Channel Incision: The Overlooked Cause of Riverbank Erosion

Channel incision, the downcutting of a riverbed, is a widespread but poorly understood cause of riverbank erosion in Quebec’s populated areas. Telling it apart from lateral river migration is essential, because conventional bank stabilization techniques perform poorly in incising channels and can even make the problem worse.

Two types of riverbank erosion that are easily confused

Riverbank erosion is one of the most common reasons clients seek advice in fluvial geomorphology. It can have several causes, each corresponding to a different type of erosion. The best known is lateral migration, or lateral mobility, of the river. This process is now built into Quebec’s Environment Quality Act (EQA) and into flood and mobility zone mapping, and it ties in with the concept of a watercourse’s freedom space.

Another type, erosion caused by channel incision, is less well understood. Yet it is very common and often more problematic, especially in rivers flowing through built-up areas. Because it is very difficult to recognize without a site visit, identifying it requires specialized expertise.

What is channel incision?

Channel incision occurs when a river’s bed is eroded/degraded. The banks then become too steep and collapse, causing bank erosion. The process stems from an imbalance between the quantity of sediment entering a river reach and the quantity of sediment leaving it. Two causes, often combined, are involved:

• A reduction of sediment supply: the river no longer receives enough sediment, either from upstream of from its banks, a condition sometimes known as a “sediment-starved” or “hungry” river.
• Increased flows: during flood peaks, the water carries away a larger share of the bed sediment.

Rivers function as a continuum, so upstream interventions strongly influence downstream conditions. For example, armouring a bank with riprap limits the amount of sediment passed downstream. During floods, bed sediment is mobilized but no longer replaced by the bank material that would otherwise have eroded. The riprap has succeeded in stabilizing the bank, but it has shifted the problem downstream. This is why analyzing fluvial transport dynamics is central to the diagnosis.

Why conventional bank stabilization fails

In an incising channel, conventional bank stabilization techniques perform poorly or not at all. Worse, they can aggravate the problem: structures tend to fail and must be rebuilt indefinitely. The problem has to be addressed at its source, with properly adapted riparian and in-stream works, to avoid recurring costs for watercourse managers.

Channel incision also has ecological consequences. It disconnects the river from its floodplain, directly affecting the natural habitats found there, reducing biodiversity and making the river less resilient to major floods.

One documented case shows the scale of the process: photos of the same house taken in 1944 and 2024 reveal up to 3 m (10 ft) of incision over 80 years. Works carried out in the river in response to bank erosion accelerated the incision. Structures had to be extended to other sections of the river and rebuilt several times, at significant cost to the community.

A fluvial geomorphology diagnosis for lasting solutions

Avizo’s expertise in fluvial geomorphology allows us to accurately diagnose watercourses and the root causes of their problems before any intervention. This integrated approach to watercourses avoids repeated interventions and the high costs that come with them. Avizo recently diagnosed and documented incision problems on several rivers, paving the way for solutions tailored to this specific issue.

Could you tell an incising channel apart from lateral bank migration? Are you confident that the experts you hired took this issue into account and that the proposed solutions will be suitable and durable?

Frequently asked questions