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Floodplain management: opportunies, challenges, and successes


A group of UC Davis graduate students in the Responding to Rapid Environmental Change IGERT program studied the consequences river control projects along the Napa River in space, time, and among social groups. Floodplains have long been favored as sites for agriculture and settlement due to their fertility, flat topography, and proximity to rivers for easy transport of goods. They also provide ecosystem services and important habitat for fish, birds, insects, and other fauna. To reduce flood risk in settled floodplains, agencies and landowners have built extensive networks of levees and other flood protection structures. These modifications have greatly reduced the ecological benefits that intact river-floodplain systems provide. Water managers now recognize that reconnecting floodplains can restore some ecological benefits while further reducing flood risk. However, few of these promising multi-purpose projects have been implemented because agencies, scientists, and the public lack both practical methodologies for estimating benefits and an understanding of social conditions likely to bolster or prevent project success.

To better understand the benefits of and challenges to floodplain restoration, REACH IGERT students used historical and biophysical approaches to examine the trade-offs and synergies of floodplain reconnection. First, they examined a broader history of river modification in California, with a focus on hydraulic mining, which filled rivers with sediment and also redistributed mining wealth from gold-panners to those who could put up capital for large sluice systems. The students found a relationship between mining-based river modification and flood control in Napa. Much of the benefit of modification went to businesses established in the formerly flood-prone downtown, while housing became harder to come by for many of the city’s residents, and flood control displaced low-income homeowners. They also combined a hydrological model used by the US Army Corps of Engineers with species’ demographic models to examine how changes in flooding patterns might affect two important species – the Sacramento splittail and Fremont cottonwood. They found that, while the change in splittail habitat was slight, much of that change occurred due to bigger floods in wet years, and this had an overall positive (projected) effect on the splittail population. They found similar results with cottonwood, though both models need to be parameterized with local data in order to have more confidence in the predictions. In the end, the students found that considering both social and ecological concerns in restoration projects was extremely important, particularly because when managers focus too much on the biophysical, there can be grave social consequences. Crossing the disciplinary boundaries between the hard sciences and social sciences allowed the students to find ways to think about restoration projects that could benefit a broader socio-ecological system.

The students subsequently hosted a workshop that included staff from environmental organizations, state and federal agencies responsible for flood control, local governments, and other stakeholders. In addition to the REACH students’ research presentation, the agenda included invited speakers and a panel discussion. Geologist Jeff Mount from UC Davis introduced the day with an overview of the scale of changes in the Central Valley and the types of challenges that had to be overcome to produce functioning floodplains. Floodplains, he emphasized, need connectivity with rivers, variability in flows and other processes, and complexity in their physical and ecological structure. Then, Mount described challenges in floodplain management. Mount thought that the scientific and geotechnical challenges were relatively easy to solve. He was less optimistic about the institutional challenges. On the economic front, work on ecosystem services and valuation is still developing. Finally, Mount emphasized that we are in an unprecedented political environment, where willingness to pay for the types of projects, environmental or other, is at an all-time low.

Subsequent speakers included historian Phil Garone, who presented a long-term view of the human relationship with California’s central valley wetlands, and John Cain, from the non-profit organization American Rivers. Cain described the work his team has been doing on modeling benefits to salmon for the proposed Lower San Joaquin River Flood Bypass, with a focus on finding ways to use hydraulic models to assess how flood control projects change off-channel habitat that juvenile Chinook salmon use for rearing. The final invited speaker was Alicia Kirchner of the U.S. Army Corps of Engineers. Kirchner described a multi-purpose river restoration project with a case study of the Hamilton City Flood Damage Reduction and Ecosystem Restoration project. This was a project in a small town in California that couldn’t be justified on flood control terms alone, but could be when ecosystem services were added to the project goals. Kirchner also explained that the Army Corps of Engineers now has an explicit mandate from Congress to perform ecosystem restoration, but appropriation from Congress happens on a case-by-case basis, and restoration usually gets only very modest amounts of financial support.

The day concluded with a panel discussion that included representatives from state and local agencies about the on-the-ground work of river project planning in California. The main points of discussion included the balance of authority between state and local authorities, and the timing and cost of restoration projects, and subsequent responsibility for maintenance. The students are preparing two manuscripts for publication, and their video and poster received IGERT judges and community choice awards.

Address Goals

In California, flood control and floodplain restoration constitute active areas of research for academics and concerns for agency personnel. The REACH students made several novel contributions to this effort. First, in their modeling work, they linked the output from hydraulic models to demographic models used by ecologists to describe population growth. Their results therefore connect flow parameters to organismal growth. This approach may aid water resource managers in developing flood control projects that maximally benefit species that depend on floodplains for growth and survival. Second, the analysis of the environmental history of river modification yielded insights into the social and political conditions that may enhance the success of a particular project. Moreover, integrating considerations from both the hard sciences and the social sciences allowed the students to find ways to think about restoration projects that could benefit a broader socio-ecological system.