The San Joaquin River is expected to flow over levees 12 miles south of Manteca sometime Thursday near Vernalis at the confluence of the Stanislaus River by the San Joaquin National Wildlife Refuge.
The river was at 28.3 feet Monday at 7:45 p.m. The National Weather Service predicts the river will rise above flood stage — which is 29 feet — sometime early Wednesday morning. As what usually happens with high river runs, there is already seepage on the land side of levees in the farming region. Stanislaus County has closed Grayson Road that’s further south across the river near the community of Grayson.
The Weather Service predicts rain and snow for five days starting Thursday. While rain is expected in the higher elevations forecasters indicated it should be colder than the last storm which means the snow melt may not be as severe.
Reclamation district and emergency personnel are monitoring the situation that has many of the same elements of what contributed to the 1997 floods save one critical element — New Melones Reservoir is nowhere near capacity. At the same time, however, the super 300-foot wide levees now protecting River Islands at Lathrop means Stewart Tract where the planned community is being built will no longer serve as a safety valve as it has in past floods including 1997. The previous levees in the past have failed turning Stewart Tract into a lake that was up to 10 feet deep in many places.
Heavy snow followed by heavy rain at the higher elevations triggered a run-off 20 years ago that threatened to overwhelm reservoirs from Friant Dam near Fresno to New Melones. State Office of Emergency Services in 1997 noted that New Melones came precariously close to being breached, prompting heavy runoffs that surged down the Stanislaus meeting with increased flows at Vernalis. that led to 70 square miles south of Manteca being flooded with 2,500 people evacuated, 800 structures damaged and loses pegged at nearly $100 million.
For all practical purposes there was no water coming out of New Melones. Releases on Monday with a miniscule 27 cubic feet per second compared to an inflow of 10,018 cubic feet perfect. The 2.1 million acre foot reservoir was only at 55 percent capacity Monday. It increased nearly 24,000 acre feet from Sunday when it was at 54 percent.
The story is much different as you head south. The following are Monday reservoir statistics:
uDon Pedro with a capacity of 2 million acre feet on the Tuolumne River was at 99 percent capacity with 12,544 cubic feet per second (CFS) of inflow and 9,961 CFS of outflow.
uMcClure with a capacity of 1 million acre feet on the Merced River was at 91 percent of capacity with 7,768 CFS of inflow and 5,775 CFS of outflow.
uEastman with a capacity of 150,000 acre feet on the Chowchilla River was at 86 percent capacity with 3,415 CFS inflow and 4,074 CFS outflow.
uHensley with a capacity of 90,000 acre feet on the Fresno River was at 54 percent capacity with an inflow of 1,789 CFS and outflow of 2,054 CFS.
uMillerton (Friant) with a capacity of 552,000 acre feet was at 85 percent capacity with an inflow of 11,066 CFS and outflow of 10,431 CFS.
The runoff from those five reservoirs/rivers joins up with the Stanislaus south of Manteca. The Bureau of Reclamation by operating New Melones as they are currently not only will have a chance at refilling the 2.4 million acre foot reservoir that is lagging between 30 and 40 percent behind other major state reservoirs in terns of capacity, but they are also helping take pressure off the San Joaquin River between Vernalis and Mossdale where the Old River splits off from the main channel to reduce water pressure.
Mossdale Crossing at the Interstate 5 bridge is expected to reach warning stage at 19.5 feet by Saturday. Flood stage is at 28.5 feet. The river at Mossdale was at 19.16 feet Monday at 8 p.m. with water flowing at 25,799 cubic feet per second.
Reclamation workers are looking for boils, or spots where water comes bubbling up. Eleven of the boils that popped up in 1997 led to levee breaks when the river was below flood stage. That is why when rivers reach warning or monitor stage it becomes mandatory for the levee maintaining agency to patrol levees.
Repair work after the 1997 flood and a more minor event in 2011 shored up sections of the levees that had been weakened. Emergency work was also done in 2008 during the spring runoff when water flows started eroding the levee. While some of the 1997 breaks were on the west side of the San Joaquin River all of the area impacted in 2008 and 2011 were on levees that protect rural south Manteca.
The area south of Manteca known as River Junction is the most flood prone area in the region.
To contact Dennis Wyatt, email firstname.lastname@example.org