River Ecology and Restoration - River Dynamics and Integrated River Management - page 705

Channelization and diversions represent forms of hydrologic modification commonly associated with
most principal land uses, and their effects should be considered in all restoration efforts. In some cases,
redesign of channel modifications to restore preexisting ecological characteristics is needed. All of the
urban channels in Beijing have been channelized with concrete bed and banks. Riparian plant community
and benthic invertebrates have lost their habitat and the water quality has become worse. It is planned to
replace the concrete bed and banks with soil and stones, providing habitat for invertebrates. It is
estimated that a high water quality may be maintained with only half of the water consumption.
Modifications of existing projects, including operation and maintenance or management, can improve
some negative effects without changing the existing benefits or creating additional problems. Levees
may be set back from the stream channel to better define the stream corridor and reestablish some or all
of the natural floodplain functions. Setback levees can be constructed to allow for overbank flooding,
which provides surface water contact with streamside areas such as floodplains and wetlands.
Instream modifications such as uniform cross sections or armoring associated with channelization or
flow diversions may be removed, and design and placement of meanders can be used to reestablish more
natural channel characteristics. In many cases, however, existing land uses might limit or prevent the
removal of existing channel or floodplain modifications.
10.4.7 Restoration of Ecosystems Disturbed by Other Stresses
Exotic species —Restoration of ecosystems disturbed by exotic species is difficult. Some land uses have
actually introduced exotics that have become uncontrolled, while others have merely created an opportunity
for such exotics to spread. Again, control of exotic species has some common aspects across land uses,
but design approaches are different for each land use. Control of exotics in some situations can be
extremely difficult and may be impractical if large areas or well-established populations are involved.
Use of herbicides may be tightly regulated or precluded in many wetland and streamside environments,
and for some exotic species there are no effective control measures that can be easily implemented over
large areas (Rieger and Kreager, 1990). Where aggressive exotics are present one should avoid unnecessary
soil disturbance or disruption of intact native vegetation, and newly established populations of exotics
should be eradicated.
Controlling exotics can be important because of potential competition with established native vegetation,
colonized vegetation, and artificially planted vegetation, in restoration work. Exotics compete for moisture,
sunlight, and space and can adversely influence establishment rates of new plantings. To improve the
effectiveness of revegetation work, exotic vegetation should be cleared prior to planting; nonnative
growth must also be controlled after planting. One must also understand the physical characteristics of
the native vegetation for successful establishment. For example, native vegetation in the midwestern U.S.
has learned to grow under conditions of low nutrient supply, and, thus, use of fertilizers often just promotes
weed growth (Neal O'Reilly, Hey and Associates, personal commun). General techniques for control of
exotics and weeds are mechanical (e.g., scalping or tilling), chemical (herbicides), and fire.
Agriculture —In agricultural areas when terraces and a waterway are installed in the nearby cropland,
the scene depicts an ecologically deprived landscape. Nutrient and water flow, sediment trapping during
floods, water storage, movement of flora and fauna, species diversity, interior habitat conditions, and
provision of organic materials to aquatic communities are just a few of the functional conditions affected
by these structural attributes.
Restoration design should establish functional connections within and external to stream corridors,
landscape elements such as remnant patches of riparian vegetation, prairie, or forest exhibiting diverse
or unique vegetative communities; productive land that can support ecological functions; reserve or
abandoned land; associated wetlands or meadows; systems; ecologically innovative residential areas;