Question

Describe how meanders form, develop, are cut off, and then are abandoned.

Short answer

Meanders form primarily due to erosion and deposition along river bends, caused by a difference in water velocity. As meanders develop, they widen and deepen due to increased erosion on outer banks and deposition on inner banks. Eventually, meander cut-offs occur when erosion causes a new, shorter channel to form between two bends, redirecting water flow and leaving the original meander loop isolated. The isolated loop gradually decreases in water flow, leading to the abandonment of the meander and the formation of an oxbow lake, which may eventually dry up and become part of the floodplain.

Step-by-step solution

1. Formation of Meanders

Meanders are sinuous, looping curves that occur along rivers or streams. They are formed primarily due to two main factors: erosion and deposition. River water flows faster on the outer bends than on the inner bends. This causes erosion to occur along the outer banks and deposition along the inner banks. The difference in water velocity between the bends is due to the centrifugal force acting on the water as it flows around the bends.

2. Development of Meanders

As meanders continue to evolve, the outer banks experience increased erosion and the inner banks continue to accumulate sediments through deposition. The force of the water flowing around the bends causes the meanders to become more pronounced and distinct. The increased erosion on the outer banks will cause the meanders to widen and deepen, further accentuating the curves.

3. Meander Cut-off

Over time, the meanders within a river system will continue to grow and change. At some point, the erosion along the outer banks of a meander may cause the river channel to become so narrow that the flow of water can create a new channel directly between the two bends. This process is known as a meander cut-off. Once this occurs, the flow of water within the river system will be redirected through the new, shorter channel, leaving the original meander loop isolated.

4. Abandonment of Meanders

After a meander has been cut-off from the main river channel, the water flow within the isolated loop will gradually decrease. Since water is no longer flowing through the meander, deposition will outpace erosion, eventually causing the meander to become shallow and filled with sediment. Over time, this process leads to the abandonment of the meander loop. The abandoned meander loop, now a semicircular or crescent-shaped body of water, is called an oxbow lake. Eventually, the oxbow lake may dry up, becoming a part of the floodplain adjacent to the river.

Key concepts

River Erosion and Deposition

Understanding the constant interplay between river erosion and deposition is essential when studying the dynamic nature of river landscapes. Imagine a river as a natural sculptor, constantly carving and shaping its path as water flows towards the sea.

Erosion primarily occurs on the outer edges of river bends where the water speed is greatest. Here, the centrifugal force exerted by the flow of water swings it against the bank, grinding away soil and rock in a process called lateral erosion. This action is most intense on the outer banks of meanders, leading to their outward expansion.

On the flip side, deposition acts as nature's construction phase. The inner banks of a meander experience slower water flow, which causes sediment to settle out of the water column and accumulate. This deposition creates gently sloping areas known as point bars.

The constant push and pull of erosion and deposition not only defines a river's meandering character but also paves the way for intricate habitats to thrive along the river's course. It's a natural balancing act where gain and loss of materials continuously reshape the river's journey.

Meander Cut-off

A meander cut-off is like a shortcut in the winding journey of a river. Over time, rivers do not follow the same path. The forces of erosion gradually eat away at the necks of large bends, and during periods of high flow, such as during floods, rivers may forge new, more direct paths.

In a dramatic reshaping event, the most eroded points of two outer banks within a meander can become so thin that it takes just a small nudge—a heavy rainfall or snowmelt, for instance— for the river to break through. This creates a new channel segment, leaving the old loop largely detached from the main flow of the river.

The inception of this cut-off is a striking example of how rivers constantly alter their courses, seeking the most efficient route downhill, a principle you could visualize as water’s relentless pursuit of the path of least resistance.

Oxbow Lake Formation

Following a meander cut-off, the abandoned river bend embarks on a slow transformation to become an oxbow lake—a serene U-shaped body of water that stands as a monument to the river’s former path.

Initially, the oxbow lake remains filled with water and can be quite deep. However, it's no longer receiving the same influx of fresh water and sediments. Thus, deposition becomes the prevailing force, while erosion takes a back seat. The reduced flow means that sediments carried by the water start settling at the bottom of the lake, gradually filling it in.

Plants also begin to colonize the still waters and edges, contributing organic matter that further accelerates the filling process. Over years or sometimes much quicker, the oxbow lake may become a swamp or a marsh, and eventually, it may dry up completely, becoming an oxbow scar on the landscape. The legacy of the meandering river is a mosaic of water bodies and fertile lands cradling a diversity of life, an ecological testament to the river's past.