Crescent-shaped depressions on river floodplains, known as oxbow lakes or abandoned meander loops, are transient landscape features. These former sections of a meandering river channel are subject to continuous natural processes that ultimately lead to their complete disappearance. The infilling of these isolated basins occurs through a long-term sequence of both geological and biological depositional events. Over periods ranging from decades to millennia, these once open bodies of water are systematically transformed into waterlogged wetlands and finally into flat, fertile ground known as a meander scar.
How River Meanders Become Isolated Features
The process begins with rivers flowing across flat floodplains developing pronounced curves, or meanders, which migrate over time. Water velocity is highest on the outer bank of the curve, leading to erosion, while the slower water on the inner bank deposits sediment, causing the bend to grow more exaggerated. This continuous erosion and deposition causes meanders to migrate, often bringing two adjacent bends closer together.
As the meander bends grow, the narrow strip of land separating the two loops, called the meander neck, becomes progressively thinner. During a major flood event, the river’s high discharge can find a shorter, steeper path by breaching this neck, creating a new, straight channel. This sudden shift in the river’s main course is known as a neck cutoff, which effectively isolates the old meander loop.
Another mechanism for isolation is avulsion, the rapid abandonment of an existing river channel in favor of a new path across the floodplain. Whether through a neck cutoff or avulsion, the result is an abandoned channel segment, disconnected from the main flow of the river. The entrances to this newly formed oxbow lake are quickly sealed off by sediment plugs, typically composed of coarse sand and gravel, deposited as the floodwaters recede.
The Role of Sedimentation in Initial Filling
The initial and most rapid phase of infilling is dominated by inorganic sediment deposition, primarily delivered during subsequent flood events. Although the oxbow lake is cut off from the river’s main current, it remains hydrologically connected to the active floodplain. This connection allows floodwaters to spill over natural levees and enter the still-water environment of the abandoned loop.
As the sediment-laden floodwaters enter the quiet, low-energy environment of the oxbow lake, their velocity drops dramatically. This sudden decrease in current speed causes the suspended load—consisting mainly of fine-grained silts and clays—to rapidly settle out. The finest particles drift farthest into the center of the abandoned channel, contributing to the thick, soft, fine-grained deposits that characterize the bottom of a young oxbow lake.
These layers of fine-grained sediment, known as overbank deposits, accumulate episodically with each major flood event. The process is particularly efficient near the mouth of the abandoned channel, where the coarse sand and gravel plugs restrict flow, promoting rapid deposition. This geological infilling quickly reduces the lake’s depth, often burying the original channel-bottom deposits, which typically consist of coarser sand and gravel. Over time, these fine-grained sediments can reach a thickness of several meters, forming a distinctive clay plug that is often buried beneath the floodplain.
Ecological Succession and Organic Infilling
Once initial sedimentation has significantly reduced the depth of the oxbow lake, the process shifts from primarily geological to predominantly biological, marking the start of ecological succession. The newly shallow basin creates ideal conditions for rooted aquatic vegetation, first around the edges and then throughout the water body. This plant life includes reeds, rushes, and other water-loving species that colonize the muddy bottom.
The subsequent infilling is driven by the accumulation of organic matter from this growing plant community. When the aquatic and riparian plants die, their remains settle to the bottom of the lake, where decomposition is slowed by waterlogged, low-oxygen conditions. This slow decay leads to the build-up of peat, a dense, dark layer of partially decomposed matter.
The rate of organic infilling can accelerate as the lake transforms from open water into a marsh or swamp, which traps more sediment and supports denser growth of vegetation. This cycle of shallowing and increased plant growth eventually replaces the open water entirely, leaving behind a meander scar. The final result is a flat area of rich, organic-laden soil, sometimes called histosols, which preserves the crescent shape of the former river bend.