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Flooding threat: Too much water filling a system with too little capacity

Storm runoff from Friant Dam passing through Mendota Pool

Millerton Lake water released from Friant Dam into the San Joaquin River passes through the Mendota Pool on its way to the Sacramento-San Joaquin River Delta.
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Millerton Lake water released from Friant Dam into the San Joaquin River passes through the Mendota Pool on its way to the Sacramento-San Joaquin River Delta.

The Kings River’s overflow system in western Fresno County cannot carry as much floodwater as it once did, so there is greater risk of flooding in this wet year, Fresno County supervisors were told on Tuesday

The problems led to approval of an emergency proclamation by supervisors that will allow officials to seek state help in an all-out effort to improve the county’s slough and levee system and any new problems that could arise from flooding over the next few months.

Steven Haugen, Kings River Water Association watermaster, said water flowing through the Fresno Slough into the Tranquillity Irrigation District area is not hitting the flows that ran through the same area in 2011, the last time there were significant releases from Pine Flat Reservoir.

Between 3.5 million and 4 million acre-feet of inflow water will have to run this year through the Kings River and the central San Joaquin Valley’s system of canals and sloughs from Pine Flat, he said. High flow problems could persist into the summer as snow in the Sierra melts, Haugen said.

More water is forecast for Millerton Lake, which is about half the size of Pine Flat, which means it will have to empty up to eight times into the San Joaquin River and diversion canals, he estimated.

The Kings and San Joaquin merge between Mendota and Firebaugh.

In 2011, water was flowing at 4,000 cubic feet per second along the Fresno Slough bypass near Tranquillity, but this year it can barely eclipse 3,100 cubic feet per second. Capacity has been reduced because of lack of maintenance, more sediment and land subsidence, and, if a solution isn’t found soon, that water will back up into the reservoir, and hundreds of thousands of acre-feet of water will still need to go somewhere, Haugen said.

In the meantime, Fresno County and Tranquillity Irrigation District employees are working on repairing the levee and Fresno Slough bypass so that more water can flow through it.

“In some instances, we’ve been within 12 inches of the tops of those levees,” Haugen told supervisors. “In 2011, it handled in excess of 4,000 cubic feet per second. ... There is some critical work needing to be done out there.”

In some instances, we’ve been within 12 inches of the tops of those levees.

Steven Haugen, Kings River Water Association, Watermaster

A lack of maintenance along the slough, additional sediment and land subsidence along the waterway have restricted the flow of water compared with six years ago, said Supervisor Buddy Mendes.

Because of subsidence, the water pools in areas where it would have flowed more freely in 2011, he said.

“If we’re diminished on what we can let out of the dam and can’t move 4,000 plus cubic feet per second, we start falling behind every day,” Mendes said.

To avoid flooding, Fresno County officials are asking the state Department of Water Resources and Department of Fish and Game to consider sending water into the Mendota Waterfowl Management Area, between Tranquillity and Mendota. The amount of water the management area can hold isn’t known yet.

Mendes said the waterfowl management area could create a lake if the levee is breached north of Tranquillity and south of Highway 180 and allow the water to flow into Mendota Pool.

Supervisor Brian Pacheco, who represents northwestern Fresno County, said 85 homes are in danger of flooding in Tranquillity if water overflows the levee. He also said he is concerned about Mendota and Firebaugh.

“We can breach the levee and flood the marsh lands,” he said. “That may not be for the best, but if you’re in one of those 85 homes it’s a pretty good alternative.”

In Mendota, the 1.28 million gallon per day wastewater treatment plant is in jeopardy if water from the nearby slough overflows its banks and mixes with water from the plant.

In Firebaugh, the city park along the slough is under water. It frequently floods in most significant wet years, but Pacheco also is concerned the flooding could spread.

Marc Benjamin: 559-441-6166, @beebenjamin

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