The Shadow of the Past: Flooding in the Central Valley
A flooded street in Merced is seen on a day when recent rain resulted in a noticeably high water level in Merced County. Photo was taken January 11, 2023, by Andrew Innerarity, California Department of Water Resources.
The Valleys
We’ve seen this kind of year before. Lots of rain and snow, similar to 1983 just forty years ago. We will see it again, with a touch of climate change to add to the mix. When you read this, I suspect that some of the rainflood events that I write about today will have already come to pass.
This week, there will be scenes of drama and pathos along the levee-controlled rivers in the San Joaquin Valley and the Central Coast of California.
In 2007, I served on the California Department of Water Resources task force on Central Valley flooding. One of my tasks was to draft a quick summary of the Valley’s flood vulnerabilities. It was an easy task.
The Sacramento Valley has deep flood/sediment basins on either side of the Sacramento River as a consequence of its Pleistocene and Holocene history (the ice ages and the modern period). Early in the 20th century, the budding floodwater management system used some of these relict basins as floodways to route large volumes of rainflood runoff between constructed levees around much of the most expensive property in the valley. Later, large dams with dedicated flood space were added to the system, and later still, huge investments were made to upgrade the quality of the Sacramento Valley’s levees. More work is underway and planned. Rivers have been expected to stay within state-designated floodways, and that remains the plan.
The San Joaquin Valley’s floodwater management efforts were not so geographically favored — or organized. The flood basins are not so deep, the levees are close to the rivers, and the floodways are incapable of conveying anywhere close to the floodflows further north. The focus in this valley has been to maximize farmland (and sometimes some portions of cities), trust in the surrounding dams (that the statisticians do not trust), and just accept that levees will be overwhelmed from time to time. No major new work is underway, the plans are modest, indeed, for most of the valley no plans exist. Valley folks are independent.
But for our story today, importantly, unlike in the Sacramento Valley, in major wet years the dams surrounding the San Joaquin Valley are unable to maintain their dedicated flood space reservations. This deficiency is because of their dramatically undersized floodways downstream. In years of generous rainfall and snowfall, the dams cannot stop from filling. The inflow-outflow math is inexorable. As a consequence, large flood releases that exceed the areas dedicated to their rivers in the modern era will happen from time to time. They are inevitable. Fortunately, but not invariably, most of the San Joaquin river basin cities upstream of the Delta are on high ground.
But there is drama in the Valley as I write this today. A good-sized rainstorm is hitting most of the state following months of wet weather. Some of the Sacramento Valley reservoirs are beginning to make minor flood releases, but there is little concern that their floodways or flood reservations will be bested.
Not so in the San Joaquin Valley. Here all the reservoirs (except the oversized New Melones Dam) are presently encroached into their flood reservations — or nearly so. Success Reservoir on the Tule River has filled up and will be unable to contain the rainflood early this week or even as I write this. There will be flooding in Porterville. More reservoirs will be faced with release dilemmas. Release some floodway-busting floodflows early to avoid making a bigger release from a full dam — or just pray that the clouds part in time. For some of these reservoirs, it is already too late for prayers.
Even after this week, reservoir rainflood inflows will continue to fill reservoirs that are just making their ordinary maximum flood releases for weeks or longer. Generous rain events may inevitably result in releases bigger than the non-system has been built for. In the Sacramento Valley, spring snowmelt floods are sleepy, languid events contained within the Valley’s floodways. In the San Joaquin Valley, it is unlikely that many reservoirs and the sometimes informal floodways will be able to contain the coming snowmelt floods. Big water is going to be heading into the San Joaquin Valley.
In the Tulare Basin (the southern half of the San Joaquin Valley), that will also mean that the pre-European-settlement terminal lakes (Tulare, Kern, and Buena Vista) are going to be resurrected again. Farming operations in the lakes will be paused for some time, perhaps for the entire growing season. Of course, that is the price of farming on Mother Nature’s lakebeds. As has happened in the past, the huge farming operations in the Tulare lakebed will be pressing for some Kings River water to be diverted into the James bypass. This diversion will put Kings River water into the San Joaquin River basin instead of into Tulare Lake.
Further north, the U.S. Army Corps of Engineers will be allowing for deviations in the operating rules for the San Joaquin River basin dams to try to optimize flows in the San Joaquin River — flows will appear in the news as floodwaters. These flood flows will result from rainfall and from the spring snowmelt. Floodwaters will shimmer across the Valley bottomlands for weeks and probably for months.
The Delta
Perched on the northern edge of the San Joaquin Valley is where the two great Central Valley rivers meet — the Delta. The major city here is Stockton. The Delta cities are “protected” by Sierran dams to the east that suffer from all the vulnerabilities of the dams that rim the San Joaquin Valley (indeed, the dams are all in the San Joaquin Valley). The flood reservations here will be encroached. The dams may be forced to make big releases as a result.
The Delta cities will also face the slowly moving wave of water coming down the San Joaquin River bottomlands from rain, rainfloods, and snowmelt. The slower the wave the better. Like New Orleans facing high water in the Mississippi River, the Delta cities rely on upstream levee failure and river flows that move around rather than through the Delta cities. That strategy may work this time. It may not.
There are plans to make the western San Joaquin River bypass route more capable. Construction may start one day. There are plans and some construction already to improve the reliability of the floodways below the dams to the east of Stockton. But to make these dams useful, the capacity of these floodways has to be expanded. That’s a so-far substantially unconfronted challenge.
“…it is our hubris that has made some of us vulnerable to the challenges of this wet year”
The levees of Delta islands mostly used for agriculture are going to experience higher-than-usual water and waves. Islands have failed sometimes in the past. They may in this week. They may in the future. Although most have eventually been reclaimed after breaks in the past, it’s a precarious place to farm.
Final Thoughts
There is much more detail to be discussed about floodplain and floodway management in California’s Great Central Valley. There is much to be discussed in other areas of the state too. But at its highest level, it is our hubris that has made some of us vulnerable to the challenges of this wet year. It is our hubris that has permitted many to experience the trauma of having their lives upended by floodwaters. It is our hubris that has left so many of our floodplain neighbors without flood insurance and the real ability to recover from flooding. It is our hubris that we recover into the same harm’s way when we rebuild.
We have sometimes learned from these experiences in the past. Perhaps we will today, in the coming weeks, and in the coming years. There is much to learn.
