Avulsion (river)

Rivers carry dirt to the sea.

This dirt builds new land. The river path gets longer and flatter. This makes the river unstable. Sometimes the river breaks its banks. It finds a new, shorter way to flow. This is called a river switch. It can move all the water. It can also move just some water. The new path carries more dirt. This builds even more land.

Rivers are always changing.

Rivers are always moving. Sometimes, a river finds a new path. This is called an avulsion. An avulsion is when water moves from an old path to a new one. This can happen in a few ways.

One way happens at a river delta. A delta is land built by dirt at the edge of the sea. As the river carries dirt, the path gets longer. This makes the slope very flat. A flat slope is hard for water to flow on. The river bed also fills with dirt. This makes the river bed higher than the land around it. During a flood, the water can break through its banks. It finds a shorter, steeper way to the sea. This builds a new part of the delta.

Another way is through erosion. This is when moving water wears away the land. Heavy rain can cause this. A river might cut a new, straight path through the ground. This can happen very fast. In 2006, the Suncook River cut a new path. It moved at 25 to 50 meters every hour! Some avulsions move all the water. Others only move a part of the flow.

[Caption: Dirt from the river builds the Mississippi delta.]

Rivers are always moving across the land. Sometimes, a river decides to change its entire path. This big change is called an avulsion. An avulsion happens when water leaves its old channel. It moves into a brand new course instead. This can happen in a full way where all water moves. It can also be a partial way where only some water moves. This process helps shape how the Earth looks over a long time.

One way this works is at a river delta. A delta is land built where a river meets the sea. Rivers carry sediment, which is dirt and sand, to the ocean. This sediment builds a shape called a deltaic lobe. As the lobe grows, the river's path gets much longer. This makes the slope of the river very flat. Water likes to flow down steep slopes. A flat slope makes the river unstable. The river bed might also fill with sediment. This makes the bed higher than the land around it. During a flood, the water can break through its banks. It finds a shorter and steeper path to the sea.

Scientists have studied how these paths change. They used to think only the slope mattered. They thought rivers only moved to find a steeper path. Now we know many other things matter too. Things like wood or beaver dams can cause an avulsion. The shape of the river channel also plays a part. Some changes happen because of how the river is built. These studies help us understand how land is made. We can see these changes in the layers of the Earth.

There are also erosional avulsions. This happens when moving water wears away the ground. Heavy rains can give a river the power to do this. In 2006, the Suncook River in New Hampshire did this. The water rose behind an old mill dam. It cut a new, shorter channel very quickly. It moved at 25 to 50 meters every hour! Another example is the Cheslatta River in British Columbia. In the 1950s, it became a spillway for a reservoir. Large spills caused the river to move in 1961 and 1972. This created a large pile of sediment called the Cheslatta Fan.

An avulsion is a lot like a person choosing a new road. Imagine you are walking a very long, winding path. If you see a straight shortcut, you might take it. A river does something very similar to save energy. It wants the easiest way to reach its goal. Small changes like a meander cutoff are also common. This is when a river cuts through a sharp hook shape. This creates a new, straighter path for the water. These events show us how powerful moving water can be.

In the study of rivers and landforms, an avulsion is a major event where water changes its course. This process occurs when flow is diverted out of an established river channel into a new path. This diversion happens through a bifurcation, which is a splitting of the channel into two branches. Avulsions can be categorized by how much water moves. A full avulsion occurs when all the flow is diverted from the old channel to a new one. A partial avulsion happens when only a portion of the river's flow moves to the new course. These events are vital because they reshape the landscape and move massive amounts of sediment.

One common setting for avulsion is in river deltas. A delta is a landform created where a river enters the ocean and deposits sediment. This process is often called delta switching. As a river carries sediment, it builds up a specific area called a deltaic lobe. The Mississippi River features a famous example known as a bird's-foot delta. As this lobe pushes further out into the sea, the river channel becomes much longer. Because the change in elevation stays the same while the distance increases, the channel gradient, or slope, becomes very small. This shallow slope makes the river system unstable.

This instability happens for two specific reasons related to physics and gravity. First, water under the force of gravity naturally seeks the most direct route downslope. If a river can breach its natural levees, which are the raised banks of a river, it will find a shorter path to the ocean. A shorter path provides a steeper and more stable slope. Second, a reduced slope decreases the shear stress on the river bed. Shear stress is the force of the water rubbing against the bottom. When this force drops, more sediment settles in the channel. This process is called aggradation, where the river bed rises relative to the surrounding floodplain.

On average, an avulsion will occur once the river bed aggrades enough to be one channel-depth higher than the floodplain. In this state, the river is superelevated above the land around it. This creates enough hydraulic head, or pressure, to force a breach during a flood. When the river finally breaks its banks, it cuts a new channel to a steeper slope. This new path then carries sediment to the ocean to build a brand new deltaic lobe. Meanwhile, the old, abandoned delta begins to subside, or sink. Over long periods, repeated channel switching creates a complex distributary network of many different river branches.

Another type of avulsion is an erosional avulsion. This occurs when moving water wears away the ground to create a straighter path. This often happens during massive floods when the new path has a much steeper slope than the old one. If the slopes are nearly equal, the river may only undergo a partial avulsion. In 2006, the Suncook River in New Hampshire provided a clear example. Heavy rains caused water to back up behind an old mill dam. This created a shallow pool that overtopped a quarry and cut a new channel at a rate of 25 to 50 meters per hour. This rapid erosion changed the river's path almost instantly.

The Cheslatta River in British Columbia shows how human activity can trigger these events. In the 1950s, the river was used as a spillway for the Nechako Reservoir. The massive amount of water released from the reservoir far exceeded the river's natural flow. This caused heavy erosion in the upper valley. These large spills forced the river to avulse in 1961 and again in 1972. The river carved a new route and deposited a large pile of sediment called the Cheslatta Fan. Eventually, a cofferdam was built to return the river to its original course.

A smaller, more common version of this process is the meander cutoff. Rivers often flow in large, winding loops called meanders. A cutoff happens when the water breaches the narrow part of the loop. This connects the two closest parts of the bend to form a straighter channel. This usually occurs when the ratio between the old slope and the new potential slope is less than 1/5. Whether caused by floods, dam removals, or obstructions like log-jams, avulsions are a fundamental way that rivers interact with the Earth's surface.