In geology, meandering streams are commonly referred to as incised meanders. A meandering stream is a waterway that has many bends and turns over its course.
The term “meander” is derived from the Greek word for “to bend”. In an incised meander, the streamside cuts away from the main channel and makes a winding path back to it.
It’s believed that most of these cut-off pieces of stream are deposited in small lakes and wetlands within or adjacent to the meander.
The phenomenon of incision in a meandering channel is typically attributed to scouring during periods of high flow (overbank flows) along with sediment load (suspended load).
When meanders grow to the point where the bankfull channel width becomes comparable to the floodplain width, a meander cut-off will form (often as a result of bank failure).
The most common method of creating an incised meander is through overbank flow. The stream will erode its outside bank and deposit it within the floodplain, forming a cutoff channel.
This process results in even greater sinuosity and lengthening of the channel system. Since the floodplain is mostly flat, the newly formed meandering stream will extend much further downstream than the original incised channel.
While streams that cut back against the meander flow direction are called contra-karst streams (or “backwater streams”), their main feature is a rapid and steep downcutting.
Meandering stream sediments are often deposited in small lakes or wetlands adjacent to the lowland floodplain, often forming an incised meander.
During periods of high flow, overbank flows can scour away more material from this floodplain than was eroded along the incised channel.
What causes meandering streams to downcut and become incised meanders?
1. Bankfull width –
In this section, we will discuss what causes meandering streams to downcut and become incised meanders.
This part of the course is devoted to describing how downcutting occurs.
As discussed in the introduction, during times of high flow (bankfull conditions) where the meander cut-off stream reaches floodplain width it begins cutting back into the floodplain.
2. Scouring –
A second factor that may cause a meandering stream to grow large enough to form an incised channel is scour.
During periods of high flow, overbank flows can scour away more material from the floodplain than was eroded along the incised channel.
The moving water moves sediment downstream of the meander; so long as the floodplain is large, this will result in an extended stream.
However, for a meandering stream to erode its entire floodplain, two conditions must be met. First, the meander must grow large enough to cross the floodplain.
Second, and most importantly, it must erode its entire floodplain. In other words, all areas of the surrounding floodplains along both sides of the meander must be eroded.
3. Rafting –
Rafting is a process that often occurs in response to scouring conditions during periods of high flow (bankfull conditions).
As a meander continues to widen, the flow may undercut the outer bank and cause that material to fall into the floodplain.
This type of process will lead to a build-up in the middle of the meander and remove sediment from both ends.
Typically, this process forms an arc shape around a sharp turn in a meandering stream.
4. Overbank flow –
Overturning or overbank flows are perhaps one of the most prominent phenomena leading to downcutting in meandering streams.
These overbank flows are forces that move sediment away from the main channel and deposit them within the floodplain adjacent to it.
During periods of high flow (bankfull conditions), meanders may grow until the floodplain and main channel cross.
At this crossing, there is a rise in water level over the main channel and a drop to the floodplain. The difference in elevation leads to an undercut of the outer bank.
5. Bank failure –
While bankfull conditions are necessary for downcutting to occur, bank failure can also play a role by causing material from the banks to tumble into the floodplain.
This process will cause material previously deposited within the floodplain, but now separated from it, to tumble into either side of the meander cut-off stream where it has been deposited previously.
As this material is carried farther downstream, it will typically deposit in a pond or pool within the floodplain.
6. Overland flow –
Overland flows, also known as overbank flows or overland flooding, occur when stream water overflows the banks of a stream and erodes material from its outer sides along with depositing sediment within the floodplain.
This type of process can be caused by rainfall that causes erosion as well as snowmelt and other sources of surface runoff.
This process has been documented for meandering streams in Cambodia, Laos, Nepal and Vietnam where extreme rainfall events have caused widespread bank erosion.
