Habitat Improvements

Boulder Placement

Description


Boulders are a common natural element of riffles, runs, rapids and to a lesser degree of pools. The location within a channel, in combination with its slope, dictates the habitat function. They are a simple, cost effective means of enhancing the complexity of habitat within a suitable reach of stream. Boulders are known for their durability and ability to maintain their original arrangement and function.

Purpose

Large angular rocks or smooth field boulders create surface turbulence and expose coarser substrates as a result of scouring. This technique can enhance the production and diversity of aquatic insects by exposing larger substrates. In addition, large boulders create more fish spawning and nursery habitat in riffles and cover habitat in pools. Commonly, this application can rsult in a significant increase in the production of juvenile salmon and trout.

Application

Typically, boulders are placed adjacent to the thalweg of the channel. They can be placed individually, in organized arrangements or in random clusters. The minimum size is dependent on the maximum velocity of the bankfull channel. They can be used in conjunction with K dams, cabled log jams, log sills or by-pass channels. Previously channelized watercourses can be visually enhanced through the placement of boulders.

A boulder placement project should begin with developing a good understanding of the channel characteristics within the selected reach. You will need to know what type of watercourse you are working in and the dynamic nature of seasonal flow, sediment transport and iceformation prior to selecting this habitat improvement technique. Sketches of the channel in plan view and cross section, indicating bankfull, low flow and thalweg will be a tremendous asset for determining suitability and positioning. Instream boulder placement is ideal in B type channels (slope between 2 to 4%) with a predominant cobble and gravel lining. They may also work in this same slope range where sand and silts dominate the channel bed. Expect to see pronounced scouring in this situation. C type channels (slope less than 2%) having a cobble dominated substrate are also appropriate but not as well suited. Boulder placements are not recommended for streams exhibiting a high bedload of fine sediments or lateral channel movement. Downstream bar formation may occur as a result.

Construction Guidelines

Once you have a good idea of the maximum channel velocities, you can then determine the best size of stones for placing in the channel. Limestone, granite or quarry stone will suffice. Sandstone has a tendency to disintegrate and is not recommended.

Typically, stones are 60 to 100 cm in diameter and placed 1.5 to 2.5 m apart adjacent to the thalweg. Twenty-five times the average diametermof the riffle substrate is a good rule of thumb for determining the size of stone you need. Avoid adding boulders that cover more than 1% of the channel bed area as this will increase hydraulic resistance, cause sediment deposition and provoke river shifting.

Boulders are usually placed at the mid point or tail of a riffle and never at the crest. In runs, they are best suited in the upper to mid reach; while in pools they are best located at the head and tail. Random placement of individual stones helps to increase habitat complexity by introducing localized scour pockets, exposing larger substrates and creating surface turbulence. Clustering and organized formations compound the effects of altering channel hydraulics by creating larger scours and more complex eddies.

Fig 3.9:
Boulder placement before crest


Boulders should be buried about one third of their depth into the stream bed as an added measure of stability. Shovels and hand levers can be used to position rocks although heavy machinery makes it easier if access is not a problem.

Materials Needed

Depending on site access and the availability of suitably sized stones, this technique can be easily implemented by a crew of four to six. You will need:

In some cases, heavy machinery may be used to move the stone into position. Access and cost will determine the most suitable means of installation.

Cost and Maintenance Needs

This is one of the most reliable techniques when applied in suitable streams and rivers. In stable channels, the expected life of the structure is indefinite.

If local stone is not available, the cost of shipping boulders to the site can be as much as $35.00 per tonne delivered. Backhoe operators charge from $45.00 to $65.00 per hour and add a float charge for moving the machine to the site.

Frequent inspection is suggested within the first full year to ensure proper placement. Annual inspection after this period is recommended.

Integration

Instream boulders can be integrated into other habitat improvement projects such as:

Demonstrations

This type of habitat structure has been applied in the following demonstration projects:

For More Information

Please refer to the following authors and their respective publications
located in the bibliography:

Buchanan, R.A. , D. A. Scruton and T. C. Anderson 1989
Fitch, L., et al. 1994
Fuller, D.D. 1990
Rosgen, D. 1996

November 5, 1998