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Principal Causes of Floods

Floods can occur in any kind of weather or season. A floods occurs when an excess of water overflows onto land. This water can come from heavy rain or from water already in rivers or lakes blocked by a dam. Knowledge of flood preparedness can prevent the costs and danger of a flood from being too overwhelming.

  1. Causes of Floods in Cold Weather

    • One of the seasonal causes of floods is the weather phenomenon known as La Niña. La Niña, the presence of very cold water in the Pacific Ocean, can affect the current of water, which can lead to overflowing rivers and floods through greater precipitation in some areas. Other cold weather floods are caused by ice jams. Ice jams are caused when cold weather begins to warm up and winter snow cover and frozen ground start to melt. This causes the frozen ice to break off into chunks, which can float downstream and cause obstructions. These obstructions prevent lakes and streams from flowing, which leads to water overflow and flooding.

    Heavy Rain and Flash Floods

    • One of the primary causes of floods is the overflow of water due to heavy rainfall. Even a few inches of rain in already full streams can lead to overflow and flooding. Flooding rates can fluctuate according to the type of weather and the season. Often floods caused by heavy rainfall occur months after the rainfall. Heavy rain in the fall will fill a stream, followed by snowfall that melts and adds to the overflow of water. On the West Coast of the United States, rainfall from November to April increases flood risks. Flash floods occur during and after the increased rainfall due to thunderstorms.

    Floods After Fires

    • Wildfires can increase the risk of flooding in western states. Washington, Idaho, Arizona, Oregon and California are most at risk for this kind of flooding. Fires burn vegetation, ground and trees, which makes it hard for the ground to soak up rainwater. Flooding risks are highest in downstream areas of the state. Charred ground leads to mudflows that accumulate after heavy rain. Flood risks will decrease over time as the ground and trees heal from the wildfires.

    Levees and Dams

    • Levees and dams are created to prevent and protect places from floods, but heavy rain, hurricanes and storms can overwhelm dams or damage these barriers. These events lead to flooding rivers and overflowing water. Dams can be naturally made or man-made, but any dam can fail and lead to flooding. The collapse of an earth-made dam in Johnston, Pennsylvania, killed more 2,000 people and was caused by heavy amounts of rain in 1889. The devastation of Hurricane Katrina was amplified in New Orleans when flooding occurred after man-made levees meant to keep out flood waters cracked and broke.

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