Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. Describe how meanders form, develop, are cut off, and then are abandoned. (D)

Short answer

Meanders form due to erosion and deposition caused by uneven water flow in a river channel, creating a series of curves. They develop as outer banks experience intense erosion while inner banks accumulate sediments. Over time, adjacent meanders may grow closer, resulting in a meander cutoff where water flows through a more direct route. Consequently, the original meander's velocity decreases, causing it to fill with sediments and eventually form an oxbow lake, abandoning the meander.

Step-by-step solution

Formation of Meanders

Meanders form as a result of the erosion and deposition of sediments that occur on the riverbanks due to the uneven flow of water in a river channel. When water flows rapidly at the outer bank, it erodes the bank and transports sediments. On the inner bank, where the flow of water is slower, sediments are deposited. This process causes the flow of the river to become increasingly sinuous, creating a series of curves or meanders.

Development of Meanders

As the meandering pattern of the river becomes more pronounced, the outer banks of the meanders are subject to more intense erosion, widening the meander loops over time. At the same time, the inner banks of each meander continue to accumulate sediments, causing the meanders to grow and develop with every successive erosion and deposition event.

Meander Cutoff

Over time, adjacent meanders can grow closer to each other as their loops widen, eventually causing their outer banks to merge. When water from the river finds a more direct route between these two meanders, it begins to flow through the newly formed channel, bypassing the original meander loop. This process is called a meander cutoff.

Abandonment of Meanders

After a meander cutoff has taken place, water continues to flow through the short, direct route between the two previously adjacent meanders, effectively forming a straightened river channel. As a result, the velocity of the water in the abandoned meander loop decreases significantly, leading to the deposition of sediments that eventually fill up the loop. Over time, the loop becomes completely cut off from the main river channel, forming what is known as an oxbow lake – a stagnant body of water that is separated from the river channel it once formed a part of. In conclusion, meanders form, develop, are cut off, and then are abandoned through a series of processes that involve the erosion and deposition of sediments on the riverbanks as water flows unevenly within the river channel. This eventually leads to the formation of a more direct route for the river flow and the abandonment of the original meander, which can result in an oxbow lake.

Key concepts

Erosion and Deposition

Erosion and deposition are fundamental processes in the formation of meanders. They work together to shape the landscape around rivers. Erosion occurs on the outer banks of a river where the water flows more rapidly. The strength and speed of the water remove materials like soil and rock, carving out the banks. This process can lead to the widening of the river
and the creation of deeper channels on the river's outer curves. On the flip side, deposition happens where the water slows down, typically on the inner banks. This is where sediments, carried by the river, settle or "deposit". Such deposits build up over time, creating gentle slopes or point bars. The interplay of these processes ensures that rivers do not remain in straight lines
but instead form bends and loops known as meanders. Over time, the cycle of erosion and deposition gives a river its characteristic winding shape.

River Channels

River channels are the pathways through which rivers flow. They play a pivotal role in the creation and evolution of meanders. As rivers move, they do not flow uniformly. Instead, variations in speed
and volume of water lead to differential erosion and deposition along the channel's course. This explains why rivers have bends and curves. A river channel can change its form over time due to these natural processes. When water moves faster, it has more energy to erode the bank it hits, digging into the landscape and altering the channel's shape. Areas where water moves slower allow for deposition,
frequently resulting in the buildup of sediments. The continuous alteration of river channels serves as a testament to the dynamic nature of rivers, showcasing how they adjust and reshape themselves over time. This adaptability is crucial for creating the well-known meandering pattern seen in many rivers.

Oxbow Lake

An oxbow lake is a fascinating consequence of the natural evolution of a meandering river. It forms when a significant meander loop is cut off from the main river flow. This occurs when two meander bends come so close that only a slender piece of land separates them. Eventually, the river breaches this divider, often during a storm or heavy flood, and takes a shortcut
instead of flowing around the lengthy loop. The once curved path becomes blocked with sediments, and the abandoned meander forms an isolated water body known as an oxbow lake.

• This lake is usually horseshoe or crescent-shaped, reflecting the original bend of the river.
• Over time, oxbow lakes can change, sometimes drying up or becoming marshy areas.
• They serve as unique habitats for various birds, amphibians, and plant species.

Oxbow lakes illustrate the ever-changing nature of rivers and the lasting impact meandering has on the landscape.