When the discharge of a stream becomes so great that it exceeds the capacity of its channel, it overflows its banks as a flood. Floods are among the most common and most destructive of all natural hazards. They are, nevertheless, simply part of the natural behavior of streams.

Rivers flood because of the weather. Rapid melting of snow and/or major storms that bring heavy rains over large areas cause most regional floods. In the spring of 2023, the upper reaches of the Mississippi River watershed experienced moderate to major flooding, with several river cities setting records for flooding. Unusually high precipitation, combined with water from the past winter’s abnormally deep and rapidly melting snowpack, caused the Mississippi River and many of its tributaries to swell to record levels by late April and early May of that year. In LaCrosse, Wisconsin, the river crested (reached its maximum flood level) at its highest since 2001. Like most other regional floods, these were associated with weather phenomena that could be forecasted with a good deal of accuracy. This allowed adequate time to warn and evacuate people who were in harm’s way. Throughout 2023, flooding disasters across the United States caused more than $195 billion in damages.

Flash floods often occur with little warning and are potentially deadly because they produce rapid rises in water levels and can have devastating flow velocities. Rainfall intensity and duration, surface conditions, and topography are among the factors that influence flash flooding. Mountainous areas are susceptible to flash floods because steep slopes can quickly funnel runoff into narrow canyons or streams. Urban areas are also susceptible to flash floods because a high percentage of the surface area is composed of impervious surfaces, such as roofs, streets, and parking lots, where runoff is very rapid. Flash flooding causes a high number of fatalities worldwide because its rapid appearance catch people off guard.

Flood Control

Several strategies have been devised to eliminate or reduce the catastrophic effects of floods. Engineering efforts include the construction of artificial levees, the building of flood-control dams, and river channelization.

Artificial Levees

Artificial levees are earthen mounds built on the banks of a river to increase the volume of water the channel can hold. These most common of stream-containment structures have been used since ancient times and continue to be used today. Artificial levees are usually easy to distinguish from natural levees because their slopes are much steeper. When exceptional floods threaten to overwhelm levees in densely populated areas, water is sometimes intentionally diverted from a river by creating openings in artificial levees. The purpose is to spare vulnerable urban areas by allowing water to flood sparsely populated rural areas. The areas that are intentionally flooded are called floodways (Figure 1).

Flood-Control Dams

Flood-control dams are built to store floodwater and then let it out slowly. This action lowers the flood crest by spreading it out over a longer time span. Since the 1920s, thousands of dams have been built on nearly every major river in the United States. Many dams have significant non-flood-related functions, such as providing water for irrigated agriculture and for hydroelectric power generation. Many reservoirs are also major regional recreational facilities.

Although dams may reduce flooding and provide other benefits, building these structures also has significant costs and consequences. For example, reservoirs created by dams may cover fertile farmland, useful forests, historic sites, and scenic valleys. Of course, dams trap sediment. Therefore, deltas and floodplains downstream erode because they are no longer replenished with silt during floods. Large dams can also cause significant ecological damage to river environments that took thousands of years to establish.

Building a dam is not a permanent solution to flooding. Sedimentation behind a dam causes the volume of its reservoir to gradually diminish, reducing the effectiveness of this flood-control measure. In the United States, over 2000 dams have been removed since 1912 to eliminate safety hazards and costs as well as to promote biodiversity. In 2023 alone,  dams were removed, including the start of the largest dam removal project to date on the Klamath River in California, where the removal of four dams is slated to be finished in 2024. Here, the removal will reopen salmon and steelhead trout habitats, which are culturally significant to local indigenous peoples.

Channelization

Channelization involves altering a stream channel in order to speed the flow of water and prevent it from reaching flood height. This may simply involve clearing a channel of obstructions or dredging a channel to make it wider and deeper.

Another alteration involves straightening a channel by creating artificial cutoffs. The idea is that by shortening the stream, the gradient and the flow velocity are both increased. By increasing velocity, the larger discharge associated with flooding can be dispersed more rapidly.

Since the early 1930s, the U.S. Army Corps of Engineers has created many artificial cutoffs on the Mississippi for the purpose of increasing the efficiency of the channel and reducing the threat of flooding. In all, the river has been shortened by more than 240 kilometers (150 miles). These efforts have been somewhat successful in reducing the height of the river in flood stage. However, channel shortening led to higher gradients and accelerated erosion of riverbank material, both of which necessitated further intervention. Following the creation of artificial cutoffs, massive riverbank protection to reduce erosion was installed along several stretches of the lower Mississippi.

A Nonstructural Approach

All of the flood-control measures described so far have involved structural solutions aimed at “controlling” a river. These solutions are expensive and often give people residing on the floodplain a false sense of security. A prime example is the failure of a dam or an artificial levee. These structures are built for flood protection and are designed to contain floods of a certain magnitude. If a dam or levee fails or is washed out, the water behind it is released and becomes a flash flood. The devastating 2023 floods in Derna, Libya, that killed more than 11,000 people were the result of two such dam failures, after flooding from a Mediterranean tropical storm overwhelmed the structures.

Today, many scientists and engineers advocate a nonstructural approach to flood control. They suggest that an alternative to artificial levees, dams, and channelization is sound floodplain management. Flood zone maps, available from the Federal Emergency Management Agency (FEMA), show areas at risk for flooding and help communities understand and mitigate their flood risk. By identifying high-risk areas, appropriate land use regulations can be implemented to minimize development and promote more appropriate land use. Further, accurate flood forecasting and warnings that estimate when floods are likely to occur, their magnitude, and duration helps residents and authorities take preventative measures to save lives.

• Floods are usually triggered by heavy rains and/or snowmelt. Sometimes human interference can worsen or even cause floods. Flood control measures include the building of artificial levees and dams. Channelization may involve creating artificial cutoffs. Many scientists and engineers advocate a nonstructural approach to flood control that involves more appropriate land use.

• flood: The overflow of a stream channel that occurs when discharge exceeds the channel’s capacity. Floods are the most common and destructive type of geologic hazard.