Greater Flood Risk

What Causes Flooding?

Warming temperatures cause snows to melt earlier in the spring and allow storm systems to generate more rain because warm air holds more moisture than cold air. As a result, seasonal floods are expected to arrive earlier in the spring, and the risk of catastrophic floods is expected to increase. These increases have already been observed in the United States – recent decades have brought more heavy summer rainfall events along with increased likelihood of devastating floods. While no single storm or flood can be attributed directly to global warming, changing climate conditions are at least partly responsible for past trends and the increasing frequency of major flood events.

How Do Changing Water Regimes Create Greater Flood Risks?

Because spring is arriving earlier, snow packs are also melting earlier. This means that many streams and rivers are reaching their peak flows earlier in the season, which, coupled with increasing numbers of high flow days, can present greater flood risks. This, in turn, affects the availability of spawning habitats for some fish species. The earlier arrival of spring floods may also contribute to worsening drought by mid-summer, stressing vegetation and wildlife, and increasing the risk of wildfire events. Because rainfall is expected to come less frequently, and via more intense storms, floods are likely to become more common and extreme. Precipitation in the United States increased by 7% over the course of the past century, and is expected to increase even more in the coming decades. Most notable is that fact that the strongest 1% of rain storms increased by nearly 20% over the past decade.

Implications for Land Trusts

The consequences of earlier and more frequent floods include:

• Potential loss of important spawning or nursery habitat for certain fishes, amphibians and other water- or wetland-dependent species;
• Increased risk of drought, due to the earlier timing of spring floods and the longer duration of summer;
• Worsening erosion, and increased runoff of topsoil and agricultural chemicals, which negatively impacts water quality and can stress or even undermine aquatic ecosystems; and
• Loss of crops or livestock to floods, which may negatively impact working agricultural lands.

These system changes can significantly alter the existing composition of water-dependent ecosystems and the species they support.

In addition to ecosystem impacts, flooding negatively affects the built environment and communities throughout the United States. The Council of Environmental Quality reports that between 1980 and 2013, the United States suffered more than $260 billion in flood-related damages. Examples of recent impacts include record flooding from excessive rainfall in central and northern Illinois in April 2013 that damaged homes and businesses and caused an estimated $1 billion in losses. President Obama’s 2013 Climate Action Plan directs federal agencies to take appropriate actions to reduce investment risks, and specifically to update flood-risk reduction standards, resulting in FEMA’s introduction of a Federal Risk Management Standard. While flooding poses significant risks to people and property as well as flood-prone ecosystems, it can also present conservation opportunities.

 Tips for Planning: What Land Trusts Are Doing

Cooperatively Working Towards Conservation Goals.

Land trusts are responding to on-the-ground impacts of changing water regimes in various ways. Some groups such as the Elkhorn Slough Foundation are prioritizing acquisitions to restore hydrologic connectivity and “reclaim the floodplains”. Others are implementing adaptation projects such as planting trees to shade streams or increasing tree cover in cities to mitigate negative effects of increasing temperatures. In addition to employing strategic conservation planning to reduce risks and enhance resilience, some land trusts are also supporting efforts to mitigate greenhouse gas emissions to reduce the extent of future climate change.

Land trusts must determine the right planning approach for their organization, however, more and more, conservation organizations are working with their communities to identify opportunities to reduce vulnerabilities and prepare for changing temperatures. Agencies are making similar strides to implement projects that reduce risks and plan for resilience by incorporating adaptation, mitigation, and engagement into their strategic goals and objectives. For example, the U.S. Fish and Wildlife Service’s Strategic Plan for Responding to Accelerating Climate Change identifies seven planning and development goals to support sustainable landscapes. Conservation groups are increasingly partnering with agencies as well as other nonprofits and for-profit organizations to respond to this global challenge at local levels.

Building resilience for multiple management objectives.

Resilience describes the ability of a system to persist through extreme change. By working to identify and reduce potential threats land managers can build resilience and achieve multiple management objectives. Assessing vulnerabilities is a critical step in the strategic conservation planning process that helps land trusts identify key threats to resources. Adaptive management planning enables conservation practitioners to implement interventions to reduce vulnerabilities and to monitor and revise strategies as new information becomes available. There is no one-size-fits-all solution to addressing management challenges associated with warming average temperatures, however, planning that acknowledges vulnerabilities can help land trusts better achieve their conservation objectives.

Learn More

• Devastating Floods and Heavy Rains, NRDC
• Flood Risk Management Guidance, Council on Environmental Quality
• Increased Flooding Risk: Global Warming’s Wake-Up Call for Riverfront Communities, NWF
• Increasing Flood Risk Linked to Global Warming, Nature