Issues in Science and Technology, Online A Publication of National Academy of Sciences National Academy of Engineering Institute of Medicine University of Texas at Dallas
  • Home
  • Back Issues
  • Search


Richard A. Haeuber

William K. Michener

Natural Flood Control

To control flooding, we need to work with the forces of nature, instead of simply trying to eliminate them.

Americans have always feared floods, and with good reason. Floods are the most common and costly large natural disturbances affecting the United States. Approximately 9 of every 10 presidential disaster declarations are associated with them. Floods took more than 200 lives between 1990 and 1995, and total flood damage costs between 1990 and 1997 reached nearly $34 billion. We have spent even more than that trying to control floods by building structures such as levees and dams to modify the ways rivers flow.

Studies of recent major U.S. floods demonstrate that flooding is critical for healthy ecosystems.

Although we understand all too well the damage floods do, we have not, until recently, understood very well the many beneficial aspects of flooding. Floods are critical for maintaining and restoring many of the important services provided to humans by riparian ecosystems. Among other things, flooding provides critical habitat for fish, waterfowl, and wildlife, and helps maintain high levels of plant and animal diversity. Floodwaters also replenish agricultural soils with nutrients and transport sediment that is necessary to maintain downstream delta and coastal areas. Indeed, recent attempts in the United States to restore riverine ecosystems have increasingly turned to "managed" floods-the manipulation of water flows from dams and other impoundments-to achieve the benefits of flooding.

As our understanding of floods has deepened, it has become apparent that floods present us with a paradox. On the one hand, we want to prevent them, because they threaten our lives and ways of life. On the other hand, we find ourselves searching for ways to allow or even reintroduce flooding, because it supports the biological infrastructure that makes valued aspects of our lives possible. Thus flood control per se cannot be effective over the long term. Rather, the key is a new, more informed kind of flood management, one that involves working with the forces of nature instead of simply trying to eliminate them.

Remaking the landscape

Recognizing the potential for catastrophe, the federal government has tried to control floods since the early 1800s. Policy has evolved as a series of scattered quick-fix solutions in response to unique events, usually with the single-minded aim of controlling water. For example, a major flood in 1850 in the lower Mississippi basin prompted an approach centering on levees-earthen embankments designed to keep water in check. Several decades of construction ensued, producing a levee system that extends from Cairo, Illinois, to the Mississippi delta.

When the levees proved unable to control the great floods of 1927, flood policy efforts were broadened. Under the Flood Control Act of 1928, the levee system was supplemented with structural measures such as reservoirs, channel improvements, and floodways, which divert spillover from the main channel. Also introduced were fuse-plug levees, which are built lower than the general levee system so as to siphon water out of the main channel at selected points. The Flood Control Acts of 1936 and 1938, which followed major floods from 1935 through 1937, continued support for these structural measures.

In 1968, the Federal Insurance Administration and the National Flood Insurance Program were created. These programs encouraged communities to explore nonstructural approaches to flood management, such as land use planning and flood-proofing of buildings, but little progress was made in implementing these measures. Flood control measures still dominated national policy.

But the focus of policy began to change dramatically in the wake of massive flooding in the upper Mississippi and lower Missouri river basins from June to August of 1993. Many of the engineering structures failed during this flooding. In response, President Clinton chartered an Interagency Floodplain Management Review Committee to investigate causes, explore how human actions might have exacerbated the situation, and determine what the nation should do to prevent a repeat event. Known as the Galloway Report, the committee's findings and recommendations marked the first time that the important ecological services provided by wetlands and upland forests, such as water and nutrient uptake and storage, were explicitly acknowledged. The committee pointed out that loss of these services through land conversion significantly increases runoff. Above all, the report recognized that large floods such as the 1993 event are natural recurring phenomena that must be accepted and anticipated.

The Galloway Report was a good first step toward major changes in national flood policy. The challenge now is to follow up on it with a comprehensive set of policies and programs grounded in our growing base of scientific knowledge.

Floodwaters and ecological health

Until recently, the environmental effects of major floods were rarely examined. Environmental scientists contributed sparingly to the development of water and floodplain management policy. But the extreme weather events of the past decade have provided opportunities to fill the gaps in our understanding. One of the most important conclusions drawn by the interdisciplinary teams of scientists who have studied them is that flooding is critical for healthy ecological systems.

The ongoing federal relicensing of dams in the U.S. provides an opportunity for creating flow regimes that closely mimic natural ones.

Flooding creates and nurtures far more diverse and complex habitats than exist when floodwaters are controlled. The production of new plant and animal tissue normally increases in response to flooding. Plants colonize new areas or take advantage of the increased light that becomes available when old vegetation is cleared away, and animals such as invertebrates and fish often find new food sources. Flooding not only leads to the dispersal and germination of plant seeds but it also results in different kinds of vegetation being able to survive in different locations. Research has found that major floods in coastal plain areas of the southeastern United States in 1994 and in the forested mountain landscapes of the Pacific Northwest in 1996 created a much more complex mosaic of habitats and biological diversity than had existed previously.

Floodwaters maintain a vital connection between rivers and the landscape through which they flow. Studies of the Illinois and upper Mississippi rivers have shown that some ecosystems depend heavily on yearly flooding cycles. Spring floods in these areas inundate wetlands, creating important spawning and nursery sites for numerous fish species. Lower summer water levels encourage the growth of wetland vegetation, which then provides food for migrating waterfowl as water levels rise with fall flooding. Supplementing the data gleaned from looking at the effects of natural floods are the results of experiments with managed flooding. For example, recent research on the middle Rio Grande indicates that consciously reintroducing floods can help restore ecosystems to a more natural state. After flood control was imposed several decades ago, native cottonwoods and willows began to decline, and nonnative species such as salt cedar and Russian olive, which favor environments that are not regularly disrupted by floodwaters, invaded. But the results of experimental flooding from mid-May to mid-June of 1993, 1994, and 1995 suggest that managed floods might help reestablish the native tree species. The experiments also triggered a wide range of other beneficial responses. Greater rates of plant respiration and decomposition reduced the tangle of wood debris that had accumulated over the years in the absence of floods, and microbial populations and specific arthropod populations, such as the native floodplain cricket, increased.

Another notable managed flooding experiment involved the Colorado River, where the Glen Canyon Dam has reduced the frequency, magnitude, and duration of floods for years. This altered important features of the river, such as the size and occurrence of sandbars and the types and composition of vegetation on the banks. The dam also interfered with conditions critical for native fish by changing water temperature and turbidity, decreasing inputs of organic material, and inhibiting migration. But a high-profile 1996 experiment involving a controlled release of water from the dam transformed a 425-kilometer reach of the river. It mobilized river sediments; reworked rapids and fans of debris; made sandbars one to two meters taller; and, through scouring, increased the size of backwater areas, providing important habitat for young fish. Populations of several nonnative fish species experienced significant declines. Riparian vegetation was minimally affected, as were two endangered species of concern to terrestrial biologists, the Kanab ambersnail and the southwestern willow flycatcher.

Still more evidence that managed flooding can be a valuable restoration tool comes from south Florida. Earlier in this century, the contiguous wetland system that originally extended from the headwaters of the Kissimmee River basin to the Everglades and Florida Bay was converted into a series of canals, levees, and other flood control structures. One project constructed between 1962 and 1971 eliminated 10,000 hectares of floodplain wetlands and seriously reduced use of the wetland system by fish, wading birds, and waterfowl. Fortunately, however, 1971 also marked the beginning of experiments with managed flooding, and by 1994 the ambitious Kissimmee River restoration plan was under way. The plan, which calls for reconstructing more than 100 square kilometers of river floodplain ecosystem, will eliminate flood control structures, including 35 continuous kilometers of canal, and reinstate historical hydrological regimes. A pilot project backfilled a 300-meter section of flood control channel and restored 5 hectares of floodplain, reestablishing communities of wetland vegetation and significantly increasing use by fish and avian species.

Lessons from these recent experiences with managed flooding should be applied as opportunities arise to review and modify operation of dams and other impoundments. Federal Energy Regulatory Commission (FERC) requirements will provide many such opportunities. FERC, an independent federal commission within the Department of Energy, issues licenses for hydropower facilities. These licenses last from 30 to 50 years, which means that many dams licensed in the 1950s or earlier are now under review or soon will be. In 1993, for example, 160 licenses affecting 262 dams on 105 rivers expired; only about 51 percent of those relicensing actions have been completed. An additional 550 more dams are due for relicensing in the next 15 years.

Because the relicensing process can entail establishing such parameters as minimum water flow levels, it allows regulators to step in and create flow regimes that more closely mimic natural ones. In some cases, FERC relicensing may well result in the complete removal of a dam, making large-scale habitat restoration possible. This is in fact what has happened with the 161-year-old Edwards Dam on Maine's Kennebec River. In other areas, such as stretches of Idaho's Snake River, restoration proponents propose removing some dams and breaching others. State and federal partnerships should be created to develop a comprehensive strategy for selecting sites, modifying dam operations, and assessing outcomes.

Renewing biological links

Whether or not the FERC relicensing process can be put to use in a particular case, the principle to keep in mind is that wherever physical and biological links between the floodplain and the main channel and backwaters have been severed, rehabilitation must focus on renewing them. After all, a common theme in the findings from natural and experimental flood episodes is the importance of connections between riverine ecosystems, their floodplains, and the broader landscapes through which they flow. It is worth noting as well that efforts to override the tendency of rivers to reclaim their floodplains rarely pay off in the long run. On the contrary, most of the $4 billion per year in flood costs over the past decade has been associated with damage to structures in the floodplain.

We have also learned that building in the floodplain increases runoff and harms wetlands, reducing their ability to hold water and retain nitrogen, phosphorus, and other contaminants. Floodplains are the sole natural defense against the harmful effects of floods. The aftermath of the 1994 flooding caused by tropical storm Alberto in Georgia has highlighted the important role they play in minimizing damage. When floodwaters eroded agricultural fields, picking up soil particles contaminated with herbicides, pesticides, and fungicides, the large, intact forested floodplains in the area filtered out those particles, so that they never entered streams and rivers where they would have harmed water quality and aquatic life.

Senate bill 2131 provides an important new opportunity to advance nonstructural floodplain management approaches.

Fortunately, although the mechanics of reestablishing floodplains are daunting, it is not an impossible task; their ecosystems are resilient, as studies on recurrent 1990s natural floods in the lower Missouri River have shown. Annual spring floods have long been controlled in this portion of the river, which stretches from Sioux City, Iowa, to St. Louis, Missouri. As a result, it was almost totally disconnected from its floodplain by 1930, and by 1990 state or federal agencies had classified 16 species of fish, 7 plants, 6 insects, 2 mussels, 4 reptiles, 14 birds, and 3 mammals within the floodplain complex as endangered, threatened, or rare. But the situation began to change with the great Midwest flood of 1993, which overtopped or breached more than 500 levees, and the record floods that followed in 1995 and 1996. Post-flood studies have demonstrated that plants, zooplankton, aquatic insects, fish, turtles, and other animals immediately exploited the reconnection with the floodplain.

Such findings suggest that essential components of river-floodplain structure and function could be restored just by reclaiming some features of floodplain habitat, such as remnant oxbows and backwaters, flood-scoured agricultural lands, and lowlands vulnerable to periodic flooding. Once the pieces of the restoration puzzle are assembled, natural forces could maintain them with little further human effort. In some cases, including areas around the Illinois and upper Mississippi rivers, existing flood control structures could be employed to manage water levels. For instance, one current approach compartmentalizes the floodplain with low levees and uses pumps and gates to create more natural flood regimes within the compartments.

Ensuring that the necessary changes in the landscape are actually made requires a good deal of thought, however. In particular, the fragmentation and lack of coordination that has characterized policy concerning floodplains in the United States must be addressed. Land use decisions that could affect floodplains are made primarily at the county and municipal levels, yet the current policy structure gives the federal government the most responsibility for floodplain management. It provides little incentive and few mechanisms for local governments to help. Nor do states have much opportunity to become involved, even though they may be best situated to coordinate the diverse actions and decisions that are needed.

Thus, one positive move would be to heed the Galloway Report's recommendation for a Floodplain Management Act. Such an act would establish a national model for floodplain management that would give states primary responsibility as floodplain managers, define responsibilities among other levels of government, and provide fiscal support for state and local floodplain management efforts.

The Galloway Report was also wise to advocate reactivating the Water Resources Council, which had been established by the Water Resources Planning Act of 1965 and dismantled by the Reagan administration in 1982. With representation from all federal water management agencies, the council served as a central clearinghouse, coordinating diverse components of floodplain management policy. Reactivating it or creating a similar body that would provide senior-level coordination of federal and federal-state-tribal water resource management is crucial if flood control policy is to become flood management policy.

Another important task is to ensure that the legislation and amendments aimed at reauthorizing the Clean Water Act explicitly address the shortcomings of Section 404. Last amended in 1977, Section 404 requires landowners to obtain permits from the Army Corps of Engineers if they wish to dispose of dredged or fill materials in waters and wetlands. However, Section 404 does not deal with the role of floods in wetland maintenance. Nor does it acknowledge the role of floodplains in flood protection. The regulation also ignores other activities that drain, flood, or reduce functional values of the areas it is intended to protect. In fact, major categories of potentially damaging activities are exempted by the 1977 amendments, including some that are routine in farming, ranching, and forestry.

Section 404 of the Clean Water Act is seriously deficient in dealing with the issues of flooding and flood protection.

Perhaps more significant, Section 404 gives the Corps authority to issue "general permits" exempting individual actions from environmental review if they fall within a general class. The most controversial general permit is Nationwide Permit 26 (NWP 26), which authorizes discharges of dredged or fill material into wetlands that are either "isolated" (that is, not adjacent to lakes or streams) or located above the headwaters of a river or stream with an average annual flow of five cubic feet per second or less. NWP 26 also automatically authorizes activities that fill less than one acre of wetlands.

Unfortunately, these features have allowed some applicants to abuse the permitting program by breaking projects into segments or combining NWP 26 with other nationwide permits, a process known as stacking. As a result, the program has been partially responsible for large amounts of unmonitored wetland losses. Currently, Section 404 addresses proposals to alter only about 40,000 acres of wetlands annually, an amount that represents about a third of the losses sustained in a year.

The practice of stacking multiple NWPs should be eliminated. Projects requiring more than one NWP for authorization should be scrutinized through the individual permitting process. Furthermore, NWP 26 should be replaced by a permit that restricts a broader range of activities and restricts them in all environmentally sensitive areas, including headwaters and isolated wetlands, which cumulatively can play a large role in water storage, flood mitigation, and other vital functions. Alternatively, NWP 26 could be continued in modified form until such activity-specific permits are developed. Responding to criticism, the Army Corps of Engineers has in fact developed plans to phase out NWP 26, continuing it in modified form until December of 1998, when it would be replaced by activity-specific permits. But the Corps has subsequently requested an extension until March of 1999, and in July of 1998 it published a proposal to modify NWP 26 and other general permits.

The recently proposed Water Resources Development Act (S. 2131) provides an important new opportunity to advance nonstructural floodplain management approaches. Introduced by Sen. John Chafee (R-R.I.) and cosponsored by Sen. Max Baucus (D-Mont.) and Sen. John Warner (R-Va.), the bill would authorize Army Corps of Engineers water resources projects and establish policy for financing them. Challenge 21, the centerpiece of the bill, mandates the use of nonstructural options in Corps projects. Projects undertaken within this program would center on restoring wetlands and relocating homes and businesses out of floodplains. They would also involve interagency cooperation among, for example, state agencies, tribal organizations, the Corps, the Environmental Protection Agency, the Federal Emergency Management Agency (FEMA), and agencies within the Departments of Agriculture and Interior.

Challenge 21 is part of an ambitious new Clinton administration effort, the Clean Water Initiative (CWI), which calls for $568 million in new funding within the president's FY99 budget and a total of $2.3 billion over five years. Intended to broaden and strengthen the Clean Water Act, the initiative sets out an "action plan" that aims to achieve a net increase of 100,000 wetland acres per year by 2005. However, this goal will be difficult to attain if wetland losses from NWP 26 development activities are not addressed as well. Also, the final form of the CWI depends largely on current budget negotiations and the actions of congressional appropriations committees, and many components of it are not faring well. Hence the effort requires continuing support.

One of the most important ideas in the CWI action plan is to strengthen buffer protection. Large floodplain ecosystems are often found on federal lands, such as national forests, Fish and Wildlife Service refuges, and national parks. However, much of the work involved in on-the-ground establishment of streamside buffers must occur locally, on private lands, and state and local governments must work with federal agencies in coordinating such efforts. Many programs and funding sources that can enable stream and riparian buffer protection are established at the federal level. Under the CWI action plan, for instance, the U.S. Department of Agriculture (USDA) would work with federal, state, tribal, and private organizations to establish 2 million miles of conservation buffers on agricultural lands. This could be achieved largely by strengthening existing USDA programs, such as the Conservation Reserve Program (CRP), the Environmental Quality Incentive Program, the Wetlands Reserve Program, the Forestry and Stewardship Incentives Program, and the Conservation Reserve Enhancement Program. Under the CRP, for example, nearly 30 million acres of highly erodible and environmentally sensitive lands have been taken out of agricultural production.

However, it is also important to reappraise policies and programs to ensure consistency. Georgia provides an interesting case in this regard. Introduced by Governor Zell Miller, the exemplary RiverCare 2000 program aims to conserve wetlands and enhance stream and river water quality by assessing important river resources, identifying more effective management tools for river corridors, and directly acquiring riverfront lands for protection, restoration, and public access. The program has already done a great deal to support stream and floodplain conservation throughout the state, and state legislation is pending to establish a Land, Water, Wildlife, and Recreation Heritage Fund that would greatly expand upon it, protecting thousands of additional acres of floodplain wetlands with revenue from an increase in the real estate transfer tax. By contrast, Georgia's efforts to protect streamside buffers have produced mixed results. Currently, only 25-foot buffer protection is afforded to Piedmont trout streams in the state. Coastal plain streams and their extensive floodplains are ignored, despite the role they play in reducing floods and safeguarding water quality and fisheries.

Finally, innovative programs that come into play in the aftermath of extreme floods can offer valuable opportunities to restore floodplains. For example, the Hazard Mitigation and Relocation Assistance Act of 1993 increased the amount of relief money targeted for voluntary buyout and relocation programs from $33 million to $125 million. According to FEMA, local governments have tapped into such funds for approximately 17,000 voluntary property buyouts in 36 states and one territory in the aftermath of the great Midwest flood of 1993. Using program money, FEMA identified 30 especially vulnerable communities and assisted them in acquiring or elevating 5,100 properties. The acquisition cost was approximately $66.3 million, or only about 35 percent of total claims from 1978 to 1995.

The 30-year-old National Flood Insurance Program has also helped, although it could be much more effective. One problem is that property owners in flood-prone communities have been reluctant to enroll, because as long as a disaster has been declared, the federal government provides assistance to rebuild homes anyway. Another problem is that premiums do not accurately reflect the amounts needed to cover losses. As a result, the program borrowed $810 million from the U.S. Treasury between August 1995 and January 1998. Claims by program participants have raised still more issues. For example, 40 percent of all insurance payments from 1978 to 1995 were for "repetitive loss properties," which had sustained $1,000 flood losses two or more times in 10 years. But of these properties, nearly 10 percent had cumulative flood insurance claims exceeding the value of the house. Clearly, then, the program needs scrutiny. At the very least, incentives for relocating homes and businesses out of flood-prone areas should be directly coupled to flood insurance rates and levels of future disaster assistance.

New questions for policymakers

The years from 1996 to 1998, a period of major floods throughout the country, have served to underscore the need for workable flood management strategies, as has the nagging awareness that approximately 150,000 square miles of U.S. land lie within the 100-year floodplain. These areas adjacent to rivers have a 1 percent chance of being flooded in any given year, and hence a 100 percent chance in any given century. However, recent experience demonstrates that 100-year floods can occur much more frequently. Thus the job ahead lies not in putting flood management on the agenda but in making certain that policymakers approach it in a constructive manner. We must reevaluate the problem, assess alternative approaches, and reframe the policy issues.

In particular, we must ask ourselves questions that are somewhat unfamiliar. How can we reconcile the need to safeguard human life and property with the need to maintain flooding as a means of supporting riparian ecosystems? How can we balance the need to reinstate aspects of natural flooding regimes with the need to retain the economic, social, and cultural benefits derived from the structures that disrupt such regimes?

The scientific community can help society address those questions by clarifying environmental issues and refining the policy discourse. It can educate managers, policymakers, and the public about the importance of the connections between river and stream channels and their floodplains. And scientists can also can help point the way forward by, for example, showing how managed flooding can be used to approximate the ecological benefits derived from flooding as a natural disturbance.

But in the end, policy decisions and the accompanying management actions are based on choices that require society to weigh competing values. In other words, they are political. Scientists alone will not and should not decide which options are appropriate. Rather, their contribution will be to spell out the implications of different policy options and management scenarios, which will include reminding society of the tradeoffs entailed in each.

Recommended reading

BioScience (March 1995 issue devoted to the ecology of large rivers; September 1998 issue devoted to natural and managed flooding).

M. Bowman, River Renewal: Restoring Rivers through Hydropower Dam Relicensing. Washington, D.C.: American Rivers, 1996.

Interagency Floodplain Management Review Committee (The Galloway Report), Sharing the Challenge: Floodplain Management in to the 21st Century. Washington, D.C.: U.S. Government Printing Office, 1994.

National Research Council, Restoration of Aquatic Ecosystems: Science, Technology, and Public Policy. Washington, D.C.: National Academy Press, 1992.

National Wildlife Federation, Higher Ground. Washington, D.C.: National Wildlife Federation,1998.


Richard A. Haeuber is associate director of the Sustainable Biosphere Initiative Project Office of the Ecological Society of America in Washington, D.C. William K. Michener is an associate scientist at the Joseph W. Jones Ecological Research Center in Newton, Georgia.