Is my stream eroding?
Usually by the time we are asked by homeowners to evaluate if a stream channel is eroding, it is. And the reason landowners know it is eroding is because they have observed a change in stream erosion and deposition in their stream. Or even worse, the stream is undermining land, roads or structures on the landscape. And the longer the erosion occurs, the more difficult and costly it could be to fix. Common signs of stream erosion include:
A noticeable and measurable change in the shoreline, such as a rapid deepening or widening of the stream channel
Steep, high, or overhanging shoreline banks with bare soil visible
Collapsed sections of shoreline
Exposed roots on the shoreline, as well as leaning or fallen trees
Water appearing to be muddier than usual, especially after a rainfall event or when there is high water
Excessive deposits of sand or sediment on the inner curves of the stream
Notably, it doesn’t always mean that there must be an intervention to stabilize or repair project just because a stream shows some of the signs of erosion above. Sediment erosion, transportation, and deposition are natural processes in a healthy stream, and a natural stream will slowly erode, develop sandbars, meander, and move over time while maintaining a stable pattern, dimension, and profile. It is important to emphasize that in a healthy stream, this is a slow and gradual process.
Healthy stream meandering Rapid erosion of the streambank (Credit: Christopher Auyeung, Source: CK-12 Foundation-CC BY-NC 3.0)
However, streams are very dynamic systems influenced by several factors like soil type, vegetation type and coverage, water volume, and water velocity. All of these factors have to balance for a stream to be stable. If just one of these factors changes, the balance can be lost and the stream will find a new balance, but something must give in order to achieve that balance.
For example, when there is a change in the watershed that delivers more runoff or faster runoff, the sudden increase in volume or velocity of water will result in an increase in erosive energy, and the channel can quickly begin to degrade as the stream tries to accommodate for the change.
Head cutting is a process of erosion that occurs in instances where there has been an abrupt change in the slope of the stream bed. The active head cut will occur at the nick point, or the point at which the slope has become steeper, and the water velocity and erosive energy will increase at that point as a result. As the head cut erodes at the nick point, it will begin to migrate upstream as it keeps cutting away at the streambed.
This process causes the channel to become deeper and eventually incised. Lateral erosion of the streambank can also occur as the channel tries to restore its meandering pattern, which is also an attempt to decrease the slope and thus the water velocity. This process leads to a widening and eventually a lengthening of the channel. The downstream effect of both processes is a raised streambed as a result of increased and excessive sediment deposits.
In sum, problems can occur when natural stream processes become accelerated because of land use, vegetation, or in-channel changes that lead to higher volumes and higher velocities of water running through the channel. Not only can this lead to the undermining of surrounding land, roads, and buildings, but it is can also lead to the degradation or loss of aquatic and shoreline habitat, a worsening of water quality, and an increased risk of flooding. Therefore, when rapid erosion of the streambank or streambed is observed, it is a sign that the stream is unstable and at risk of becoming unhealthy if it cannot restore its balance.
Thus, if a stream channel changes in the middle of a prairie with native grasses, there is little concern because there is room for the natural corrective processes to occur. However, sometimes a stream channel tries to change next to the foundation of a building that would be cost prohibitive to relocate or rebuild and it makes more sense to intervene and find an alternate location to jumpstart the natural rebalancing process.
Our experts can look at your site and investigate not just that location, but the larger dynamic system to help you determine if your stream is eroding and if it is a problem. And if it is a problem, what to do about it.
Mitigation Partners, Inc. Founders Dax Dickson & Tory Christensen
