Here’s what to know about weirs and how they’re used in Sacramento

If you're driving north of downtown Sacramento on Highway 5 or 99, you may notice flooding in off-road areas that seem a little unusual. What you’re likely seeing is one of many weirs in the Sacramento region that help prevent flooding during intense weather. Since New Year’s Eve, many weirs in the area have diverted water that would have otherwise overflowed out of the American, Sacramento and Feather rivers, among others.

Unlike dams or levees, weirs aren’t created to store water or hold it back. Instead, weirs divert water. When a river begins to overflow, like it might during a storm with a lot of rain, a weir carries that excess water from the river elsewhere. 

Aside from the Sacramento Weir, which has a gate, most weirs don’t require anyone to operate them, passively diverting flow only after water levels in a river rise enough to pass over the weir structure. 

CapRadio spoke to Todd Bernardy, a manager for the Division of Flood Management with the state’s Department of Water Resources, to learn more about the history of weirs in our area, and how they operate. 

The interview has been edited for clarity and length. 

Can you describe the basic function of a weir? 

It's typically a concrete structure that has a flat, concrete surface. As the water and the rivers rise, that serves as an overflow point [and] allows that water to be diverted off of that waterway and into a bypass or a different waterway. 

Why did we first start building weirs in California? 

Historically, both the Feather and the Sacramento River cannot contain the amount of water that comes down the watershed. And when I say historically, I mean all the way back before the settlers existed. 

So, [these rivers] have always overflowed their banks, and those overflow areas became these basins. We know them as the Butte Basin, the Sutter Basin and the Yolo Basin. And when the river overflowed, it always went into those basins and it was really marshy land that existed in those basins. It’s those basins where we see a lot of our farmlands today. 

Well, what we’ve done is utilize those areas… those basins, as a way to convey those very high flood flows that we see [during] these very rare, extreme events. What the weirs do is [take] the water from those rivers and [convey] that water into those basins. Those basins then convey much larger flows. 

How often are weirs in the Sacramento Region actually in use? 

Weirs [are] not utilized very frequently. They are dependent on the water surface elevation getting to a very high level, so it's really only in those extreme events that they're going to be used. It’s rare, actually, to have all of the weirs operating. We typically see the utilization of those weirs anywhere between 5 to 10 years.

As an example, the Sacramento Weir was built in 1917. It's more than 100 years old. The last time we had opened the gates of the Sacramento weir was 2017. So here we are in 2023 and again, even with this storm … it didn't quite get there. So we have not opened it now in roughly five years. 

How many weirs are in use at the moment? 

Five of the six weirs, actually, we're seeing flow during the storm event. And all that means is that we're seeing a significant amount of precipitation and the system is working exactly the way it's supposed to work. 

Are there any concerns for wildlife or farmland when these weirs are in use, or after? 

No, there’s no concerns. In fact, we would like the bypass to actually be inundated more frequently. There's a project right now that we call “Building the Big Notch” that actually is intending to inundate the Yolo Bypass more frequently. 

As the waters recede, it's excellent for agriculture. You're adding a ton of nutrients into that farmland and into that habitat. 

But from a maintenance perspective, all floods carry things like logs and trees and sediment. So, there is a certain amount of cleanup that needs to happen afterward. It could affect some of the irrigation channels … and so those will all need to be looked at after the fact. 

But it's usually fairly flood-proofed, meaning we're used to seeing water in these areas and there's not a massive recovery effort. It’s more beneficial than anything else.

Any chance the Sacramento Weir will have to be opened up? 

There's very specific criteria that's required in order to open that weir. The guidance needs to show that … those rivers are going to continue to rise. And as of right now, that criteria doesn't exist, not even in the next seven-day forecast. 

You would see flow over Fremont Weir more frequently than you’d see the Sacramento Weir having to be opened. That's because [the Fremont Weir] is right at the confluence of two major rivers, the Feather and the Sacramento. 

So far, our flood system is acting exactly the way it was designed to act … and an indication of that is the fact that we are seeing some of the flows over the weirs and that the bypasses are being utilized. The way that these storms have come in so far has been very manageable from that perspective. 

But again, things can change dramatically based on how the weather systems approach and then affect California.