Erosion Control

Reconnecting Floodplains

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It’s getting harder to predict the weather. As this New York Times article points out, “Researchers say it is unclear whether climate change will make California drier or wetter on average. What is more certain is that the state will increasingly whipsaw between extremes, with drier dry years, wetter wet ones, and a rising frequency of intense periods of precipitation.”

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Warmer temperatures mean more moisture in the atmosphere, and there will also likely be more precipitation falling as rain rather than snow. Rather than saving up that snowpack to slowly melt in the spring, we’re going to get more water earlier—and more flooding along with it. That includes both more frequent, smaller floods, and risks of major disasters like last year’s damage to Oroville Dam, which caused the evacuation of 200,000 people.

Although the state has a complex (and aging) water management and flood control network of dams and levees, many parts of it are likely to be overwhelmed. (Voters just approved another $300 million for floodplain projects.) Reservoir operators might have to release water during periods of intense precipitation, causing downstream floods.

So if flooding seems to be inevitable, the thing to do is to shore up the main tool for dealing with it: the floodplains themselves. The president of the group River Partners, John Carlon, notes that recently restored floodplains did their job during recent storms: “They just absorbed that floodwater beautifully—they acted like a shock absorber…. It was a big test for this concept and we’re really pleased with how it worked.”

The “concept” he’s referring to involves moving levees away from the rivers to give them some breathing room, as well as planting trees—lots and lots of trees. River Partners is taking farmland in the state’s Central Valley and returning it to a more natural condition, providing new habitat along the way. One project is known as Dos Rios, a 3-square-mile where the San Joaquin and Tuolumne Rivers meet. As many as 30 similar projects—some done by the state and some jointly with groups like River Partners—are underway.

The small dams and levees farmers had built over the years to control some of the flooding is no match for the larger floods that are expected to come, so those structures are being removed, reconnecting the rivers with their floodplains and allowing water to flow where it will be expected. Much of the restored floodplain has been purchased from farmers who, as the article notes, are tired of dealing with the flood damage. Once it enters the floodplain, the water can sit and infiltrate down to the aquifers, replenishing some of the region’s depleted groundwater.

The projects are returning the farmland to riparian woodland, planting species that are water-tolerant, such as cottonwood and black willow, as well as shrubs and grasses that can survive months of being waterlogged. It’s a carefully planned mix, designed to direct the flow of water: “Sometimes the goal is for the floodwater to slow down, as this can reduce downstream levels even more than floodplains alone. So in those areas…the mix will include species with relatively stiff stems or trunks that help impede the flow.” EC_bug_web

 

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Comments

  1. What a wonderful approach to looking at flood mitigation from the perspective of how the earth actually works and not trying to force it to people’s whims or ideas. Kudos to these projects and forward thinking land managers.

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