The spillway at Don Pedro Reservoir has closed.
At least for the time being.
On Monday dam operators at the seventh largest lake in California closed the controlled spillway that was opened a week ago for the first time in two decades to alleviate a lake level that just inches from overflowing.
Despite releasing more water through the spillway to complement the water already being released through the dam, the lake level at Don Pedro has actually increased by 1.5 feet since Feb. 20 when the spillway was first opened – an event that drew people from Stanislaus, Mariposa and Tuolumne counties who came to see the rare sight of water flowing through the canyon that was carved in 1997 when the spillway was first used.
A request by the Turlock Irrigation District to continue to release water through the spillway until the lake level falls more than 13 feet was denied by the United States Army Corps of Engineers citing flood concerns on the lower San Joaquin River as its rationale for ordering the spillway to be closed.
And as South San Joaquin County farmers brace for a possible disaster – aging, water-logged levees that are holding back amounts of water that haven’t been seen in over a decade – the water coming from Don Pedro indicate that the Tuolumne River will be a formidable opponent for the foreseeable future.
With a record snowpack ample enough to completely fill that reservoir sitting in the snowpack above the reservoir, Monday’s announcement that the gates would be closed was made with the qualifier that there’s a very good chance they’ll have to be opened again at some point this spring to reduce the lake level and properly brace for the spring melt.
The immediate forecast for the San Joaquin River, however, is more favorable than it was a week ago when the threat of a levee breach somewhere in South Manteca was viewed as inevitable.
With the river level at Vernalis hovering just above the 30-foot mark, the National Weather Service is forecasting that the level will decline steadily for the next seven days – dropping below the danger stage mark of 29.5 feet between Wednesday and Thursday, and dipping below flood stage early Saturday morning.
The focus of dam operators is now to get the reservoir down to 815 feet, which could take two weeks, before setting the sights at 801.9 feet which is the preferred mark set in the flood-control manual prepared for that particular reservoir by the Army Corps of Engineers.
And New Melones Reservoir – which has cut its releases down to double-digits to alleviate pressure on the San Joaquin River – has increased its water in storage by more than 20 percent by holding back water that would ultimately join with the San Joaquin River at the river junction. That particular area has been the intense focus of levee upgrades and enhancements in the last two weeks as crews prepare to reinforce what one engineer calls one of the “weakest” spots in the miles of levees that protect rural farmland south of the Highway 120 Bypass.
So what will the tributaries that feed the San Joaquin River look like when the period of high water – which could last months – look like when they return back to normal flows?
According to Modesto Junior College geology professor Gary Hayes, the floodwaters will serve as a natural cleanser to rivers that have ravaged by drought and ultimately will bolster the health of the rivers themselves once everything settles down.
“I have a feeling that floods like this rejuvenate rivers. Many old trees will be lost to the high water, but others will grow in their place,” Hayes said. “The hyacinth has been washed downstream for the first time in five years, and it had been a serious problem.
“This gives us a chance to prevent the spread upstream. Stream gravel has been redistributed, and silt flushed away which should help the salmon and other fish spawn with better success.”