A floodplain is a flat expanse of land adjacent to a river channel that is built by the river itself and is periodically inundated by floodwaters. This landscape acts as a temporary storage area for water and sediment during high-flow events. Floodplains are agriculturally significant because the deposited sediment creates deep, nutrient-rich soils that support productive ecosystems. Understanding how these areas are constructed requires looking at the conditions that allow a river to transform its surrounding terrain.
Prerequisites The Dynamic River System
Floodplains form when a river transitions from a steep-gradient environment to a lower-energy, gentler-sloped environment. This reduction in the channel’s slope causes the river’s flow velocity and stream power to decrease significantly. In high-gradient sections, the river primarily cuts downward, deepening its valley, but in low-gradient sections, the energy is redirected toward lateral movement and deposition.
The river carries a significant load of sediment, known as alluvium, which includes everything from large pebbles to fine silts. When the energy of the flow drops below a certain threshold, the river can no longer transport all of this material. The system shifts from being primarily erosional to predominantly depositional, setting the stage for floodplain construction.
This depositional phase means the river begins to drop its sediment load across the surrounding landscape during high-water events. This balance between the river’s transport capacity and its sediment load is the requirement that permits the river to build the flat expanse of the floodplain over geologic time.
Horizontal Growth Through Lateral Accretion
The primary mechanism for creating the wide, flat floor of the floodplain is lateral accretion, driven by the river’s tendency to form meanders. A meander is a bend that continuously shifts its position across the valley floor, eroding one bank while simultaneously building up the opposite bank. This process is responsible for the vast width of mature floodplains.
In a meander bend, water velocity is highest on the outer curve due to centrifugal force, leading to intense erosion that undercuts the bank, creating the cut bank. Conversely, the water slows down on the inner curve, causing sediment to be dropped and forming a gently sloping accumulation of alluvium called a point bar.
Through the simultaneous erosion of the cut bank and deposition on the point bar, the entire meander loop slowly migrates sideways across the landscape. This continuous lateral shifting carves out the broad, flat valley floor, which is composed entirely of layered alluvial deposits.
The accumulated deposits from this lateral migration are responsible for the majority of the sand and gravel found beneath the floodplain surface. This process effectively widens the river’s corridor, defining the maximum extent of the future floodplain and creating the foundational layer.
Vertical Growth Through Overbank Sedimentation
Once the valley floor is established by lateral migration, the floodplain builds vertically through overbank sedimentation, or vertical accretion. This mechanism relies on periodic flood events that cause the river to spill out of its normal channel onto the surrounding land. Vertical accretion adds height to the floodplain surface, counteracting compaction and subsidence.
When floodwaters leave the main channel and spread out over the floodplain, their flow velocity drops dramatically. This sudden loss of energy forces the water to drop the suspended sediment it was carrying, especially fine particles like silts and clays. Only the finest materials are carried far from the main channel during these events.
These fine materials are deposited in thin, successive layers across the entire flooded area. Over centuries, the accumulation of flood layers slowly raises the overall elevation of the floodplain surface. This process ensures the floodplain maintains its low elevation relative to the river channel, allowing future floods to continue the deposition cycle.
The sediments deposited vertically are much finer than the sands and gravels deposited laterally. This fine-grained material gives floodplain soils their characteristic fertility and layered structure.
Characteristic Features of a Mature Floodplain
The combined actions of lateral migration and vertical accretion create several distinctive landforms that characterize a mature floodplain. Immediately adjacent to the main river channel are natural levees, which are subtle, elongated ridges of slightly coarser sediment.
These levees form because floodwaters lose their velocity most rapidly right at the bank edge upon leaving the channel, causing the heaviest suspended sediment to be dropped first. Because of their height, levees are the least frequently flooded parts of the plain and act as a barrier to subsequent floodwaters.
Further away from the active channel, where only the finest silts and clays are carried, lie the low-lying backswamps. These areas are poorly drained and often remain saturated because they are the lowest points on the floodplain, accumulating fine-grained material and organic matter.
Another common feature is the oxbow lake, which marks a former path of the river. An oxbow forms when a meander loop becomes so exaggerated that the river cuts across the narrow neck of land to find a shorter path. This action abandons the old loop, leaving a crescent-shaped body of water isolated from the main flow.