River Ecology and Restoration - River Dynamics and Integrated River Management - page 692

10.4.2 Instream Structures for Habitat Restoration
Artificial instream structures have been used to modify fish habitat conditions from about the mid 1930s
in the U.S. Since that time the use of these structures has gradually increased worldwide. Such techniques
have been increasingly used to rehabilitate habitat in rivers impacted by channel works or adversely
affected by regulation (Brookes and Shields, 1996). It appears that the techniques were initially used to
improve rivers, which had a recreational value for salmonids, but more recently have been extended to
degraded rivers also containing non-salmonid species. Instream structures are just one of the more
localized methods of improving habitat. Common types of instream structures include weirs, deflectors,
random rocks, bank covers, substrate reinstatement, fish passage structures, and off-channel ponds and
coves. Habitat structures have been used more along cold water streams. In 1995, 1,234 structures were
evaluated according to their general effectiveness, the habitat quality associated with the given structure
type, and actual use of the structures by fish (Bio West, 1995). The study concluded that instream habitat
structures generally provided increased fish habitat, but 18 percent of the structures need maintenance.
Where excessive sediment delivery occurs, structures have a brief lifespan and limited value in terms of
habitat improvement. The typical structures are listed in Table 10.9.

T able 10.9 Structural techniques used for instream habitat improvement (after Brookes and Shields, 1996)

Type Functions

Deflectors To direct flow and eliminate accumulated sediment or to narrow a channel,

thereby increasing the velocity and creating a scour pool with a corresponding
downstream riffle

Small weirs or sills Diversify habitat by impounding a greater depth of flow above the structure

and by increasing the velocity downstream to erode a scour pool

Substrate placement Placement of new substrate to enhance the habitat for fish and macro

invertebrates

Devices providing direct cover Fixed to the bed or banks of a channel to float and adjust their level with

varying discharge

Species-centered restoration —Many angling organizations have used instream structures to improve
habitat for maximum production of salmonids and other game fish. In the U.S. there has also been Federal
involvement through agencies such as the U.S. Forest Service and the U.S. Fish and Wildlife Service
since the 1930s. There are a considerable number of publications, which demonstrate how effective such
projects can be for increasing fish numbers and biomass. However, sometimes this type of work has been
detrimental to other species, such as beaver (NRC, 1992). Gore (1985) argues that restoration of macro
invertebrate communities is essential since they often form a substantial portion of the food supply for
fish. Maximizing habitat value for any single species is not the same as recreating the biotic structure and
function of a stream, involving consideration of a number of species (NRC, 1992).
A key characteristic of a productive stream is physical habitat diversity. It is essential that there are
appropriate ranges of water depths, water velocities, and substrate types. Since cover is also important,
consideration needs to be given to the riparian plant community (Hunt, 1988). Suitable depths and
velocities are needed for spawning habitat, but if an appropriate substrate is absent then the habitat value
is diminished for species, which spawn on the bed. Likewise, if substrate has been reinstated without
consideration of holding pools for mature fish then the habitat value is again questionable. Many
approaches have now been developed to quantify habitat value. The approaches are based on the concept
that abundance of a particular species can be correlated with particular habitat requirements. For example,
the U.S. Fish and Wildlife Service Physical Habitat Simulation System Model is useful for crude
prediction and analysis of potential habitat improvement (Bovee and Milhous, 1978).