73
conveyance of the logs. Naturally occurring large wood, large boulders, and bedrock knobs were blasted out, and floodplains and secondary channels were blocked
to prevent floating logs from becoming trapped in them (Fig. 3.8) (Young et al.
1994; Miller 2010). Log floating occurred wherever there was commercial timber
harvest and commonly involved the movement of thousands of logs each year.
Historical references describe these activities from the second century BC in the
Italian Alps (Comiti 2012) and at least as far back as the fifteenth century in central,
lowland Europe (Schama 1995). Log floating continued into the twentieth century
in regions such as Siberia and northern Sweden (Nilsson et al. 2005b).
Floodplain drainage lowers the riparian water table, usually to facilitate cropping
or other land uses in the floodplain. Drainage typically involves installing tile drains
and cutting a grid of channels into the floodplain to convey water to the natural
channel (Fig. 3.9). This activity, too, has a long history. The English invited Dutch
engineers to help develop floodplain drainage during the seventeenth century, for
example, and the library of early American agriculturalists such as George
Washington included English manuals of land drainage (Simco et al. 2009).
Floodplain drainage facilitates compaction and drying of floodplain soils, changing
nutrient retention and habitat abundance. This type of drainage was typically undertaken in connection with other alterations of the river corridor, such as construction
of levees and flow regulation (Wohl 2013).
3.2.2 Alterations of Fluxes of Materials and Movements
of Organisms
This section briefly reviews levees and flow regulation as two particularly widespread, human-induced alterations of fluxes and connectivity within river corridors.
Levees are naturally occurring depositional features at the boundary between the
channel and floodplain. Levees form where flow velocity declines as peak flows
move from the confined channel to the larger cross-sectional area and greater
hydraulic resistance of the floodplain. Artificial levees are linear mounds built along
the channel to completely or largely contain peak flows within the active channel.
Artificial levees reduce or eliminate lateral connectivity within the river corridor,
leading to drying and simplification of the floodplain. Artificial levees also result in
greater velocity and erosive energy within the active channel during peak flows.
This can enhance bed and bank erosion and downstream sedimentation. Levees also
facilitate human settlement or development within flood-prone zones, exacerbating
flood damages when the levees fail or are overtopped (Tobin 1995; Pinter 2005).
Flow regulation includes dams and diversions, although this section focuses on
dams. Construction of dams has a long history: the first record of a dam comes from
Jordan in 3000 BC (Smith 1971). The construction of large dams (> 15 m tall) and
major dams (> 150 m tall) greatly accelerated during the latter half of the twentieth
century (Gleick 2003) and numerous large dams continue to be built in Asia, Africa,
3.2 Direct Alterations of River Networks and River Corridors
conveyance of the logs. Naturally occurring large wood, large boulders, and bedrock knobs were blasted out, and floodplains and secondary channels were blocked
to prevent floating logs from becoming trapped in them (Fig. 3.8) (Young et al.
1994; Miller 2010). Log floating occurred wherever there was commercial timber
harvest and commonly involved the movement of thousands of logs each year.
Historical references describe these activities from the second century BC in the
Italian Alps (Comiti 2012) and at least as far back as the fifteenth century in central,
lowland Europe (Schama 1995). Log floating continued into the twentieth century
in regions such as Siberia and northern Sweden (Nilsson et al. 2005b).
Floodplain drainage lowers the riparian water table, usually to facilitate cropping
or other land uses in the floodplain. Drainage typically involves installing tile drains
and cutting a grid of channels into the floodplain to convey water to the natural
channel (Fig. 3.9). This activity, too, has a long history. The English invited Dutch
engineers to help develop floodplain drainage during the seventeenth century, for
example, and the library of early American agriculturalists such as George
Washington included English manuals of land drainage (Simco et al. 2009).
Floodplain drainage facilitates compaction and drying of floodplain soils, changing
nutrient retention and habitat abundance. This type of drainage was typically undertaken in connection with other alterations of the river corridor, such as construction
of levees and flow regulation (Wohl 2013).
3.2.2 Alterations of Fluxes of Materials and Movements
of Organisms
This section briefly reviews levees and flow regulation as two particularly widespread, human-induced alterations of fluxes and connectivity within river corridors.
Levees are naturally occurring depositional features at the boundary between the
channel and floodplain. Levees form where flow velocity declines as peak flows
move from the confined channel to the larger cross-sectional area and greater
hydraulic resistance of the floodplain. Artificial levees are linear mounds built along
the channel to completely or largely contain peak flows within the active channel.
Artificial levees reduce or eliminate lateral connectivity within the river corridor,
leading to drying and simplification of the floodplain. Artificial levees also result in
greater velocity and erosive energy within the active channel during peak flows.
This can enhance bed and bank erosion and downstream sedimentation. Levees also
facilitate human settlement or development within flood-prone zones, exacerbating
flood damages when the levees fail or are overtopped (Tobin 1995; Pinter 2005).
Flow regulation includes dams and diversions, although this section focuses on
dams. Construction of dams has a long history: the first record of a dam comes from
Jordan in 3000 BC (Smith 1971). The construction of large dams (> 15 m tall) and
major dams (> 150 m tall) greatly accelerated during the latter half of the twentieth
century (Gleick 2003) and numerous large dams continue to be built in Asia, Africa,
3.2 Direct Alterations of River Networks and River Corridors