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Placer and aggregate mining refer to removal of sediment from the river corridor.
Placer metals are precious metals—usually gold—disseminated through alluvial
sediment in the channel and floodplain. Aggregate is sand and gravel used in various
forms of construction. In each case, mining involves displacing or removing massive quantities of channel and floodplain sediment from the river corridor. This displacement and removal cause changes in valley-bottom topography and increase
sediment supply and mobility. This in turn decreases habitat abundance and stability, as well as biomass and biodiversity (Van Nieuwenhuyse and LaPerriere 1986;
McLeay et al. 1987; Gilvear et al. 1995). Placer and aggregate mining can be conducted on an industrial scale with heavy machinery (Fig. 3.7) or as artisanal mining
with hand tools. Artisanal mining does not necessarily have a lesser impact on the
river corridor, however, if it involves hundreds or even thousands of individuals
working simultaneously in a concentrated portion of the river (Cordy et al. 2010).
Log floating refers to the use of channels to convey cut timber downstream to
collection points or sawmills. Although this might sound relatively benign, log
floating historically involved substantial modification of flow regime and channel
boundaries. Log floating was conducted throughout river networks, including headwater channels barely wide enough to contain the diameter of a single log (Cowan
2003; McMahon and Karamanski 2009).
Splash dams modified flow regime. These temporary dams collected water and
cut logs. Once filled, a splash dam was dynamited, sending an outburst flood and
Fig. 3.7 2016 aerial view of aggregate mining within the river corridor of the Rio Maipo near
Santiago, Chile. Yellow arrow indicates flow direction. Inset photo at upper right is an upstream
view and photo at lower right is a downstream view from ground level, both taken in 2012. (Aerial
photograph courtesy of Google Earth)
3.2 Direct Alterations of River Networks and River Corridors
Placer and aggregate mining refer to removal of sediment from the river corridor.
Placer metals are precious metals—usually gold—disseminated through alluvial
sediment in the channel and floodplain. Aggregate is sand and gravel used in various
forms of construction. In each case, mining involves displacing or removing massive quantities of channel and floodplain sediment from the river corridor. This displacement and removal cause changes in valley-bottom topography and increase
sediment supply and mobility. This in turn decreases habitat abundance and stability, as well as biomass and biodiversity (Van Nieuwenhuyse and LaPerriere 1986;
McLeay et al. 1987; Gilvear et al. 1995). Placer and aggregate mining can be conducted on an industrial scale with heavy machinery (Fig. 3.7) or as artisanal mining
with hand tools. Artisanal mining does not necessarily have a lesser impact on the
river corridor, however, if it involves hundreds or even thousands of individuals
working simultaneously in a concentrated portion of the river (Cordy et al. 2010).
Log floating refers to the use of channels to convey cut timber downstream to
collection points or sawmills. Although this might sound relatively benign, log
floating historically involved substantial modification of flow regime and channel
boundaries. Log floating was conducted throughout river networks, including headwater channels barely wide enough to contain the diameter of a single log (Cowan
2003; McMahon and Karamanski 2009).
Splash dams modified flow regime. These temporary dams collected water and
cut logs. Once filled, a splash dam was dynamited, sending an outburst flood and
Fig. 3.7 2016 aerial view of aggregate mining within the river corridor of the Rio Maipo near
Santiago, Chile. Yellow arrow indicates flow direction. Inset photo at upper right is an upstream
view and photo at lower right is a downstream view from ground level, both taken in 2012. (Aerial
photograph courtesy of Google Earth)
3.2 Direct Alterations of River Networks and River Corridors