An oxbow lake is a crescent-shaped body of water that forms when a wide curve in a river is separated from the main channel. The specific type of stream associated with this phenomenon is known as a meandering stream. These rivers do not flow in a straight line but instead follow a sinuous, winding course across a flat plain.
Characteristics of Meandering Streams
Meandering streams are characterized by high sinuosity. This pattern typically develops in the middle and lower sections of a river where the valley gradient is low and the stream flows across a wide, flat floodplain. The continuous shift in the channel’s position is driven by the interplay between erosion and deposition of sediment.
The river flow within a meander bend is asymmetrical, leading to two distinct features: the cut bank and the point bar. The outer bend, known as the cut bank, experiences the fastest water velocity and is the site of continuous erosion. Conversely, the inner bend, where the water slows down, is a site of deposition where sediment accumulates to form a gently sloping point bar.
The lateral migration causes the meander loops to grow larger and migrate both sideways and slightly downstream over time. This process is highly dependent on a high percentage of fine, cohesive sediment in the riverbanks and a low overall slope. As the loops become more pronounced, they set the stage for oxbow lake formation.
The Dynamic Process of Oxbow Lake Formation
The process of channel migration eventually brings the outer banks of two adjacent meander loops closer together. This creates a narrow strip of land, often called the meander neck. Ongoing erosion at the cut banks causes this neck of land to become thinner.
Oxbow lake formation occurs during a major flood event. The powerful force of the floodwaters is able to breach the narrow meander neck, cutting a new, straighter channel across the floodplain.
Once the new, shorter path is established, the majority of the river’s flow is diverted, leaving the long, curved meander loop abandoned. The river begins to deposit sediment rapidly at the entrance and exit points of the old loop. This deposition of alluvium effectively seals off the former meander from the main channel, creating the oxbow lake.
Evolution of the Oxbow Lake
The oxbow lake is a still body of water detached from the river’s flow. Without the constant flushing action of the main channel, the lake begins a process of infilling and ecological succession. Fine sediments and organic matter from surrounding vegetation begin to settle on the lakebed.
During subsequent flood events, floodwaters can still overflow the banks and deposit additional alluvial sediments into the lake. This continuous, slow accumulation of material causes the lake to become progressively shallower. As the water body shrinks and becomes richer in organic material, it transitions into a swamp or marsh environment.
The final stage is the complete filling of the depression with sediment and vegetation. Geographers refer to this visible geological trace on the floodplain as a meander scar.