Modesto area flooded exactly 25 years ago. Could current winter be anything like it?
The Modesto area last got catastrophic flooding around New Year’s Day of 1997. A series of warm storms melted much of the already deep snowpack in the Tuolumne River watershed.
Snow abounds once again 25 winters later, but water managers do not see the same danger. They said Don Pedro Reservoir has plenty of unused space for runoff from above, and their tools for monitoring the flows have advanced.
The assurances came from the Modesto and Turlock irrigation districts, which share the huge impoundment in the Sierra Nevada foothills.
“Given Don Pedro Reservoir’s current level and available capacity, a repeat of the 1997 event is highly unlikely,” said a joint statement from the districts in response to emailed questions from The Modesto Bee.
They also do not foresee a repeat of 2017, a record year for Tuolumne runoff. Don Pedro filled up earlier than usual, forcing high releases into the lower river for several months. It was unsafe for swimmers and boaters, but it did not cause major flooding.
The 1997 flooding damaged about 1,800 homes in Stanislaus County and many more around Northern California. Most of the toll was in neighborhoods close to the rivers.
The Tuolumne flows into the San Joaquin River, which posed trouble in ‘97 for the Manteca area and points downstream. The Stanislaus River ran high but did not overwhelm New Melones Reservoir, which is even bigger than Don Pedro.
The current winter follows two years of below-average rain and snow, which drew down many reservoirs. The snowpack feeding the Tuolumne was 165% of average as of Wednesday, MID and TID said. The The storm season started strong in late October, and it typically runs through March.
Don Pedro was at 54% of capacity Tuesday and 79% of the historical average for the date. The managers have to release some water to aid salmon spawning in the lower river. They also have to maintain enough space as part of the Central Valley’s system for dealing with sudden runoff from the mountains.
New Melones was at 39% of capacity and 70% of its historical average as of Tuesday. It supplies the Oakdale and South San Joaquin irrigation districts, as well as the federal pumps that send water south from the Sacramento-San Joaquin Delta.
Restored floodplains help too
The past 25 years have brought increased understanding of the importance of flooding to the Valley environment. The rivers spread widely most every spring in the time before dams and levees.
Restoration projects by nonprofit groups and public agencies have mimicked these processes in some places. They sustain fish and other life while easing the flooding risk to downstream homes.
In a Dec. 28 email to supporters, the Tuolumne River Trust said this stream alone has had more than 25 projects. They are on nearly 50 miles of river from La Grange to the confluence with the San Joaquin.
“When rivers are connected to their floodplains and able to spread out, the water has time to seep into the ground and recharge groundwater supplies during dry seasons, which can mitigate the impacts of drought,” the group said.
The effort also involves River Partners, which works around the state, and the Tuolumne River Conservancy.
Don Pedro stands ready for runoff
Irrigation districts have helped with floodplain restoration. But they also take part in the regional system for assuring that large expanses are not inundated. Dams are the biggest part of it.
MID and TID said Don Pedro has enough room as 2022 starts to handle the expected runoff of about 565,000 acre-feet by the end of April. The water surface is at about 740 feet above sea level, 36 feet lower than in 2017.
“When you have a large snowpack (165% of average), there is always a possibility that a series of warm storms could come in and melt it rapidly, causing us to encroach into Don Pedro’s flood control space,” the agencies said. “If such conditions were forecast, then the districts would start releasing the necessary volume of water in a controlled fashion.”
A key measure for the Modesto area’s flood risk is at the Ninth Street Bridge. The river crested at 71 feet above sea level in the disaster of 1997. It got as high as 59 feet in 2017 but was only 39 feet as of Wednesday.
MID and TID said high-tech tools give them a better sense of what the river is up to.
“We are using a real-time model that was instituted after the 1997 flood and have spent the last 20-plus years fine tuning it and bringing on more forecasting tools. These state-of-the-art forecasting tools can assist us in responding to any unprecedented conditions.”
What if winter turns dry?
Despite the ample storms so far, the central Sierra watersheds are still at only 58% of the historical average through April 1. The recovery from drought could stall if the rest of the winter is drier.
“As we monitor California’s dynamic weather patterns, we are always prepared and planning for the possibility of a drought,” MID and TID said.
The National Weather Service forecast a dry but frosty start to 2022, with a chance of more rain and snow starting late Monday.
