Random Boulders
Random Boulders are habitat improvement devices whose basic design is completely explained by their name. They are large stones, or groups of stones, randomly placed in the stream channel to provide overhead cover for fish, encourage scour pools and create eddies (areas of slow moving water downstream of unmovable objects in moving water). Random Boulders can be used in most stream habitats such as riffles, runs and pools. Random Boulders utilize two principles of flowing water to create habitat. First, as water flows in a stream channel and into an immovable object, it will be pushed to the side and re-circulating back behind that immovable object. This area is ideal for fish to rest, not being forced to fight the stream's current constantly. The second principle utilized involves higher stream flows. As stream flows rise above immovable objects, they will flow over the top of the object and straight down the other side. This places all of the stream's horizontal energy in a vertical direction, which will scour the stream bottom. This scour pool creates deeper areas of the stream for fish to hide from predators. Random Boulders will create habitat diversity by forming eddies (backwater areas) and scour pools, but caution should be used when placing the rocks in order not to accelerate bank erosion. Boulders are not recommended in streams with a sandy substrate because they will tend to bury themselves and function improperly.
