San Joaquin County’s clogged river

The swollen San Joaquin River finally began receding last week, but its brief surge above flood monitor stage rekindled concerns that a far more serious flood is becoming more likely each year.

Why? Tens of thousands of dump truck loads worth of dirt wash down the San Joaquin toward the Delta each year. Scientists have found that much of that dirt, or sediment, is deposited on the bottom of the river as it flattens out between Vernalis and Stockton.

That means the bottom of the river may be rising — perhaps 5 to 10 feet in places, according to one levee engineer.

And if the bottom of the river is rising, that means there is less room in the channel to push flood waters out through the Delta.

Like a clogged pipe, the river could more or less burst.

With a snowpack that is 171 percent of normal, and plenty of time for more storms, it could happen even this year.

“If we get higher flows this winter, we’re going to get a blowout much sooner than we did in 1997,” said Chris Neudeck, a levee engineer from Stockton. More than two-dozen levee breaches occurred on the San Joaquin that year.

“It’s going to get very problematic in short order,” he said.

If the river is gradually filling in, the channel elevations used to determine flood stage and gauge the risk to the public may no longer be accurate, he said.

Some areas likely are more susceptible than others. Silt sometimes accumulates around bridges or other structures. While the recent high flows may scour some of the mud away, trees and brush growing within the riverbed itself may stubbornly anchor some of it place.

The issue isn’t limited to the San Joaquin itself. The state Department of Water Resources has estimated that three-fifths of the channels in the San Joaquin watershed cannot convey all the water that their levees were designed to hold.