River Ecology and Restoration - River Dynamics and Integrated River Management - page 695

the flow, enabling organic debris to fall out and enhance invertebrate production (Wesche, 1985). Such
structures extend across the entire width of the channel, although some incorporate a notch to locally
concentrate flow. This type of structure is perhaps most effective for producing a pool-riffle pattern in
low energy streams.
Habitat provided by weirs can be particularly valuable in channelized streams experiencing relatively
high levels of erosion and deposition. Cooper and Knight (1987) compared fish catches from pools
below weirs used as grade control structures and natural scour pools in unstable, channelized streams in
Mississippi. U.S. Grade control pools produced greater catches by weight, more fish of harvestable size,
and more stable length frequency distributions than natural scour holes. Cooper and Knight (1987)
speculated that habitat created by grade control structures was more stable than natural scour holes,
which undergo frequent cycles of filling and scouring. This stability resulted in more consistent successful
spawning and recruitment and ultimately higher yields.
Weirs have been used successfully in many countries to increase fish population within relatively short
periods of time (Gard, 1961; McCall and Knox, 1978; Carling and Kloslewski, 1981; Shields et al., 1995).
Artificial pools created by weirs in New Mexico, U.S. were up to 70% greater by volume than natural
pools and held 50% more trout with twice the biomass.
There are also many examples of failures (Wesche, 1985). The structure may fail in high-energy rivers,
particularly at high flow and they tend to be unsuccessful in enhancing habitat where there is an
excessively high sediment load (Keown, 1981). Structure failure may not necessarily equate to failed
habitat improvement. For example, rock scattered from a failed deflector may provide substrate for
invertebrates, resting areas, or ambush sites for stream fish. Other reasons for failure to enhance biological
populations include the blockage of fish passage (Johnson, 1971) and the lack of an adequate food supply
(Rockett, 1979).
Modification of substrate —Boulders have been placed in channels to provide cover for fish, to
improve the pool-riffle characteristics, to provide additional habitat for rearing fish or to protect banks
from erosion. Randomly placed boulders can enhance fish habitat substantially (Knox, 1982; Lere, 1982).
Diamond-shaped clusters of four boulders are often used. A field experiment was conducted by replacing
the substrate with gravel, stones, sand and silt. Figure 10.50 shows the experiment in the Juma River in
the suburbs of Beijing. The highest species richness was obtained with gravel substrate, proving that
replacing substrate with gravel may effectively improve the aquatic ecology.
In lowland streams where no endemic rock exists, large logs or wooden pilings may have ecological
advantages over boulders. Woody debris is a key component of aquatic habitats in sand-bed rivers
(Shields and Smith, 1992). Although woody structures do not last as long as rock, they provide a carbon
source and may be more acceptable to organisms that have evolved to live on submerged woody debris.
Wooden substrate may also be cheaper and more readily available.
From an ecological point of view the placement of more natural bed sediments may speed recovery
(Gore, 1985). Placement of artificial materials such as crushed limestone and quarry rejects may also
improve the habitat for fish and for macro-invertebrates (Stuart, 1960). For example, on the Afon Gwyfai
in Wales decolonization of invertebrates on a reinstated gravel bed was a gradual process, taking about a
year (Brookes, 1988). The stability of reinstated gravel is a key issue. If the gravel is unstable, then the
diversity and abundance of species will be less than for a stable bed. In a high-energy environment it may
be necessary to install structures to retain gravels in situ (Claire, 1980). Also, where sediment loads are
too high due to upstream channel modification or land use change then spawning gravels may become
smothered. This is a particular problem in low-energy lowland streams.
Devices Providing Cover —Under natural circumstances undercut banks and overhanging vegetation
are important habitat features referred to as “cover”. Fish utilize cover for shade and shelter. Figure 10.51