A Watershed Moment: Aysha Massell on Floodplains, Recharge, and Collaboration for the Future

As we chart a path for our joint effort to expand our Merced River Watershed study to four additional watersheds in the San Joaquin Valley, we checked in with Water for the Future Program Director Aysha Massell to learn more about why the project is so critical for California’s water and people.

Massell gave a broad overview of the watershed studies, and went into detail on exciting, small-scale floodplain recharge opportunities that could bolster our water supply, ecosystem health, and community flood security. We also talked about how floodplains really work, the unique geology and hydrology of the San Joaquin Valley, and what Massell’s looking forward to during this exciting work!

Why are the upcoming Flood-Managed Aquifer Recharge (Flood-MAR) watershed studies important, in your opinion?

Aysha Massell (AM): The scale of the groundwater overdraft problem in the valley is so big that we need to look at all options. The watershed studies incorporate aquifer recharge, flood reduction, reservoir reoperations, climate modeling, and ecosystem and community improvements. In my opinion, this type of integrative thinking is what is needed to prepare for the challenges ahead of us.

So, what does this watershed expansion mean in the larger context of California water management?

AM: It’s an evolving process, but we have a chance to integrate flood management and groundwater replenishment. The San Joaquin River system, from a bit north of Fresno to the Bay Delta, drains the Sierra Nevada foothills and is at high risk of catastrophic flooding. As our climate warms, there’s going to be less snowpack, and the San Joaquin mountains are a snow-dominated region. More of that precipitation is going to fall as rain, and it’ll come earlier in the season and in much bigger amounts in periodic flood events. We’re also preparing for droughts that are going to be episodic, longer, and more extreme. Floods and droughts can even occur in the same year because we can have one extreme storm event within an otherwise dry year.

Like the rain in December of last year.

AM: Yes, exactly. And we’re looking at these climate extremes that are just going to get more dire as we go. This watershed study combines some of the important points of sustainably managing our groundwater and trying to plan for and head off our major flood risk, and to do so in a way that everybody supports. So, the key is getting buy-in.

And for downstream communities, higher flows are a significant risk, right?

AM: Absolutely. When you look at how the flooding in 1997 impacted Stockton and Manteca, for example, that’s the region that likely will be most affected. We can’t plan for everything, but we can look at our system as a whole and prepare. With the Merced study, we looked at proof of concept in one watershed, and now we are extending it to four more watersheds.

We’re so focused on drought right now that we don’t always consider flood emergencies at the same time, but they are linked. When we get down into real high flood-prone areas, we don’t have a lot of land to work with. Stockton faces multiple stressors, like sea level rise. But maybe an upstream community has some land and dwindling groundwater levels. So, what can we do to reduce flood risk and improve groundwater levels, at the same time?

That’s the concept of Flood-Managed Aquifer Recharge (Flood-MAR.) The idea is to spread the flood water out on the land to reduce flood risk, percolate into soils, and recharge aquifers.

What about the fear that pulling water off rivers to use as recharge can harm the environment?

AM: This is a real concern, and many people express skepticism that Flood-MAR will be protective enough of riparian and aquatic ecosystems. There are a few safeguards embedded in the Merced watershed study that likely will be replicated in the upcoming studies. The “water available for recharge” only includes flows in excess of the 90th daily percentile, and with excess Delta conditions.

The study also utilizes elements of the Environmental Flows Framework, which characterizes rivers by key events in their annual life cycle, such as migration flows for fish, and determines thresholds to support ecosystems during these important times. Finally, the extreme flood flows due to climate change that we may face in the future are likely to be just as devastating for ecosystems as for communities. In other words, spreading water over the land in a planned fashion during these big flood events is probably a good thing for both people and nature.

We should also keep in mind that the San Joaquin River watershed once had a lot of floodplains in it. It regularly flooded and had droughts. It’s a land of extremes, and it was shaped by those extremes. We need to understand the hydrology, the geology, and how our groundwater got there in the first place if we want to get the water back into the ground.

Another way to say this is that our rivers aren’t just what’s in between levees. We’ve installed levees to manage our rivers so they minimize the extremes between floods and droughts. But in this process, we have disconnected the rivers from their natural floodplains.  They can’t expand and contract anymore and they can’t recharge water into the aquifer to the extent they once did. The rivers need room to spread out, slow down, and recharge aquifers and when they are constrained between levees those big, fast flood flows scour the bottoms of rivers which then become incised.

If the water level in the rivers drops too much, it can’t reach its natural floodplain anymore?

AM: Exactly. But what if we could bring back some of those floodplains, and allow them to recharge water once again? In the Merced study, we applied this idea by looking at areas next to small creeks, like Bear Creek that comes through Merced – which periodically floods Merced, by the way. We call it the flow through basin approach. These are essentially little floodplains or recharge basins on the sides of those smaller creeks.

Those banks are easier for floodwater to access because the creeks are much less incised than the big rivers, so when the creek floods it happens more frequently. The water can dip over into a floodplain, infiltrate for recharge, and shave the peak flow off the creek so there’s less flood risk downstream. And we could design these smaller basins and floodplains to be habitat areas so they serve more than one purpose. There are things we can do that are compatible with nearby farming, like planting pollinator habitat to attract Monarch butterflies or a riparian forest to provide habitat for song birds. EDF’s Building multibenefit recharge basins guide is a great resource for inspiration.

And these recharge basins aren’t the big floodplains, with all the infrastructure, that take months or years to plan and construct?

AM: That’s right. Floodplain projects on larger rivers are very worthwhile, but they cost a lot of money and take a lot of time. The idea that we’re trying to promote is a farmer transforming what could be a problem area for them – land that often floods or land they might not be able to irrigate with limited groundwater allocations under the Sustainable Groundwater Management Act (SGMA)– and installing a multibenefit flow-through basin. In addition to the ecosystem services and flood risk reduction we discussed, this could potentially be an economically worthwhile option via conservation, flood easements, and other funding programs that are just now getting started under SGMA.

Aysha Massell headshotThe scale of the groundwater overdraft problem in the valley is so big that we need to look at all options. The watershed studies incorporate aquifer recharge, flood reduction, reservoir reoperations, climate modeling, and ecosystem and community improvements. In my opinion, this type of integrative thinking is what is needed to prepare for the challenges ahead of us. – Aysha Massell

Let’s dig in and talk more about floodplains, because we tend to picture those large-scale projects, like the Yolo Bypass or giant swaths of flooded land. You said they can be small or large, right?

AM: Floodplains look different depending on where you are in a watershed. They are smaller up in the high meadows, various sizes in the tributary rivers, and biggest in the flat valley bottom. They’re often formed from what we call alluvial materials – sand, gravel, things like that – that sort out from coarser to finer as you move from the mountains to the Valley floor. The valley geology is complex, heterogenous, and built up layer by layer over millennia. In the Flood-MAR world, we’re looking for those veins of coarse and fast-recharging materials. You can put water there and you know it’ll go into the aquifer rather than sit on a clay layer.

The floodplains on the Valley floor tend to be made up of those finer materials that limit recharge speed, or clay or clogging layers, but they still might be a good place for a project.

Why would a floodplain with a clay layer or less suitable soils still be considered for recharge?

AM: There are probably a lot of different types of soil in these floodplains on the Valley floor, which may or may not be good recharge zones. But what we call “clogging layers” are not impervious unless they’re pure rock. They contribute to recharge. It’s just a matter of how much, and how fast. A site that has soil with lower percolation rates will tend to pool up water, which can be important habitat for birds and fish. In the Merced study, we actually targeted some fields with poor percolation for exactly this reason – to provide habitat for shorebirds during their migration through the valley.

Going deeper into some of the Valley floor geohydrology, we talk a lot about the Corcoran Clay layer and how on-farm recharge can be hard in some regions.

AM: The Corcoran Clay is the remnant of an ancient lakebed and extends 4,000 square miles, covering most of the San Joaquin Valley bottom. It ranges from 50-120 feet thick and is anywhere from 200–800 feet beneath the surface. With this fundamental knowledge we can think of our aquifers as two types: those that recharge above the Corcoran Clay, and those that recharge below. When planning for recharge, it is essential to understand the purpose of the recharge and the hydrogeologic context. Both of these will inform the project and determine its success.

What’s fascinating is that recharge on the eastern side of the Valley can slip under the clay layer and provide deep aquifer recharge. This means that communities in the foothills can engage in recharge projects that benefit communities way down in the valley.

Water will find the lowest point eventually.

AM: Exactly. It’s like any water anywhere. It just moves slower and has some impediments because of this complex geology. If you do recharge on top of the clay layer, it will sit on top and, if not pumped out, will eventually move through the system via rivers or evapotranspiration. In any project we should be asking ourselves what type of recharge we are intending, whether it’s shallow or deep, and if it’s for domestic, agricultural, or environmental purposes. For example, we could place a recharge site over the Corcoran Clay for the express purpose of flushing out legacy nitrates that are also pooling on top of the clay layer, or focus recharge projects further east, off the Corcoran Clay, to augment our deeper aquifers.

It’s tempting to think that groundwater released into rivers is a loss. But we know that the slow release of cool water into the rivers deep into the dry season is essential for salmon survival. It also benefits downstream water users and water quality. We’re getting all these algal blooms, especially in the Delta, and they’re really linked to temperature and excess nutrients.

So, looping back to the watershed study after we’ve covered the fascinating complexity and potential of floodplains and what makes the Valley so unique, what excites you most about working with these new watersheds? What are you looking forward to?

AM: During the Merced study, I had an amazing experience working with DWR staff and all their consultants. They’re such good problem-solvers, and they can be very powerful allies. The initial model results from the flow through basins showed impressive recharge potential and flood risk reductions, and I think through this process the engineers became more intrigued with ecosystem-based approaches. I’m looking forward to institutionalizing the idea of using natural infrastructure as a first-tier approach to solving our most intractable climate change problems. Ecosystems are often an afterthought when conducting these types of big engineering studies, but their inclusion might be the key to our success.

I’m also excited about the time we’ve built in for community engagement moving forward. We’re working in close collaboration with local irrigation districts, and we also want to include engagement from local non-profits who know the community and the on-the-ground realities that they’re facing. Those considerations need to inform our work. Just like ecosystems, community engagement can often be an afterthought, but I think it is one of our most powerful strategies to get projects done and with broad public support.

What’s Next?

This is our watershed moment. Join us!

We’ll dive into more details on the watershed study and hear from partners and community advocates on why recharge needs to be done with care in the Central Valley. We’ll also introduce you to our full Water Team and hear what they’re looking forward to as we expand the study. We’ll chat with a Sustainable Conservation supporter on why she gives to empower our water management work, and we’ll deliver some more in-depth looks at California’s water complexity, community engagement, and potential policy future.