Floodplains and Floods Key idea: River floods are naturally occurring events that sometimes threaten human populations.

Learning Objectives: 1. Describe the features of a floodplain. 2. Explain why floods occur. 3. Summarize natural and artificial methods of flood prevention and control.

Features of a Floodplain The floodplain of a river is the valley that is covered with water when the river overflows its banks. The floodplain of a river is the valley that is covered with water when the river overflows its banks. As the river approaches the base level, its gradient and velocity decrease. The river current is more easily deflected sideward, and it course becomes winding. As the river approaches the base level, its gradient and velocity decrease. The river current is more easily deflected sideward, and it course becomes winding. The banks and the valley walls are slowly eroded by the river and valley floor widens into a floodplain. The banks and the valley walls are slowly eroded by the river and valley floor widens into a floodplain.

Floodplain

Floodplain

Meanders A river flowing through a floodplain typically forms broad curves, called meanders. A river flowing through a floodplain typically forms broad curves, called meanders. Meanders form because the erosion is most rapid on the outside of a river bend. In time, the channel is deepened there. Meanders form because the erosion is most rapid on the outside of a river bend. In time, the channel is deepened there. Swift currents send the water across the channel and downstream into the opposite bank. The process repeats itself, forming a series of meanders. Swift currents send the water across the channel and downstream into the opposite bank. The process repeats itself, forming a series of meanders.

How Meanders Form

Meanders Change Constantly Meandering rivers change all the time. As a meander swings wider and wider, it becomes a loop that the river can break through Meandering rivers change all the time. As a meander swings wider and wider, it becomes a loop that the river can break through The river usually breaks through during a flood, forming a cutoff. The river then deposits mud and silt at the ends of the abandoned meander. The river usually breaks through during a flood, forming a cutoff. The river then deposits mud and silt at the ends of the abandoned meander. A curved body of water formed when the deposits separate the meander from its river is an oxbow lake. A curved body of water formed when the deposits separate the meander from its river is an oxbow lake. During a flood the river carries large amounts of sediments and deposits them on the banks of the river, forming natural levees. During a flood the river carries large amounts of sediments and deposits them on the banks of the river, forming natural levees.

Meanders and Oxbow Lake

Floods Floods are naturally occurring events which temporarily relieve the water and sediment overload of a river channel. Floods are naturally occurring events which temporarily relieve the water and sediment overload of a river channel. Most floods occur due to long lasting rains, rapid melting of snows, or both. Most floods occur due to long lasting rains, rapid melting of snows, or both. Small rivers, with narrow floodplains can overflow after one heavy rain, creating a flash flood. Small rivers, with narrow floodplains can overflow after one heavy rain, creating a flash flood.

Floods Created by Large Rivers Large rivers such as the Mississippi, Ohio, Missouri, create floods after many days of steady rainfall over large parts of their vast drainage system. Also, floods occur in the spring time, when the ground is still frozen, increasing run-off, and snow thaws rapidly. Large rivers such as the Mississippi, Ohio, Missouri, create floods after many days of steady rainfall over large parts of their vast drainage system. Also, floods occur in the spring time, when the ground is still frozen, increasing run-off, and snow thaws rapidly.

Humans Activities and Floods Although most floods have natural causes, some human activities can worsen or even create floods. Such activities are: Building dams for reservoirs Building dams for reservoirs Urban development increases the runoff Urban development increases the runoff Removal of vegetation Removal of vegetation Agricultural activities Agricultural activities Modifications of the course of the river Modifications of the course of the river

Flood in Caracas Venezuela (Dec. 19 th 1999) killed over 10,000

Flood in Cedar Rapids, Iowa in June 2008

Flood Prevention and Control People rely on many different measures to control and prevent floods. Each of these measures has its own advantages and disadvantages. People rely on many different measures to control and prevent floods. Each of these measures has its own advantages and disadvantages. One good way to that people can lessen the impact of flooding is to restore natural flood protection that human activity has altered. One good way to that people can lessen the impact of flooding is to restore natural flood protection that human activity has altered.

Flood Prevention If vegetation has been removed, replanting helps to control runoff. If vegetation has been removed, replanting helps to control runoff. Dams are built to control floods, but in long run they affect the natural balance of the river, because they trap in all sediments. Without deposition, the erosion accelerates downstream, and this worsen floods. Dams severely alter the rivers ecosystems. Dams are built to control floods, but in long run they affect the natural balance of the river, because they trap in all sediments. Without deposition, the erosion accelerates downstream, and this worsen floods. Dams severely alter the rivers ecosystems.

Flood Control Dam

Spillways Spillways are channels that are parallel to the main course of a river. Spillways are channels that are parallel to the main course of a river. At critical points on the floodplain the water is guided into the spillways, and help control floods. At critical points on the floodplain the water is guided into the spillways, and help control floods. On the lower Mississippi River there are numerous channels running parallel to the main river through the back swamps that then flow into the Gulf of Mexico. On the lower Mississippi River there are numerous channels running parallel to the main river through the back swamps that then flow into the Gulf of Mexico.

Flood Control on Mississippi River