 at the eastern entrance to Yosemite National Park.
The snow level could be as low as 5,000 feet.
The second storm arriving on the first day of spring on Monday and lasting through Wednesday may add as much as 48 inches of snow on the Sierra crest from Tioga Pass north to Ebbetts Pass on Highway 4 according to the National Weather Service.
The snow level Monday and Tuesday will be between 4,000 and 5,000 feet with it dropping down as low as 3,000 feet on Wednesday.
In the valley, between 1 and 2 inches of rain could fall in Manteca. Sonora could receive 4 inches of rain and Yosemite Valley 5 inches.
Winds could reach 25 mph Sunday night in the valley.
The National Weather Service is anticipating additional storm system to hit Northern California throughout the rest of March.
In 1997, New Melones and Don Pedro were operated not for flooding but to maximum water storage when warm weather hit in late December and January with a significantly above average December snowpack that is minuscule compared to what is now accumulating.
By the time they switched to flood mode both dams came perilously close to being breached hence increased river flows that pounded levees south of Manteca and Lathrop.
The levees along the Stanislaus and San Joaquín ended up failing in 11 spots. That led to the flooding of 70n square miles between Tracy and Manteca with more than 2,000 people evacuated and $100 million in property damage.
To contact Dennis Wyatt, email email@example.com