An overbank is an

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...

deposit consisting of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

that has been deposited on the

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

of a river or stream by flood waters that have broken through or overtopped the banks. The sediment is carried in

suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in mathematics * Suspension (chemistry), small solid particles suspend ...

, and because it is carried outside of the main

channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austral ...

, away from faster flow, the sediment is typically fine-grained. An overbank deposit usually consists primarily of fine sand, silt and clay. Overbank deposits can be beneficial because they refresh valley soils. Overbank deposits can also be referred to as floodplain deposits. Examples include natural

levee A levee (), dike (American English), dyke (English in the Commonwealth of Nations, Commonwealth English), embankment, floodbank, or stop bank is a structure that is usually soil, earthen and that often runs parallel (geometry), parallel to ...

s and

crevasse splay A crevasse splay is a sedimentary fluvial deposit which forms when a stream breaks its natural or artificial levees and deposits sediment on a floodplain. A breach that forms a crevasse splay deposits sediments in similar pattern to an alluvial f ...

Geomorphology

Floodplains are far wider than the channel they border, reaching widths of up to 100 kilometers, and their length is 10 times that. They are thin and roughly planar in shape. Unlike channel bars, which often build horizontally, overbank deposits build vertically.

Depositional processes and facies

Overbank deposits are fine-grained and accumulate vertically. The disturbance of adjacent environments during flooding events leads to deposits containing terrestrial organic debris such as plant matter, and the intervening dry periods allow subaerial

bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...

by roots and burrowing animals. Notable sub-environments within the floodplain include natural levees and crevasse splays.

Natural levees

Natural levees are sloped deposits which form on the banks of channels during flooding events, serving as barriers to future floods. The slope of a levee is primarily a function of its grain size. Levees tend to be steeper when they first form and are close to the channel, then gradually level out as they grow and their grain size decreases. In the stratigraphic record, natural-levee deposits typically consist of thinly-layered sandstones overlying mud- to clay-sized beds.

Crevasse splays

Crevasse-splay deposits form during flooding events when a river cuts a levee to form a smaller channel away from the main channel. These crevasse channels are essentially miniature distributary systems and can have many of the features that larger fluvial bodies possess, like levees. A crevasse-splay sequence typically begins with an erosive base, followed by the deposition of coarse

bed load The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load. Bed load moves by rolling, sliding, and/or saltating (hopp ...

sediment and transitioning to finer suspended sediment as energy decreases, resulting a

graded bedding In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into pro ...

pattern when viewed in cross-section. Crevasse channels are ephemeral, and their deposits commonly show terrestrial or desiccation features near the top such as

mudcrack Mudcracks (also known as mud cracks, desiccation cracks or cracked mud) are sedimentary structures formed as muddy sediment Desiccation, dries and contracts.Jackson, J.A., 1997, ''Glossary of Geology'' (4th ed.), American Geological Institute, Ale ...

s or roots.

Relation to paleosols

Because overbank deposits often overlie areas that are normally exposed to

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...

, they can bury soils, allowing those soils to be preserved as

paleosol In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...

s. Paleosols can serve as bounds for overbank depositional sequences or alternate with overbank deposits where flooding is episodic. Paleosols tend to show more maturity at a greater distance from the channel, where there is less sediment flux. The degree of soil horizon development can be used as a proxy for this process.

Controls on depositional system evolution

When a river changes course ( avulsion), former floodplains can be stranded far from their former channel. They can be covered by new overbank deposits, cut by a channel, eroded completely, or converted into non-fluvial terrestrial deposits like soils. Overbank deposits are climate-dependent. Of course, the frequency of floods has a major impact on overbank deposits. The controls on flood frequency are complex, but rainfall frequency is a major contributing factor. In humid environments, crevasse channels may empty into long-standing lakes or marshes, whereas in arid environments any drainage areas can dry up between flooding events.

Tectonism Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

can also affect the fluvial system by altering

relative sea level Relative sea level (abbreviated as RSL) is defined as the sea level that is observed with respect to a land-based reference frame. It is often contrasted with eustatic sea level, which is a measure of the total mass or volume of the oceans. Relative ...

, exposing floodplains or covering new areas with overbank deposits.

References

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