For centuries, humans have relied on rivers for food, freshwater, and fertile land for growing crops. While water is essential to life, it can also be destructive. In many cases, the flooding of rivers can have terrible effects.
The Most Common Natural Disaster
Flooding is the most common natural disaster, and the results are often deadly. 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 aftereffects were terrible too. The floodwater carried deadly diseases, and many who survived the initial flood struggled to find food.
Floods can have big impacts on humans, but floods affect the environment too. The results are not always bad, however. In fact, some ecosystems even rely on flooding each season.
Drowning, Erosion, Sedimentation
Flooding can harm wildlife. Floods can spread disease, drown animals, and destroy habitats. In 2012, hundreds of animals were killed in floods in eastern India.
Floodwater can also change the landscape. It can cause riverbanks to collapse in a process called erosion. Floodwater carries material like dirt and mud from the riverbanks. Known as sediment, these particles can reduce the water quality and lead to overgrowth of harmful water plants.
The sediment eventually settles out of the water in places like the bottom of a river or stream. This process is called sedimentation. It can clog riverbeds and streams, kill water-dwelling organisms, and destroy habitats.
Floodwater can be contaminated with pollutants, such as garbage, sewage, chemicals, and pesticides. If contaminated floodwater enters the ocean it can negatively 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 disease. Deadly diseases transmitted through contaminated water, like hepatitis A and cholera, are more likely to spread after flood events. Malaria is another disease that can emerge following a flood. That is because mosquitoes, which transmit malaria, breed in pools of standing water, and such pools often remain after flooding.
Rich Soil, Groundwater Supplies
While floods can be devastating, they can also give new life to ecosystems. Floods carry important nutrients to the surrounding land. When the water dries up, it leaves sediment and nutrients behind on the ground. This rich, natural fertilizer improves soil quality and encourages plants to grow.
Ancient civilizations thrived along seasonally flooded rivers because they provided fertile soil for farmland. The ancient Egyptians, for example, relied on the seasonal floods of the Nile River.
Floods can refill underground water sources. Floodwater gets absorbed into the ground and 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.
During dry spells, this groundwater may be the only supply of freshwater available. A good supply of groundwater improves soil health. It also leads to more productive crop and pasture lands.
Animal Breeding and Migration
Floods can trigger some organisms to breed or migrate. In 2016, thousands of waterbirds flocked to the Macquarie Marshes in Australia. Flooding had filled their wetland habitat for the first time in years. This triggered a mass breeding, or mating, event.
In the Southeast Asian country of Cambodia, rains cause a yearly flood on the Mekong River. When the floodwater enters the Tonle Sap lake, it is a signal to the fish. They begin to migrate down the river.
Small seasonal floods can be good for fish. Floods deposit sediment on riverbeds, providing a place for baby fish to grow. Nutrients carried by floodwater can support aquatic food webs.
Many Animals Rely on Wetlands
Wetlands are extremely important ecosystems. These ecosystems include marshes and swamps. About 40 percent of the world's species rely on wetlands. 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, which causes a flood that replenishes the wetlands during the dry season.
Floods are a force of nature, and they have positive and negative consequences on the ecosystems they affect. Floods can be destructive to humans and the natural environment. However, they are also essential to many ecosystems.