Boulder Clusters

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


Boulders clusters are groups of boulders placed in the stream channel to create pools for fish to take refuge. Boulder clusters also provide surfaces for insects to attach their eggs to. They provide:

• Aquatic habitat

• Insect habitat

Typically, boulder clusters are not used in sandy streambeds because they have a greater chance to become dislodged in stream systems with bed material that is much smaller than the boulder itself.

Photo courtesy of: Geosyntec

Boulder clusters naturally occur in stream systems and they provide valuable benefits to the corridor. By breaking up the flow of the stream, deeper pools are formed downstream of the boulders. These pools are used by fish for shelter and rest. The surface of the boulders also provide a surface on which insects can lay their eggs.

Because a boulder cluster project involves placing material in the stream, there will be permitting associated with the project. Often, a properly qualified river constructor will be able to guide you through the permitting process without having to go through expensive engineering processes.

If the project is larger or has more potential to cause negative effects, you may need to hire an engineer to make sure the project will be beneficial for the stream corridor, neighbors and yourself.

Boulder clusters are intended to promote aquatic habitat. Before contacting outside help, it is important to understand what type of aquatic species you have in your stream. It will also help to observe if there are boulders in the channel upstream or downstream of you. If you are in a sandy plains stream, it is not very likely that there are boulder clusters in your area. Likewise, installing boulder clusters in your stream will not be effective for aquatic habitat and it could have negative effects on the stream system.

Placement of boulder clusters will require heavy equipment and some excavation. The river constructor should be able to discuss the project schedule and what the final product will look like with you.

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