The following Questionnaire will assist you in determining what types of stewardship and recovery strategies relating to bank stability are the most applicable for your property.
Streambanks, or ‘banks’, are the sloped areas alongside streams, creeks and rivers that connect the stream to its floodplain. Banks contain the flow of the stream on either side of the channel and often dictate its behavior. Shaped by the complex processes of river dynamics (discussed more in Chapter 1: Living Along a Stream), banks can vary from bedrock or large boulders barren of vegetation to finer soils lush with trees, shrubs and grasses. Banks will also vary in their height and slope, as well as their distance from the river.
Bank Stability
Bank Stability refers to the ability of a streambank, including its soils and vegetation, to resist erosion from water flows and gravity.
Gradual streambank being created by sediment being deposited by the stream.
Faster water is eroding the streambank in this area, creating an unstable bank.
Floodplain; this area is above the streambank but water may still run over it during higher flows. When a stream is well-connected to its floodplain, seasonal high flows will run through the floodplain area. This is normal and healthy for a stream corridor.
Streams will naturally change their course by shifting and meandering. These changes are the result of the stream working to maintain a balance, or equilibrium, between the movement of water and the movement of sediment. As this happens, both the stream and the streambanks will change. For example, a slow moving section of stream may deposit sediment along a streambank. On the other hand, a faster moving section of stream may intensify the erosion of the adjacent streambanks and carry more sediment downstream.
Typically, a streambank will erode when the soils and vegetation cannot stand up to the flows of water from the stream. In other words, if the water is moving faster than what the vegetation can support, erosion will take place. For example, a dense stand of trees with roots in stable soils can generally resist greater flows than a gravelly area with little or no plants. In addition to the type of soils and vegetation that are visible on the surface, a streambank’s stability will also depend on factors such as the frequency and force of stream flows, nearby land uses, health of the plants and the types of soil layers underneath the surface.
While it may be difficult for you to identify all of these factors, there are many indicators of bank instability that you can watch for. These include exposed soils, the bottom of the streambank falling into the stream and eroding vertical walls along the stream. In addition, the vegetation and how it might be growing/positioned could be an indicator. For example, a tree leaning across a river with its roots exposed could be a sign that the bank is not stable.
Vertical walls are a giveaway that a streambank is unstable. The white deposits on the soil indicate that the erosion may be depositing very salty soils into the stream. Some wildlife, such as bank swallows, will use steep or vertical cutbanks for shelter.
Because this isn’t threatening a structure, the landowner may consider leaving the bank alone, especially if bank swallows are using the bank. Another option would include re-grading to create a more gradual slope down to the channel and adding vegetation to create a stable riparian bench
Vertical walls are a giveaway that a streambank is unstable. The white deposits on the soil indicate that the erosion may be depositing very salty soils into the stream. Some wildlife, such as bank swallows, will use steep or vertical cutbanks for shelter.
Because this isn’t threatening a structure, the landowner may consider leaving the bank alone, especially if bank swallows are using the bank. Another option would include re-grading to create a more gradual slope down to the channel and adding vegetation to create a stable riparian bench
To evaluate a streambank, you should consider what ‘degree’ of stability the bank is offering. Often times, banks have the strength to withstand the forces produced by annual high flows, but are unstable under the forces of larger flood flows. It is important to understand that even though a bank can appear stable under average daily flow conditions, it may not be able to withstand a 100-year flood event. For this reason, it is always recommended that structures be built away from the streambanks wherever possible.
The ultimate goal behind improving bank conditions is to encourage the stability and resiliency of a stream. However, the appropriate approach to improving bank conditions is highly dependent on each site’s conditions. If there is a structure nearby, such as a house or road, it may be most appropriate to armor unstable banks with large rock. On the other hand, if an unstable streambank is not threatening a structure, simply revegetating the banks may be the best option for achieving this goal. The revegetation will still allow for greater resiliency and stability, but it will also allow the stream to migrate over time. This is much more in line with what the stream would naturally do.
Because many of the bank stability strategies will require working in the stream, floodplain and wetland areas, you may be required to work with outside help such as engineers, environmental consultants or river constructors to design, permit and install the project. This will ensure that your project is successful for you and for your upstream and downstream neighbors.