The Many Effects of Flooding

The Many Effects of Flooding

Floods can be destructive to humans and the natural environment, but they also help to drive biodiversity and are essential to the functioning of many ecosystems.


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Climatology, Earth Science, Geography, Physical Geography


1931 Yangtze River Flood

In 1931, water overwhelmed the banks of the Yangtze and Huai Rivers, resulting in the Central China flood. Killing at least hundreds of thousands and potentially millions of people, it was one of the worst flooding events in recorded history. Here, people near the Yangtze River are shown.

Photograph from Adrienne Livesey, Elaine Ryder, and Irene Brien
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It is hardly surprising that rivers have been an important part of human history: They provide food, freshwater, and fertile land for growing crops. While water is essential to life, it can be a destructive force too. When rivers flood, the effects can be catastrophic.

Flooding is one of the most common types of natural disaster, and the results are often fatal. The Central China flood of 1931, for example, was one of the worst flooding events in recorded history. The Yangtze and Huai Rivers broke their banks, killing as many as several million people. The aftermath was devastating; deadly waterborne diseases like dysentery and cholera spread quickly, and those who survived faced the threat of starvation.

The human cost of flooding can be large, but events like this have a big impact on the natural world too, and the effects are not always negative. In fact, some ecosystems rely on seasonal flooding to drive ecological processes.

Floods Can Harm Wildlife

Flooding can have a negative effect on wildlife, causing drowning, disease proliferation, and habitat destruction. In 2012, hundreds of animals, including many vulnerable one-horned rhinos (Rhinoceros unicornis), were killed in floods that swamped Kaziranga National Park in the Indian state of Assam. Unpredictable floods can be harmful even to aquatic life. For example, fish can be displaced and their nests destroyed.

Floods Cause Sedimentation and Erosion

Floodwater can also alter the landscape, for instance, by eroding riverbanks and causing them to collapse. As floodwater carries material from the eroded banks, it suspends sediment in the water, which can degrade water quality and lead to harmful blooms of algae. Suspended sediment eventually settles out of the water in a process called sedimentation, which can clog riverbeds and streams, smother aquatic organisms, and destroy habitats. Erosion and sedimentation have a more negative impact on ecosystems that are already degraded or heavily modified.

Floods Carry Contamination

Floodwater can be contaminated with pollutants such as agricultural pesticides, industrial chemicals, debris, and sewage. If contaminated floodwater enters the ocean it can affect water quality and disrupt delicate ecosystems, such as coral reefs. In February 2019, marine biologists feared for the safety of the Great Barrier Reef off the coast of Queensland, a state in Australia, after it was inundated with polluted floodwater.

Floods Spread Diseases

Floods are the leading cause of weather-related infectious disease outbreaks. Flooding events increase the chance of spreading waterborne diseases, such as hepatitis A and cholera. Receding floodwater can create stagnant pools of water, which provide the perfect breeding ground for mosquitoes, which can transmit malaria and other diseases. Flood events also lead to an increase in some forms of zoonosis, such as leptospirosis.

Floods Carry Nutrients

While floods bring hazards, they also bring nutrients and essential components for life. Seasonal floods can renew ecosystems, providing life-giving waters in more ways than one.

Floods transport vital nutrients, such as nitrogen, phosphorus, and organic material, to the surrounding land. When the water recedes, it leaves sediment and nutrients behind on the floodplain. This rich, natural fertilizer improves soil quality and has a positive effect on plant growth, thus increasing productivity in the ecosystem. Ancient civilizations first arose along the deltas of seasonally flooded rivers, such as the Nile in Egypt, because they provided fertile soil for farmland.

Floods Recharge Groundwater

Floods can replenish underground water sources. Floodwater gets absorbed into the ground then percolates through layers of soil and rock, eventually reaching underground aquifers. These aquifers supply clean freshwater to springs, wells, lakes, and rivers. Ecosystems rely heavily on groundwater during dry spells when it may be the only supply of freshwater available. A good supply of groundwater has a positive impact on soil health and leads to more productive crop and pasture lands.

Floods Can Trigger Breeding Events and Migrations

Floods can trigger breeding events, migrations, and dispersal in some species. In 2016, thousands of water birds flocked to the Macquarie Marshes in the Australian state of New South Wales. Flooding had filled their wetland habitat for the first time in years, triggering a mass breeding event.

In Cambodia, monsoon rains cause an annual flood pulse on the Mekong River that prompts migrations for some animals. The floodwaters cause the Tonle Sap river, which connects the Mekong River to Tonle Sap lake, to reverse its flow, filling the lake. When floodwater enters the lake, it triggers fish migrations, supporting one of the world’s most productive fisheries.

Floods Can Boost Fish Stocks

Small seasonal floods can be beneficial to native fish stocks and can help those fish outcompete invasive species that are not adapted to the river’s cycles. Sediment deposited on riverbeds during floods can provide a nursery site for small fish. Nutrients carried by floodwater can support aquatic food webs by boosting productivity.

Floods Bring Life to Wetlands

Wetlands are an extremely important ecosystem; approximately 40 percent of the world’s species rely on them. They filter water, mitigate flooding, and act as a carbon sink. The Okavango Delta in Botswana is a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site and one of the world’s largest, most important wetland habitats. The river captures rainfall from far to the north in the highlands of Angola. This causes a flood pulse that replenishes the wetlands at the height of the dry season, providing a lush oasis in the Kalahari Desert. National Geographic Explorer Steve Boyes, with a team of scientists and Explorers, has participated in a series of expeditions to trace the Okavango from source to sand to protect the waters of this unique habitat.

Floods are a force of nature, and their consequences, both positive and negative, are strongly felt by affected ecosystems. Floods can be destructive to humans and the natural environment, but they also help to drive biodiversity and are essential to the functioning of many ecosystems. Whether you regard floods as good or bad, one thing is for certain: The world would be a very different place without them.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Tyson Brown, National Geographic Society
National Geographic Society
Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
Clint Parks
Last Updated

September 8, 2022

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