River Ecology and Restoration - River Dynamics and Integrated River Management - page 706

neighboring springs and stream systems; ecologically innovative residential areas; and fauna (field
borders, windbreaks, waterways, grassed terraces, etc.) offer opportunities to establish these connections.
An edge (transition zone) that gradually changes from one land use into another will soften environmental
gradients and minimize disturbance.
Urbanization often is the strongest disturbance to river ecosystems. The development of residence
areas may severely impair the riparian vegetation and aquatic biocommunities. Seven restoration tools
can be applied to help restore urban streams (FISRWG, 1997; Schueler, 1987). The best results are
usually obtained when the following tools are applied together.
Tool 1: Partially restore the predevelopment hydrological regime. The primary objective is to reduce
the frequency of bankfull flows by constructing upstream stormwater detention ponds that capture and
detain increased storm water runoff for up to 24 hours before release (i.e., extended detention).
Tool 2: Reduce urban pollutant pulses. A second need in urban stream restoration is to reduce
concentrations of nutrients, bacteria, and toxics in the stream, as well as trapping excess sediment loads.
Generally, three tools can be applied to reduce pollutant inputs to an urban stream: storm water retrofit
ponds or wetlands, watershed pollution prevention programs, and the elimination of illicit or illegal
sanitary connections to the storm sewer network.
Tool 3: Stabilize channel morphology. Over time, urban stream channels enlarge resulting in severe
bank and bed erosion. Therefore, it is important to stabilize the channel, and if possible, restore equilibrium
channel geometry. In addition, it is also useful to provide undercuts or overhead cover to improve fish
habitat. Depending on the stream order, watershed impervious cover, and the height and angle of eroded
banks, a series of different tools can be applied to stabilize the channel, and prevent further erosion. Bank
stabilization measures include imbricated riprap, brush bundles, and soil bioengineering methods, such as
willow stakes and bio-logs and rootwads.
Tool 4: Restore instream habitat structure. Most urban streams have poor instream habitat structure,
often typified by indistinct and shallow low flow channels within a much larger and unstable storm
channel. The goal is to restore instream habitat structure that has been blown out by erosive floods. Key
restoration elements include the creation of pools and riffles, deepening of the low flow channels, and the
provision of greater structural complexity across the streambed. Typical tools include the installation of
log check-dams, stone wing deflectors and boulder clusters along the stream channel.
Tool 5: Reestablish vegetative banks and riparian cover. Vegetative banks and riparian cover are essential
components of the urban stream ecosystem. They stabilize banks, provide woody debris and detritus, and
shade the stream. Therefore, the fifth tool involves reestablishing the vegetative banks and riparian cover
plant community along the stream network. Figure 10.60 shows an urban stream in Beijing where a layer
of soil is placed on the hardened channel bed and banks, thus a vegetation bank and aquatic high plants
have developed.
Tool 6 : Protect critical stream substrates. A stable, well sorted streambed is often a critical requirement
for fish spawning and secondary production by aquatic insects. The bed of urban streams, however, is
often highly unstable and clogged by fine sediment deposits. It is often necessary to apply tools to restore
the quality of stream substrates at points along the stream channel. Often, the energy of urban storm
water can be used to create cleaner substrates—through the use of tools such as double wing deflectors
and flow concentrators. If thick deposits of sediment have accumulated on the bed, mechanical sediment
removal may be needed.
Tool 7 : Allow for recolonization of the stream community. It may be difficult to reestablish the fish
community in an urban stream if downstream fish barriers prevent natural recolonization. Thus, the last
urban stream restoration tool involves the judgment of a fishery biologist to determine if downstream