There are a number of reasons for rivers flooding. Heavy rainfall, such as that in tropical regions during the monsoon season, will produce more water than a river is able to hold, making it burst its banks. A flood can also happen if a river is blocked by something, such as a landslide. Tidal rivers can flood when very high tides and strong winds combine to force more water upstream. In cold countries, melting snow has the same effect as heavy rainfall, while ice melting upstream before it does further downstream can cause ice blockages in the channel, resulting in a flood.
Any pulse of high water that overwhelms a river channel can create a flood, large or small. Common causes include heavy rainfall, including peak seasonal rains in tropical river systems such as the Amazon – the extensive annual flooding of which is a defining feature of this biggest river basin in the world – and more unpredictable torrential downpours caused by tropical cyclones making landfall and other storms.
In mid- and high-latitude rivers as well as lower-latitude rivers draining high, alpine mountains, seasonal snowmelt can also cause flooding due to large volumes of meltwater. Rapid meltoff due to a dramatic spike in temperatures or “rain-on-snow” events are particularly apt to cause rivers to overspill their banks.
Ice jams, where river current backs up behind accumulations of river ice, are another significant cause of flooding on higher-latitude rivers, mainly in the Northern Hemisphere. Major rivers most prone to large ice jams are those that flow north, because, during spring, their upper and middle courses may thaw out and run ice-free while their lower reaches are still icebound. This is the situation, for example, for the Lena River in Siberia, the Mackenzie River of northwestern Canada and the Red River of the U.S. Upper Midwest and Manitoba. Besides backing up waters behind them, ice jams can also produce river floods downstream if they are abruptly breached.
Human-caused (anthropogenic) changes to river basins around the world have profoundly affected the nature of flooding as well as other hydrological characteristics. Constructed levees are meant to confine floodwaters and protect floodplain communities, though they can also result in larger floods by backing up flows above their bottlenecks and by limiting the lateral spread of high-volume discharges, sometimes forcing water levels up enough that levees are overtopped. Failures of both levees and dams can also result in catastrophic flooding.
Riparian (riverside) and floodplain wetlands such as marshes, swamps and bottomland forests historically control flooding by slowing runoff and soaking up overflow. Where humans have removed such wetlands, destructive river floods may become more likely because water levels can rise more rapidly and because the landscape manipulation results in less suitable habitat to sponge up floodwaters.
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