The swollen Mississippi has already inundated more than 3 million acres in Arkansas, Tennessee and Mississippi, once again showing the power of this mighty river. Yet as bad as these floods are for property owners in their path, the damage could have been much worse if the federal government hadn't created designated floodways along the Mississippi's banks to serve as emergency relief valves.
Designated floodways are historic flood basins or floodplains, typically farmed, that are used only during the biggest of big floods. In Missouri, the U.S. Army Corps of Engineers used explosives to break a levee to inundate one of these designated floodways. By doing so, the Corps helped save the town of Cairo, Ill., from inundation, albeit with significant costs to farmers who till the normally "dry side" of the levee.
Here in the Sacramento Valley, we have yet to set up a system of designated emergency floodways. It is long past time to have that discussion. True, we do intentionally store and convey water on some parts of our floodplains, most specifically in the Yolo and Sutter bypasses, which work well in most floods. But these are inadequate for very large floods, the kind that come perhaps once in a generation or two.
For large floods, our relief valves are uncontrolled, ad hoc levee failures. In some cases, the failure of weak levees reduces pressures on other levees. However, in most cases, these unplanned levee failures cause random damage. They also provide little benefit elsewhere because their failure is not timed or located in ways that are helpful.
In the Central Valley, those in the floodplain operate under an old adage: You hope that the other guy's levee fails first. For political, economic and legal reasons, our institutions have been unable to move beyond this ad hoc system toward a new paradigm – designating areas suitable for emergency floodwater storage and bypass, and providing adequate compensation as an incentive. The historic legislative package of 2007 that guides current Central Valley flood planning failed to fully address this issue.
Our current flood-control system needs an overhaul. To the state's credit, there is ongoing planning. However, there is the ever-present temptation, if not pressure, to "fix" the system largely by beefing up our century-old levees. The Mississippi River floods, and our own recent history, show this is unlikely to be adequate.
The cost of building up levees is simply too large and the resulting benefits would be too low. The Department of Water Resources estimates that $20 billion to $25 billion is needed for modest improvements in the existing levee network just to bring it up to its original design standards, excluding the Sacramento-San Joaquin Delta. Even this investment would still rely on ad hoc levee failures to manage some floods. The volumes of water are too vast to squeeze reliably between the existing levees, even if strengthened.
Changes in reservoir operations, or new reservoirs, are also touted to hold floodwaters between our levees. While these may help, they do not solve the problem, particularly because of conflicting demands on reservoirs to serve many purposes.
In debates over the Delta, conflict over water supply and ecosystems in the Delta tends to command all the attention. Yet our flood-control system is connected to the fate of this estuary.
Although rarely discussed, the flood-control system upstream of the Delta has transformed the Central Valley, disconnecting the river from more than 90 percent of its wetlands and riparian forests. These floodplains were the source of food for the Delta and provided spawning and rearing habitat for its native fishes. Flood management infrastructure is a major stressor in this system that has multiple stressors. Bigger levees will do nothing to help this chronic problem.
Flood management in the Central Valley has no simple, elegant or cheap solution. As pointed out in our recent book "Managing California's Water: From Conflict to Reconciliation" (www.ppic.org ), we need to embrace a portfolio of approaches.
Diverting water from extreme floods into designated floodplains should be part of our plans. We need to identify areas that could serve as designated floodways, promote appropriate land uses, and most importantly, compensate those who accept waters on their land to spare others. These emergency flood storage areas would be kept in farming indefinitely through coordinated purchases of easements coupled with land-use zoning. When these lands are flooded, perhaps once or twice a century, those who benefit – mainly cities – would compensate those who farm.
Once land is designated, the temptation will be to depend too greatly on these emergency flood storage areas during floods. To minimize overuse of these areas, we would need to steadily increase the capacity of the existing network of floodways. But not by building up. Wherever possible, we should build out through levee setbacks, strategic levee breaches, and new or expanded bypasses.
This expansion of capacity improves overall flood performance, even during flood emergencies. It also improves water supply by increasing the flexibility of reservoir operations, especially when coupled with aggressive floodplain groundwater banking and recharge programs.
Building out, not up, expands the seasonal connection between rivers and floodplains. This benefits the ecosystem while reducing regulatory pressures on flood and water supply management. Those unfamiliar with the environmental miracle that occurs when rivers regularly flow onto floodplains should visit the Cosumnes River Preserve.
Finally, like the Mississippi River system, floods in the Central Valley seem to be increasing in frequency and magnitude, which is why we've seen many record floods in a short period of time. Whether this change is from global warming, long-term regional climate shifts or bad luck is irrelevant. The trajectory of change is real and, when combined with increasing numbers of people living in floodplains, catastrophic risk increases. A combination of larger floodways and emergency flood storage areas is a promising hedge against this dangerous trend.
Floods on the Mississippi remind us of our local vulnerabilities in the Central Valley. The Central Valley needs a long-term strategy that employs floodplains for storage and conveyance of floods, whether as emergency storage for extreme floods, or increased flood capacity for more frequent floods. That's our best lesson from the Mississippi, a python that is now attempting to swallow a pig.
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Jeffrey Mount is a geology professor at the University of California, Davis. Jay Lund is a civil and environmental engineering professor at UCD. Mount is the founding director of the Center for Watershed Sciences, and Lund is the director.
