RIVERS AND STREAMS - Water Quality Engineering in Natural Systems - page 132

Figure 4.23. Riprap stabilization of a stream bank.

Figure 4.24. Fish ladder at the Bonneville dam on the Colum-
bia River (oregon). Source : Bonneville Power Administra-
tion (2012).

and/or to sustain proper ecological conditions for
fish and aquatic life. The source of the diluting flow
may include upstream reservoirs, pumping from
nearby water bodies, or recycling (pumping) flows
of cleaner and more diluted downstream flows.
Using highly treated effluents in effluent-
dominated streams is also a feasible and accept-
able source of base flow.
In-stream and sidestream aeration may be feasible
for streams that exhibit low Do levels. In-stream
aeration is accomplished by turbine aeration in
power plants or by installation of floating or sub-
merged aerators. More natural in-stream cascades,
spillways, and water falls may also provide addi-
tional oxygen.
Aeration is mostly a temporal measure that can
be used in the following circumstances: (1) when
summer, low-oxygen levels drop for a short time
(such as during the night and early morning hours)
below the dissolved-oxygen limit for fish protec-
tion (typically 4-6 mg/L depending on the type of
fish); or (2) during winter, when stream aeration is
reduced by ice cover. Aeration may be particularly
useful for streams draining wetlands that exhibit
dystrophic conditions (i.e., low Do levels caused
by high rates of decomposition of organic matter
in the wetlands) and as a supplement when the
waste assimilative capacity of the water body is
exhausted because the water body receives loads
of biodegradable organics that cannot be con-
trolled by removal at the source. The efficiency of
aeration increases as the oxygen deficit increases.
Fish ladders are cascades of small basins that have
an elevation differential that migrating fish can

easily overcome when moving upstream and also
make it possible for the fish to rest in the basins.
A typical fish ladder is illustrated in Figure 4.24.
Fish ladders have been installed worldwide;
however, experience with these installations on
the Columbia River in the Pacific northwest (in
the United States) is mixed and have proven insuf-
ficient to restore full salmon migration up the
river.
Removal of river impoundments can be considered
restoration techniques in addition to construction
of new structures or modification of existing struc-
tures. In the United States, many dams were built
on streams for various purposes more than 100
years ago, and, in Europe, building stream weirs
(low-head dams) for providing head to mills dates
back several centuries. Sediment has accumulated
behind these dams from urban and rural diffuse
sources, wastewater discharges, and from com-
bined sewer overflows. often, the impoundment
today is filled with sediments, has ceased to func-
tion, and the sediments contain toxic pollutants.
The removal of the McGoldrick dam on the
Ashuelot River (new Hampshire) is shown in
Figure 4.25.
Removal of contaminated sediment is an important
option in many cases. Some sites with contami-
nated sediments have been declared as hazardous
contaminated sites that must be cleaned up, and
the most common cleanup methods are sediment
removal and sediment capping. Extreme caution