68
3.2 Direct Alterations of River Networks and River
Corridors
Direct alterations are those that occur within the river corridor. These alterations
can affect the form of the channel or floodplain, as in channel engineering, mining
within the river corridor, log floating, and floodplain drainage. Direction alterations can also change the fluxes of materials and the movement of organisms within
the corridor, as in levees and flow regulation. Finally, direct alterations can change
the presence of particular species within the corridor, as in the case of beaver
removal, introduced species, and commercial extinction of species. Recognition of
direct alterations is important for river management because these activities may
have occurred so long ago that, although their effects persist, they are no longer
immediately apparent, as in the case of abandoned, historical mill dams discussed
in Sect. 3.2.2.
Each of the types of direct alteration of river corridors listed above commonly
affects the other types. Channelization, for example, simplifies and homogenizes
the channel. This increases downstream fluxes and limits lateral and vertical fluxes,
which may decrease habitat availability and stability and thus alter aquatic and
riparian biomass and biodiversity (Schoof 1980; Fig. 3.5). Introduced riparian vegetation may substantially increase hydraulic and erosional resistance of stream
banks and reduce stream flow because of increased water uptake and transpiration
(e.g., tamarisk (Tamarix spp.) in the southwestern United States (Graf 1978) and
willow (Salix spp.) in Australia (Schulze and Walker 1997; Cremer 2003)).
Introduced riparian vegetation can also change nutrient cycling and habitat available for riverine species (Greenwood et al. 2004). As with the effects of indirect
alterations, direct alterations of the river corridor seldom have just one effect.
Instead, the initial alteration triggers a cascade of interrelated changes that can make
prediction and management of the river corridor very challenging (Fig. 3.6).
3.2.1 Changes in Channel and Floodplain Form
This section briefly reviews channel engineering, mining within the river corridor,
log floating, and floodplain drainage. Although other types of direct changes in
channel and floodplain form exist, the four categories of change discussed here are
among the most common and widespread.
Among the forms of direct alterations, channel engineering has a particularly
long history. Channel engineering refers to activities designed to simplify and
homogenize the channel form and to ‘harden’ the channel boundaries in order to
make them more resistant to erosion and to limit lateral or vertical channel changes.
Types of channel engineering include dredging of the channel bed, usually to
enhance navigation or downstream conveyance of flood waters or waste products.
Another form of channel engineering is stabilization of the channel banks, usually
3 Human Alterations of Rivers
3.2 Direct Alterations of River Networks and River
Corridors
Direct alterations are those that occur within the river corridor. These alterations
can affect the form of the channel or floodplain, as in channel engineering, mining
within the river corridor, log floating, and floodplain drainage. Direction alterations can also change the fluxes of materials and the movement of organisms within
the corridor, as in levees and flow regulation. Finally, direct alterations can change
the presence of particular species within the corridor, as in the case of beaver
removal, introduced species, and commercial extinction of species. Recognition of
direct alterations is important for river management because these activities may
have occurred so long ago that, although their effects persist, they are no longer
immediately apparent, as in the case of abandoned, historical mill dams discussed
in Sect. 3.2.2.
Each of the types of direct alteration of river corridors listed above commonly
affects the other types. Channelization, for example, simplifies and homogenizes
the channel. This increases downstream fluxes and limits lateral and vertical fluxes,
which may decrease habitat availability and stability and thus alter aquatic and
riparian biomass and biodiversity (Schoof 1980; Fig. 3.5). Introduced riparian vegetation may substantially increase hydraulic and erosional resistance of stream
banks and reduce stream flow because of increased water uptake and transpiration
(e.g., tamarisk (Tamarix spp.) in the southwestern United States (Graf 1978) and
willow (Salix spp.) in Australia (Schulze and Walker 1997; Cremer 2003)).
Introduced riparian vegetation can also change nutrient cycling and habitat available for riverine species (Greenwood et al. 2004). As with the effects of indirect
alterations, direct alterations of the river corridor seldom have just one effect.
Instead, the initial alteration triggers a cascade of interrelated changes that can make
prediction and management of the river corridor very challenging (Fig. 3.6).
3.2.1 Changes in Channel and Floodplain Form
This section briefly reviews channel engineering, mining within the river corridor,
log floating, and floodplain drainage. Although other types of direct changes in
channel and floodplain form exist, the four categories of change discussed here are
among the most common and widespread.
Among the forms of direct alterations, channel engineering has a particularly
long history. Channel engineering refers to activities designed to simplify and
homogenize the channel form and to ‘harden’ the channel boundaries in order to
make them more resistant to erosion and to limit lateral or vertical channel changes.
Types of channel engineering include dredging of the channel bed, usually to
enhance navigation or downstream conveyance of flood waters or waste products.
Another form of channel engineering is stabilization of the channel banks, usually
3 Human Alterations of Rivers