Overflow Channels and Backwater Areas

* Potential permits required: CWA 404, Threatened & Endangered Species, NHPA, Floodplain

* Heavy equipment may be required: Depends on project size.

Materials and Tools needed
varies by project.

Overflow channels are sections of stream that do not normally carry water during average daily flows, but will become active during larger flow events to carry excess water. They serve many benefits to the stream system and the surrounding ecosystems, including:

• Reduces velocity in main channel
• Unique backwater ecosystems
• Provides water ‘storage’ during high flows
• Provides location for sediment to be stored

Little Thompson River on left, overflow/backwater area on right.

Overflow channels are a natural part of many stream systems and they can provide numerous benefits. You may already have overflow channels on your property. If that is the case, you will want to make sure that the overflow channel stays clear enough for higher flows to pass through them. You also want to make sure that the water has a path to return to the main channel of the stream. If you are concerned that the path back to the stream may be blocked by sediment or a landform, contact an engineer, river constructor or your local watershed to help you evaluate the situation. Lastly, if your overflow channels are eroding, you may want to consider stabilizing them with willow stakes and/or other riparian plantings. See the riparian plantings strategy sheet for ideas.

If you don’t currently have overflow channels on your property, you may be interested in adding them. Developing overflow channels can require extensive outside help to design a properly functioning system that does not have negative impacts on the floodplain or private properties. Overflow channels should be set at or near the annual high water level. When designed this way, the overflow channel will see active flows during spring runoff and during storm events. At other times of the year, the channel will be mostly dry. Care also has to be taken in designing the location of the overflow channel to reduce the chance of erosion or sediment deposition.

If you are interested in creating a new overflow channel, you will want to discuss a number of factors with the engineers designing the project. This includes understanding how often the overflow channel will carry water, how much water will flow through it and where it will be aligned on the property. Overflow channels should be provided to reduce the flow in the main channel but they should not direct flows towards a structure.

More Strategies to help accomplish Objectives you identified in the Questionnaires: