Classifying Your Primary Streambed Material

Determine your stream’s primary bed material type below.

Understanding the primary bed material of your stream will help you to determine specific details for some of the stewardship and recovery strategies. For example, certain strategies will make different recommendations based on whether your streambed is primarily sand or primarily cobble. When determining your stream’s primary bed material, you will likely encounter more than one of the categories described below. Select that material/size that makes up the majority of your streambed. If your streambed is mostly cobble but has some boulders mixed in, select cobble as your stream’s primary bed material.

Determine your stream’s primary bed material:

Sand is made up of fine sediment that is difficult to pick up individual pieces of.
<2mm across

Sand streambeds are the most susceptible to erosion and changes in path and channel shape. Finer bed material like sand can better host riparian vegetation, which may provide some additional channel stability.

Gravel is smaller than a baseball, but can be picked up individually.
2mm – 2.5” across

Gravel bed streams can be found throughout a watershed. While they are less likely to transport fine sediment such as sand, they often do transport gravel size sediment.

Cobble is between the size of a basketball and a baseball.
2.5” – 10” across

Cobble streams also have a higher resistance to erosion. Like boulders, cobble streams are typically found in mountainous areas where the energy within the stream is relatively high. Cobble has a lesser ability to dictate the path, so changes in stream alignment are still possible in less entrenched areas.

Boulders are larger than a basketball.
>10″ across

Streams with a primary bed material of boulders are usually found high in the watershed, in mountainous areas with steep hillsides. When stable, boulders have a strong resistance to erosion and usually dictate the path of the stream. Boulders are often associated with high energy streams that are frequently found in an entrenched setting with steep slopes.

What is your primary streambed material?___________________________

What does your stream’s primary bed material tell you?

The primary bed material of a stream refers the predominant size of rock or sand within the channel and along the streambanks. The streambed material affects the stream’s capacity to resist erosion, transport and deposit sediment and the general shape of the channel. Generally, streams with gravel and sand bed material are more susceptible to changes during high flow events than streams with larger bed material such as cobble and boulders. Streams with finer bed material are more likely to erode and increase the amount of sediment in the water under high energy conditions. Similar to its slope, a stream will naturally adjust with its bed material to try to find a state of stable equilibrium.