Floodplains are hard-working landscapes when they’re allowed to “act naturally.” But their flood-taming, habitat-feeding abilities are compromised when they’re paved over or constricted by levees. We talked with Josh Viers, a watershed scientist at UC Merced and a member of the PPIC Water Policy Center research network, about restoring floodplains.
PPIC: What happens when rivers are given more room to flood?
Josh Viers: River systems are highly dynamic—they’re always changing in time and space. As they rush from their headwaters, they break down mountains and transport a lot of sediment. By the time they reach the lowlands, they’ve started to deposit that sediment. It’s that dynamic between water and land that creates floodplains. Humans like to inhabit floodplains because of their rich soil and flat land. So we’ve engineered ways to protect people from floodwaters. But ecosystems evolved to have functions and processes that depend on floods. For example, flood flows are slower and warmer on floodplains. That creates good conditions for a lot of creatures―bugs, fishes, and birds, for example. When we cut off rivers from their floodplain, we’re cutting off these essential habitat functions.
We now recognize that giving rivers room to flood can have many beneficial outcomes. Dynamic river systems that are allowed to flood result in more complex habitats and more productive ecosystems. There’s a growing effort to reconnect floodplains to their rivers —for example, by setting back or breaching sections of levees. The most immediate benefit is a reduction in flood hazards downstream because you store some floodwater and some of the sediment that constricts channels on the floodplain. There are also ecosystem benefits. As more nutrient-rich sediments are deposited on the floodplain, they’re colonized by riparian plants, feeding whole communities of insects, birds, and other animals. Floodplains are also incubators for native fishes. Fishes on floodplains get much fatter than those that remain in the river, which can translate into better reproductive success. Floodwaters in floodplains can also recharge local groundwater supply, and they facilitate the long-term sequestration of carbon by burying plant matter with sediments deposited on the floodplain.
PPIC: Where is floodplain restoration underway?
JV: I’ve been part of a team of scientists looking at floodplain restoration on the Lower Cosumnes River near Sacramento. This river has no major dam on it so it has a relatively natural flow regime, including relatively frequent floods. We’ve studied how its floodplain responds to reconnection with flood waters after its levees were set back. And we’ve documented that setting back levees and allowing rivers to flood have multiple benefits for habitat and groundwater recharge while maintaining flood protection. The lessons from the Cosumnes are informing similar efforts in other locations—the Sacramento, San Joaquin, and Feather rivers are having some levees set back in strategic locations.
One of the more remarkable recent floodplain restoration projects is located at the confluence of the San Joaquin and Tuolumne rivers. Federal and state agencies have worked with landowners and conservation organizations to integrate flood protection and habitat creation through levee enhancements and the acquisition of key floodplain lands. The results have not only benefitted fish and bird populations, but also provided room for the rivers to behave more naturally in a location that has historically had large river meanders and complex features such as oxbows and sloughs―habitats that are now rare in the San Joaquin Valley.
PPIC: What’s slowing floodplain restoration in California?
JV: There are a number of obstacles. A big goal for these projects is to maintain or improve flood hazard reduction. So permitting processes can require fairly sophisticated studies that take time and money. They also have to undergo the same environmental review as any other construction project. And financing is a big hurdle. A few decades ago, floodplain restoration projects were fairly simple. Today, projects are bigger and more sophisticated, so costs have gone up. Also, projects can be hampered by urbanization, unwilling landowners, or poor conditions on the ground. That said, there are thousands of miles of levees and hundreds of miles of rivers, and many more places where we could do floodplain restoration.
