Throughout history, humans have depended on rivers to provide food, freshwater, and fertile land for growing crops. While water is essential to life, it can also be destructive, and the flooding of rivers can have terrible effects.
One of the Most Common Natural Disasters
Flooding is one of the most common types of natural disaster, and the results are often fatal. The Central China flood of 1931 was one of the worst flooding events in recorded history. The Yangtze and Huai Rivers broke their banks, killing up to several million people. The aftermath was devastating. The floodwater carried deadly diseases, and the people who survived the initial floods faced starvation.
The effects of flooding on humans can be severe, but events like this have a big impact on the environment too. The effects are not always negative, however, and some ecosystems even rely on flooding each season.
Drowning, Erosion, Sedimentation
Flooding can have a negative effect on wildlife, spreading disease, destroying habitats, and causing animals to drown. In 2012, hundreds of animals, including many vulnerable one-horned rhinos (Rhinoceros unicornis), were killed in floods in eastern India.
Floodwaters can also change the landscape. For example, they wear away riverbanks in a process called erosion and cause them to collapse. Floodwater carries material like dirt and mud from the riverbanks. Known as sediment, these particles can worsen water quality and lead to an overgrowth of water plants like algae.
The sediment eventually settles out of the water, in places such as the bottom of the river or stream. This process is called sedimentation, and it can clog riverbeds and streams, smother water-dwelling organisms, and destroy habitats.
Pollution, Disease
Floodwater can be contaminated with pollutants, such as garbage, sewage, chemicals, and pesticides. If contaminated floodwater enters the ocean it can affect the quality of the water and may also harm delicate ocean ecosystems such as coral reefs. When polluted water filled Australia's Great Barrier Reef in February 2019, scientists worried about these things.
Floods can lead to outbreaks of diseases. Flooding events increase the chance of spreading deadly diseases spread through contaminated water, such as hepatitis A and cholera. Malaria is another disease that can emerge following a flood. That is because mosquitoes, which can transmit malaria, breed in pools of standing water that are often left behind after a flood.
Rich Soil, Groundwater Supplies
While floods can be devastating, seasonal floods can also bring new life to ecosystems. That is because floods carry important nutrients to the surrounding land. These nutrients include nitrogen, phosphorus, and other organic material. When the water dries up, it leaves sediment and nutrients behind. 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 seasonally flooded rivers, such as the Nile in Egypt. The river provided fertile soil for farmland.
Floods can refill underground water sources. Floodwater gets absorbed into the ground and then drips through layers of soil and rock. Eventually, the water reaches underground bodies of water, called aquifers. These aquifers supply clean freshwater to springs, wells, lakes, and rivers.
Animal Breeding and Migration
Ecosystems rely heavily on underground water during dry spells. That's because it may be the only supply of freshwater available. A good supply of underground water, known as groundwater, can improve soil health and leads to more productive crop and pasture lands.
Floods can trigger some species to breed, migrate, or scatter. In 2016, thousands of waterbirds flocked to the Macquarie Marshes in Australia. Flooding had filled their wetland habitat for the first time in years, triggering a mass breeding event.
In the Southeast Asian country of Cambodia, rains cause a yearly flood 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.
Small seasonal floods can be beneficial to local fish. The floods can help them outcompete nonnative species for resources. In addition, sediment on riverbeds can provide a place for baby fish to grow, and nutrients in the water can support aquatic food webs by boosting productivity.
Many Animals Rely on Wetlands
About 40 percent of the world's species rely on wetlands, which are ecosystems that consist of marshes and swamps. The Okavango Delta in the southern African country of Botswana is one of the world's largest, most important wetland habitats. The river captures rainfall from faraway highlands of Angola to the north, which causes a flood that replenishes the wetlands during the dry season. It provides a lush oasis in the Kalahari Desert.
Floods are a force of nature, and they have positive and negative impacts on the ecosystems they affect. 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.
