Every two hours they are walking the levees protecting the San Joaquin River Club.
The volunteers look for anything amiss: Puddled water. Erosion. Boils.
This is what happens when Vernalis reaches flood stage.
The enclave of roughly 400 homes is nestled against the San Joaquin River on the northwest quadrant of Airport Way’s bridge crossing of the San Joaquín River.
It is here — and all the way downstream just beyond Mossdale Crossing in Lathrop before the river splits into the “Old River” and the new channel — that a tourniquet exists on the 366-mile San Joaquin River that drains 12,250 square miles of watershed.
Today the 12th “atmospheric river” — meteorology jargon for a heavy and fairly sustained storm system with significantly more moisture than typical — is expected to arrive and slam Northern California. Its heaviest impacts are predicted tonight. It will then weaken Wednesday before continuing east.
The National Weather Service forecasts that the latest atmospheric river will:
*Dump between 12 and 18 inches of snow from Ebbetts Pass on Highway 4 to Tioga Pass on Highway 120.
*Drop up to an inch of rain on Manteca, Lathrop, and Ripon.
*Drop up to 2 inches of rain on Yosemite Valley and Sonora.
*Will bring winds reaching 25 mph to 30 mph later today.
*Between noon and 8 p.m. today there is a chance for thunderstorms, high winds, lightning, and hail in a large swath of the Northern San Joaquin Valley although Manteca and Ripon are just barely included in the area where they are excepted to occur.
Vernalis — the name given the water gauge just north of the Airport Way bridge 10 miles south of Manteca — had the San Joaquin River still at flood stage at noon Monday. It was at 29.56 feet. Floods stage is 29 feet.
It had reached a high of 29.83 feet on Sunday at 11 a.m.
Flood stage means constant levee patrols every two hours.
Close to the Delta the situation was just notch below flood stage. That said, the mandatory watch stage still requires daily levee checks though not with as high a frequency. Out of abundance of caution, Reclamation District 2026 that protects River Islands with the strongest, widest, and newest levees around is still conducting patrols as if they were in flood stage.
The slight drop off in water level at Vernalis came as the operation of New Melones and Don Pedro reservoirs had outflow cut back ever so slightly.
As of 12:01 a.m. Monday, New Melones was releasing 1,336 cubic feet per second of water (cfs) — a cubic feet is roughly the volume of water that a basketball could hold. That was 60 cfs less than on Friday. Inflow into New Melones Monday was 5,832.
Don Pedro was inflowing and outflowing 10,419 cfs on Monday. The outflow was about 100 cfs less than on Friday.
The operation has accomplished two things.
It released water in a timely manner that river flows past Vernalis have peaked before the new storm hits putting levees in a somewhat of a better position. It also has allowed both reservoirs to have room in the event warm rains accelerate the snow melt.
The 2.4 million acre-foot New Melones was 55 percent full on Monday with 1,311,485 acre feet. That is 89 percent of normal for March 20.
Don Pedro on the Tuolumne River on Monday was 88 percent full with 1,728,201 acre feet of a top capacity of 2,030,000 acre feet. It is 118 percent of normal for March 20.
The snowpack at Dodge Ridge Ski Resort’s highest lift at 8,200 feet off of Highway 108 above Sonora was at 160 inches on the first day of spring on Monday.
Bear Valley Ski Resort, at 7,100 feet off of Highway 4, had 233 inches of snowpack at its highest lift.
Putting current SJ River
water flow in perspective
Levees can fail in other ways than being topped.
That is why with 8 feet to go before major flood stage — a point where levees are in danger of being breeched — there is a heightened guard in the reclamation districts that protect both sides of the San Joaquín River as it passes though the rural areas of the county, skirts Manteca, and cuts through Lathrop, and passes Stockton.
Based on the Vernalis gauge, 36,999 cfs of water was passing beneath the Airport Way bridge on Sunday.
Last year on March 19, there roughly a sixth of that water volume at 6,692 cfs.
The six highest recorded flows at Airport Way on the San Joaquin River were:
*Dec. 9, 1951 with 79,000 cfs, at 27.75 feet.
*Jan. 5, 1997 with 75,600 cfs at 24.88 feet.
*Jan. 27, 1965 with 52,600 cfs at 34.55 feet.
*March 1, 1938 with 51,200 cfs at 26.64 feet
*Dec. 25, 1958 with 50,000 cfs at 26.89 feet.
*March 7, 1983 with 45,100 cfs at 31.48 feet.
While there is without a doubt sediment build up that may have reduced the river channel volume, the fluctuation in cubic feet per second with somewhat lower water levels handling significantly more water is directly tied into the speed of the water movement.
A contributing factor is how “clean” the “table or shelf” — that area between a normal flowing river and levees that usually is not underwater — is in terms of brush and trees.
Such vegetation slows down the flow rate.
The catch-22 is removing it can devastate the ecological system for wildlife. But it can also undermine erosion control and end up sending more sediment into the channel during heavy flows and reduce the capacity of the river downstream that in turn raises flooding threats elsewhere.
To contact Dennis Wyatt, email dwyatt@mantecabulletin.com
