Classifying Your Stream Entrenchment
Determine your stream entrenchment type by following the steps and classification types.
Entrenchment is a way of judging how well the stream is connected to its floodplain. In a not entrenched stream system such as the picture at right, the stream will flow into the floodplain multiple times a year, meaning the stream is well connected to its floodplain. This allows the water to spread out and slow down, typically causing less damage than higher speed flows. In an entrenched stream system (shown below), the water is confined and will move much quicker. While every stream system will have varying types of entrenchment, identifying the entrenchment type of your reach of stream will allow you to better understand the high flow risks on your property and how to address them. It will also give you more information and insight if you decide to work with outside help to alter your stream’s entrenchment type.
How to determine your stream entrenchment:
“Entrenched” stream example
Step 1
Standing near the water, look at the banks and changes in plants to determine the annual high water level
Step 2
Double the height of the annual high water level to estimate event high flow.
Step 3
Is the event high flow within the banks? Choose your stream’s entrenchment type:
Entrenchment Types Defined:
If 2x of your annual high water level stays in the channel, it is entrenched.
If it barely reaches the top of the channel, it is moderately entrenched.
If it spills over and out of the channel, it is not entrenched.
What is your entrenchment?___________________________
An understanding of the relationship of a stream to its floodplain is very important in developing a broader understanding of the processes that shape the stream. Entrenchment is a way of determining if a stream is ‘connected’ to its floodplain. A stream that is connected to its floodplain will allow water to flow onto the floodplain during seasonal runoffs and storm events. This is considered a natural and healthy function of a stream system.
Similar to a confined valley, an entrenched stream is one that cannot readily access its floodplain during flood flows. This confines the energy of the stream to the main channel, rather than allowing the energy to slow down and spread out into the floodplain. Conversely, streams that can easily access the floodplain are considered not entrenched and are able to disperse this energy across a greater area. This reduces the erosive forces within the main channel.
What does your stream’s entrenchment tell you?
An entrenched stream will often have high rates of erosion within its main channel during high flow events. The risk for erosion could be worsened when coupled with a finer streambed material, such as sand. Entrenched streams often naturally occur in mountainous areas with steeper valleys. Entrenched streams are also found in areas where a stream has experienced significant erosion.
A moderately entrenched stream will often exhibit erosive forces similar to entrenched streams. However, they can be minimized if and when flow spills onto the floodplain. Moderately entrenched streams can be seen throughout a watershed, but are most common in eroded areas. They will also be seen in transitional areas, where the mountains meet the plains.
Streams that are not entrenched are most often found along the plains. These reaches disperse the power of high flows across a floodplain, reducing the potential for erosion. They can also be found in wider valleys and pockets between more confined stretches of the stream.