By Jeffrey Mount

For more than a century, California has sought to separate floodplains from rivers.  An elaborate array of levees and dams usually confine, divert or capture winter floods, supporting agriculture on rich floodplain soils and unreliably protecting urban growth in flood-prone areas.

Nowhere is this approach more evident than the Central Valley.  One of the world’s most complex flood management systems—involving thousands of miles of levees, several bypasses, and a dozen large, multi-purpose dams—supports the Valley’s urban and agricultural economy.  This system, conceived more than a century ago, has proven inadequate to meet flood management needs today and in the future (Hanak et al., 2011).  In addition, this flood management system has severely degraded the quality of riverine and floodplain habitat, with a host of unintended economic and environmental consequences.

Recognizing that the Central Valley has major flood management problems, the Legislature called for an analysis and overhaul of the system in 2007.  One tool likely to be used to better manage floods involves storing and conveying water on floodplains, either through new flood bypasses or setting back levees.  This approach eases pressure on levees by lowering the elevation of floods (Mount, 1995).  It also allows floods to pass more slowly, reducing damage to bridges, levees and other infrastructure.

Although there are flood management benefits associated with floodplain reconnection, there are other underappreciated benefits as well.   These include:

Water Supply Improvements—Reconnected floodplains improve water supply in two ways: first, well-managed reconnection increases groundwater recharge, adding to the state’s largest reservoir of water (Fleckenstein et al., 2004); second, reconnection increases flexibility in reservoir management, allowing greater water supply storage (Opperman et al., 2009, 2010). Both of these aspects are essential for adapting to climate change.

Water Quality Improvements—Riparian forests and floodplain wetlands provide water treatment benefits, reducing sediment, nutrients and pesticides in river water, reducing the costs of water treatment for these constituents. However, wetlands can also release forms of carbon that are difficult to treat. (Naiman et al., 2005; Mitsch and Gosselink, 2007).

Air Quality Improvements—Riparian forests enhance air quality by trapping airborne particulates and by cooling summer air temperatures.  This has its greatest impact near urban areas (Smith, 1990; Nowak, 2006).

Recreation Benefits—Activities ranging from hunting, bird watching, ecotourism, hiking and more can occur on reconnected floodplains.  These activities significantly boost local recreation economies (Golet et al., 2006) and enhance property values when located near urban centers.

Agricultural Benefits—Reconnecting floodplains may reduce the total amount of agricultural land, but the associated flood management and water supply benefits will increase economic certainty for many other agricultural producers in the valley.  In some reconnection efforts, flood-compatible agriculture, involving annual crops like rice, corn and tomatoes, can be grown sustainably, taking advantage of seasonal flooding.

Ecosystem Benefits—All of the above provide direct economic benefit or reduced costs to communities on or near floodplains and rivers.  There are also broader environmental benefits from floodplain reconnection (Opperman et al., 2009).  The plant and animal communities that occupy floodplains, riparian corridors and rivers of the Central Valley are all dependent upon and adapted to seasonal flooding (Naiman et al., 2005).  Elimination of 95 percent of Central Valley riparian and floodplain wetland habitat, along with the channelization of rivers, is one of the prime causes of a decline in native biodiversity in the region (Lund et al., 2010).  This decline bedevils water supply and flood management today due to permitting issues.

Oaks benefit from Cosumnes River floodplains

Experiments conducted up and down the Central Valley show that riparian and wetland vegetation responds immediately to river reconnection (Golet et al., 2006).  Riparian plants in particular take advantage of periodic flood disturbances, along with higher soil moisture conditions (Florsheim and Mount, 2002; Trowbridge, 2007).  These disturbances create a patchwork of plant communities of different ages—ideal for supporting riparian insects, birds and other animals. This also promotes better competition with non-native, invasive plants.

The most surprising success story of reconnection comes from how it supports native fish (Jeffres et al., 2008; see blog on fat floodplain fish). As floodwater moves slowly across floodplains and through riparian forests, it warms, cycles nutrients, and produces abundant plankton, zooplankton and aquatic insects.  The juveniles of all four races of Chinook salmon in the Central Valley, including the endangered spring and winter Chinook runs, made historic use of floodplains.

Comparison of juvenile Chinook salmon raised on a restored Cosumnes River floodplain (right) and in the river’s main channel. Photo by Jeff Opperman, 2006

Finally, the water that flows off of these Central Valley floodplains is rich in plankton, coarse organic matter, and other sources of food for river and estuarine fishes and insects (Ahearn et al., 2006).  In this way, floodplains subsidize the productivity of rivers, promoting healthy and abundant fish populations.

River reconnection is a tool in modern flood management around the world.  While mostly focused on reducing flood risk by storing or conveying water on the floodplain, this tool creates broad, often unaccounted for economic and environmental benefits.  To be successful, the Central Valley needs to incorporate the benefits of reconnection into flood planning efforts.

Jeffrey Mount is a geology professor and founding director of the UC Davis Center for Watershed Sciences.

Further reading:

Ahearn, D.S., J.H. Viers, J.F. Mount and R. A. Dahlgren (2006), “Priming the productivity pump: flood pulse driven trends in suspended algal biomass distribution across a restored floodplain,” Freshwater Biology 51:1417-1433.

Fleckenstein, J., M. Anderson, G. Fogg and J. Mount (2004), “Managing surface water-groundwater to restore fall flows in the Cosumnes River,” Journal of Water Resources Planning and Management-ASCE 130:301-310.

Florsheim, J.L and J.F. Mount (2002), “Restoration of floodplain topography by sand-splay complex formation in response to intentional levee breaches, Lower Cosumnes River, California,” Geomorphology 44:67-94.

Golet, G.H., M.D. Roberts, E.W. Larsen, R.A. Luster, R. Unger, G. Werner and G.G. White (2006), “Assessing societal impacts when planning restoration of large alluvial rivers: A case study of the Sacramento River Project, California,”  Environmental Management 37, 862-879.

Hanak E, J. Lund, A. Dinar, B. Gray, R. Howitt, J. Mount, P. Moyle and B. Thompson (2011), Managing California’s Water: From Conflict to Reconciliation, San Francisco: Public Policy Institute of California.

Jeffres, C.A., J.J. Opperman and P.B. Moyle (2008), “Ephemeral floodplain habitats provide best growth conditions for juvenile Chinook salmon in a California river,” Environmental Biology of Fishes 83:449-458.

Lund, J.R., E. Hanak, W.E. Fleenor, J.F. Mount, R. Howitt, B. Bennett and P.B. Moyle (2010), Comparing Futures for the Sacramento-San Joaquin Delta.  Berkeley: University of California Press and Public Policy Institute of California.

Opperman, J.J., G.E. Galloway, J. Fargione, J.F. Mount, B.D. Richter and S. Secchi (2009), “Sustainable Floodplains Through Large-Scale Reconnection to Rivers,” Science 326:1487–1488.

Opperman, J.J., R. Luster, B.A. McKenney, M. Roberts and A.W. Meadows (2010), “Ecologically Functional Floodplains: Connectivity, Flow Regime, and Scale,” Journal American Water Resources Association 46: 211-226.

Mitsch, W.J. and J.G. Gosselink (2007), Wetlands, 4th ed. Wiley, Hoboken, NJ.

Mount, J.F. (1995), California Rivers and Streams: The Conflict between Fluvial Process and Land Use, Berkeley: University of California Press.

Naiman, R.J., N. Décamps and M.E. McClain (2005), Riparia: Ecology, Conservation, and Management of Streamside Communities. Elsevier/Academic Press.

Nowak, D.J., D.E. Crane and J.C. Stevens (2006), “Air pollution removal by urban trees and shrubs in the United States,” Urban Forestry and Urban Greening 4: 115-123.

Smith, W. H. (1990), Air pollution and forests, New York: Springer-Verlag.

Trowbridge, W. B. (2007), “The role of stochasticity and priority effects in floodplain restoration,” Ecological Applications 17:1312-1324.