RIVERS AND STREAMS - Water Quality Engineering in Natural Systems - page 131

structures that can easily be knocked down or
washed away by large surges of water or material
that result from heavy rainfall. Therefore, they
should be used as one tool available to prevent
erosion and not considered as the only solution.
They may be removed once a site is permanently
stabilized and the threat of accelerated erosion no
longer exists. In front of the silt fence shown in
Figure 4.21 is a turbidity barrier, which provides
secondary containment of sediment that might not
be intercepted by the silt fence. Streams that are
affected by nutrient loads from lawn fertilization
can be improved by changing the type of fertilizer
used or the frequency and timing of fertilization.
nonstructural pollution abatement activities also
include use of fencing around streams and riparian
zones, which is effective in excluding livestock and
humans.
Propagation facilities are used to propagate aquatic
species through incubation and spawning, and is a
common approach where it is desirable to main-
tain fishing in an area that otherwise would not
have a sustainable sport-fish population. A fish
hatchery on the Columbia River is shown in Figure
4.22. Hatchery-raised fish may not always survive
or propagate in the wild.
Land acquisition can protect a watercourse through
maintenance of buffer zones and prevention of
potentially destructive land uses in the watershed.
Land acquisition approaches can lead to the estab-
lishment of greenways, buffer strips, and parks.
These can be purchased by government or special
foundations and trusts set up to provide such
protection.
Land use regulation in the riparian zone and water-
shed is an effective legislative approach to control-
ling pollution sources.

Biomanipulation is a fish-management technique
that involves direct manipulation of the fish com-
munity and other organisms that serve as prey,
predators, or competitors with the fish species of
interest. Activities include game-fish stocking,
control of undesirable fish species, and prey
enhancements to supplement food supplies.

4.8.2 Structural Techniques

Structural techniques for the restoration and manage-
ment of streams are those that require some type of
physical alteration of the watercourse and may include
alterations to existing human-made structures, such as
dams and levees. Structural stream restoration tech-
niques include bioengineering techniques, bank armor-
ing techniques, aquatic habitat improvement methods,
low-flow augmentation, in-stream and sidestream aera-
tion, fish ladders, removal of river impoundments, and
removal of contaminated sediment. Descriptions of
several of these techniques are given below.

Bioengineering techniques use plants to replace
natural stream bank stabilization in situations
where the stream bank has been eroded or lacks
vegetation due to some destructive process. Plant-
ing can provide more ecological benefits than
erosion control through addition of stream cover,
shade, and improvement of bank soil conditions.
native plant species are recommended because
they are generally adapted to local environmental
conditions.
Bank armoring techniques use rock, wood, steel,
and other conventional construction materials to
stabilize stream banks. An example of riprap
(rock) stabilization of a stream bank is shown in
Figure 4.23. Bank armoring methods rarely provide
significant ecological benefits other than erosion
control unless they are combined with bioengi-
neering techniques.
Aquatic habitat improvement methods involve
improving aquatic habitat through installation of
certain in-stream structures. Disturbed streams
often lack diverse morphological features, and
habitat improvement structures can add gravel
beds, structural complexity, restricted flow, and
riffles and pools. These features are important to
allow spawning and rearing areas for aquatic life.
Selection of a particular technique or combination
of techniques depends on current habitat deficien-
cies, watershed conditions, and the current mor-
phology and hydrology of the watercourse.
Low-flow augmentation provides cleaner diluting
flow during times of water-quality emergencies

Figure 4.22. Fish hatchery on the Columbia River. Source : U.S.
Army Corps of Engineers (2005a).