What is a flood?
Floods are high water levels above the banks of a stream channel, lake shore, or ocean coast that submerge areas of land that are usually dry. They are natural, reoccurring events in every stream, lake, and coastal environment. The duration of floods ranges from minutes to months. A flood can be caused by intense or prolonged precipitation from thunderstorms, storm surges during a hurricane, melting snow, or even dam collapses. Flooding is controlled by soil conditions as much as it is by how much precipitation is received; moderate rainfall can produce flooding in already saturated soils, while heavy rainfall may produce no flooding if the soil is relatively dry.

Since most of the world's population lives near coasts and on floodplains, floods are a hazard for billions of people. Indeed, more deaths are caused by flooding in most years than by any other natural hazard. In the United States, more than half of all flood victims die in their automobiles. Floods can cause loss of life by drowning or disease, while also causing extensive damage to property, electricity and communication outages, contamination of drinking water, and destruction of crops and fields. They can also trigger other hazards such as slope failures and even fires. A single flood event can submerge an entire city and require many years of rebuilding. In extreme cases, entire counties and large portions of states can experience flood conditions for many weeks.

Where do floods occur?
Floods occur in dry and wet climates, highlands and lowlands, and urban and rural areas. Although they are usually less common in dry climates and highlands, rain falls or snow melts nearly everywhere on Earth leaving few places completely safe. In dry environments, especially those situated at the base of mountain ranges, flash flooding can occur. This extremely dangerous type of flooding results from intense rainfall in a short period of time and moves downslope at high speed. Occasionally, flash flooding events are caused by spring rain falling onto winter snowpack in mountainous areas. The warmer raindrops rapidly melt the snow while making their own contribution to the mountain streams. This situation has resulted in several large floods on the Colorado Front Range. Coastal flooding is usually caused by storm surges associated with hurricanes, or subsidence of land near the coast.

Urban areas are at high risk of flooding because of how they are constructed. Roadways, sidewalks, and buildings are not permeable, so all of the rain that falls accumulates on the surface. Combined with a high density population, this can quickly create a hazardous situation. In the United States, urban flooding has cost billions of dollars in damage in just a few years. In contrast, rural areas have permeable soil, fewer roads, and more vegetation to intercept precipitation. When floods do occur in rural areas, loss of life is usually very low because fewer people live in these areas. However, the economic costs of rural flooding can still be high due to agricultural losses.

In general, the areas at highest risk of flooding are those within the 100 year floodplain. Floodwaters submerge the 100 year floodplain once every one hundred years on average. However, it is possible to have several 100 year floods within a single year. This basic rule of statistics needs to be considered in planning if structures are placed within or near the 100 year floodplain.

When do floods occur?
In mid-latitude regions around the world, flooding is common in the Spring when winter snow melts and thunderstorms bring intense rain. Summer months can bring ample surface heating that generates thunderstorms and the potential for torrential rain. However, floods can occur at any time of the year, depending on location. The timing of floods is largely dependent on climate, especially seasonal weather patterns. In some areas, such as monsoonal India, floods can be relatively predictable events. In most areas, especially those affected by mid-latitude cyclones, floods can be difficult to predict more than 24 hours in advance.

How do we cope with floods?
The United States Geological Survey (USGS) maintains an extensive system of stream gauges throughout the United States to monitor stream flooding. Although floods are geologic hazards, flood watches and warnings are issued by the U.S. National Weather Service when unusually intense or prolonged periods of precipitation are expected. Traditionally, floods have been viewed as extreme acts of Nature that could be combated by building structures such as dams, levees, and bank reinforcements to contain, redirect, and otherwise control floodwaters. In 1968, the United States Congress developed the National Flood Insurance Program. This program came in response to the billions of dollars of destruction caused by floods even when protection structures such as dams, levees, and bank reinforcements were in place. In essence, this policy directs development away from flood-prone areas (floodplains). Today, protection structures are still used to mitigate floods throughout the United States, but development within floodplains is limited. Simply stated, the best form of flood mitigation is to not build in the floodplain. Natural vegetation, especially wetlands, is usually a better flood mitigation solution than any structure created by people.