Don Pedro spillway opens, could nearly triple Tuolumne River flow

The spillway gates opened at Don Pedro Reservoir at 3 p.m. Monday, and over the next four or more days could nearly triple the flow of the Tuolumne River as it comes through Stanislaus County and Modesto.

The amount released will vary from 18,000 to 30,000 cubic feet per second, said officials with the Turlock Irrigation District, which manages the dam’s operations. The first of the increased flow should reach the Tuolumne at Ninth Street in Modesto 23 hours after the gates opened – which means 2 p.m. Tuesday.

Updated flood inuation maps from the Stanislaus County Office of Emergency Services: Click here and here.

At the highest flow, the Tuolumne as measured at Ninth Street should reach 65 feet – 10 feet above flood stage, officials said.

For comparison, the river was at 54.96 feet at 2:30 p.m. Monday, and flow was 10,800 cfs. Flood stage of the Tuolumne is 55 feet.

TID officials assured that the flows will come nowhere near those that resulted in widespread flooding in 1997. Flows then reached were 60,000 cfs, and the river hit 71 feet, said district spokesman Herb Smart.

The National Weather Service’s Advanced Hydrologic Prediction Service estimated Monday afternoon that the Tuolumne would peak at 62.90 feet at midnight Tuesday, with a flow of 24,900 cfs.

TID is required to open the spillway when the water level at Don Pedro reaches 830 elevation. At 4:30 p.m. Monday, the water surface elevation at Don Pedro was 826.54 feet. The decision to open the spillway before hitting 830 was made to “minimize downstream flows that could be damaging if we waited,” said Jason Hicks, incident commander for TID.

There are no mandatory evacuations at this time, Stanislaus County Sheriff Adam Christianson said Monday afternoon, but deputies, Modesto police officers and other emergency workers were out to ensure that residents in areas likely to be affected know the risks. The sheriff said no estimate is available on the number of people who could be affected.

The increased flow on the Tuolumne will affect the San Joaquin River, Dry Creek and other tributaries, Christianson said. “We want to give people the opportunity to collect their belongings. ... We’re strongly encouraging people to seek shelter and move to higher ground now before the river levels get to what TID has projected.”

Stormy weather in recent days, combined with timed releases from Don Pedro to stave off opening of the spillway, already have caused the San Joaquin River on the county’s West Side to spill over its banks, flooding agricultural land and some RV parks. The Tuolumne flooded a homeless camp at the Santa Fe Avenue bridge.

The sheriff had a warning for criminals who might seek to take advantage of evacuations. “I will tell you, if people are forced to leave their homes because of rising water, guaranteed there will be deputy sheriffs that will be watching, and if you dare victimize anyone who has to leave their home – if you decide you want to exploit them – guaranteed there will be a strong response from the sheriff’s office. You will be arrested; you will be prosecuted.”

TID and the Modesto Irrigation District have begun turning off power to meters in eight zones along the Tuolumne that are expected to flood. TID said the zones run from La Grange to the base of the Tuolumne where it converges with the San Joaquin River. Three zones had been shut down early Monday afternoon.

About 1,000 of TID’s 100,000-plus accounts will be affected. Not all 1,000 accounts are residences, said Hicks. The number includes pumps and other services.

MID spokeswoman Melissa Williams said about 1,000 of its customers also will likely be affected – again, not all residential. “About 75 percent have been turned off as of 2:30 p.m. today,” she said Monday. MID serves about 121,000 electrical customers – 97,000 of them residential.

The floodgates will remain open a minimum of four days, Hicks said. Once the gates are closed, he said, “then we have to wait for the water to safely recede, and each meter site has to be inspected for safety.” So while there’s no time estimate of how long the power will be off, “once we reach peak flow, if any areas are not affected that have had their power cut, then we will inspect and restore those first.”

By the third day of increased flows, said Hicks, TID should be able to provide more specific information on when and how it will shut the spillway.

Prior to opening the gates, a stretch of Bonds Flat Road was deconstructed to minimize debris flow, Hicks said. TID also closed the headgates at La Grange to protect the infrastructure of its upper canal system. The lower canal system has some normal winter operations, he said, and will be used to manage the water level at Turlock Lake.

Don Pedro is closed to boating and water use, though a few hardy campers remained at the recreation area Monday. Several dozen people also gathered Monday afternoon to watch as TID officials opened the gates.

This story was originally published February 20, 2017 at 5:25 PM with the headline "Don Pedro spillway opens, could nearly triple Tuolumne River flow."