A number of different conditions can cause floods, but the overall reason why an area is flooded is because the amount of water exceeds the ability of the land to drain away that water, whether through natural seepage or by run-offs in the form of rivers. Most floods are caused when a greater than average amount of rain falls in a catchment area. Torrential thunderstorms can cause flash floods that rush down dried up river beds. Heavier than usual rain can cause rivers to overflow their banks when they are unable to carry off the excess water. In the case of a large river such as the Mississippi, flooding upstream can slowly and predictably move down with the flow of the river.
Somewhat predictable flooding can occur after a particularly bad winter that has brought heavy snowfall. When the big thaw comes around in springtime, especially if the thaw is a rapid one such as those occurring alongside global warming, excess water running off of mountains can be so voluminous that creeks, streams and rivers run over their banks, causing flooding. This type of flooding usually starts close to the mountain run-off areas, and can affect large areas downstream.
Coastal flooding is not as predictable as the previous two examples, and can sometimes come as a complete surprise. Hurricane Katrina threw a flood surge at the coastal areas near New Orleans in 2005, causing severe damage and hardship. When such a storm surge occurs coincidentally with a new or full moon, when tides are at their highest, the storm surge can be particularly devastating. The 9.0 magnitude Japanese earthquake of March 11, 2011 is an excellent and deadly example of flooding that was followed by a 10-meter-high tsunami that devastated the coastal plains and valleys of northeast Japan. Unlike flooding from a river, which is usually slow and creeping, a tsunami flood hits at 150 mph or more, sweeping away everything in its path.
When Mount St Helens erupted in 1980, part of the devastation occurred when the heat of the eruption melted not just the snow on the mountain, but also entire glaciers. These were carried with ash and debris from the volcano in the form of giant mudslides, blocking rivers and killing everything in their way. The resulting devastation and flooding spread as far as the Columbia River, 50 miles to the south of Mt St Helens. Steps have since been taken to prevent Spirit Lake on the mountain from being as unsafe by installing overflow tunnels.
