76
their propagules via water transport (Jansson et al. 2000; Merritt and Wohl 2006;
Burke et al. 2009).
The fourth primary effect of dams is to substantially alter water chemistry by
changing water temperature, dissolved oxygen, and solute concentrations
(Humborg et al. 1997; Bednarek and Hart 2005). Many dams only release water
from the base of the dam. This water is likely to be colder and to have smaller suspended sediment concentrations than undammed flow on some rivers, which sometimes favors introduced aquatic species over native species (King et al. 1998;
Clarkson and Childs 2000).
The details of how a particular dam affects upstream and downstream portions of
a river depend on at least three factors (Ligon et al. 1995; Poff and Hart 2002). First
of these is the physical and ecological characteristics of the river. Details such as
tributary inputs downstream from the dam, the presence of erosionally resistant layers in the channel bed or banks (Jiongxin 1996), or the pre-dam form of the river
(Musselman 2011) influence physical responses to the altered water and sediment
regime below the dam. Ecological details such as the specific habitats required by
downstream organisms, the migratory and reproductive patterns of those organisms,
or the degree of endemism among organisms influence the ecological responses
below the dam.
Second, the size and operating regime of the dam influence the dam’s effects on
the river ecosystem (Poff and Hart 2002). Dams can be operated primarily for water
storage or for hydroelectric power generation, for example, which results in very
different patterns of water release from the dam and differences in the degree to
which flow regime downstream the dam is altered.
Finally, whether the dam is isolated or part of a series of dams along the river
governs the dam’s effects on the river ecosystem (Skalak et al. 2013). The effects of
changes in water and sediment supply, in particular, vary with distance downstream
from the dam. For the large, historically braided Missouri River in the USA, Skalak
et al. (2013) document consistent downstream changes over distances of 100 to
140 km below each dam, but multiple dams with closer downstream spacing interrupt this spatial sequence.
A secondary effect of dams is that, by destabilizing the river corridor downstream, the dam can result in additional channel engineering. Sediment storage
behind a dam can result in accelerated channel erosion, for example, leading to bed
and bank stabilization as mitigation measures (Gendaszek et al. 2012).
Dam removal or changes to the operating regime of a dam can mitigate the negative effects created by the dam. Dam removal has been undertaken at many sites in
the United States, with varying degrees of success in restoring river ecosystems
(Bednarek 2001; Stanley and Doyle 2003). Although more than 1200 dams had
been removed in the USA as of 2016, fewer than 10% of these removals have been
scientifically evaluated and most evaluations involve only short-term (1–2 years)
monitoring (Bellmore et al. 2016). Although restoration of longitudinal connectivity
and a more natural flow regime may be immediate following dam removal, adjustments in sediment transport and channel and floodplain form both upstream and
downstream from the former dam may continue for decades (Pizzuto 2002).
3 Human Alterations of Rivers
their propagules via water transport (Jansson et al. 2000; Merritt and Wohl 2006;
Burke et al. 2009).
The fourth primary effect of dams is to substantially alter water chemistry by
changing water temperature, dissolved oxygen, and solute concentrations
(Humborg et al. 1997; Bednarek and Hart 2005). Many dams only release water
from the base of the dam. This water is likely to be colder and to have smaller suspended sediment concentrations than undammed flow on some rivers, which sometimes favors introduced aquatic species over native species (King et al. 1998;
Clarkson and Childs 2000).
The details of how a particular dam affects upstream and downstream portions of
a river depend on at least three factors (Ligon et al. 1995; Poff and Hart 2002). First
of these is the physical and ecological characteristics of the river. Details such as
tributary inputs downstream from the dam, the presence of erosionally resistant layers in the channel bed or banks (Jiongxin 1996), or the pre-dam form of the river
(Musselman 2011) influence physical responses to the altered water and sediment
regime below the dam. Ecological details such as the specific habitats required by
downstream organisms, the migratory and reproductive patterns of those organisms,
or the degree of endemism among organisms influence the ecological responses
below the dam.
Second, the size and operating regime of the dam influence the dam’s effects on
the river ecosystem (Poff and Hart 2002). Dams can be operated primarily for water
storage or for hydroelectric power generation, for example, which results in very
different patterns of water release from the dam and differences in the degree to
which flow regime downstream the dam is altered.
Finally, whether the dam is isolated or part of a series of dams along the river
governs the dam’s effects on the river ecosystem (Skalak et al. 2013). The effects of
changes in water and sediment supply, in particular, vary with distance downstream
from the dam. For the large, historically braided Missouri River in the USA, Skalak
et al. (2013) document consistent downstream changes over distances of 100 to
140 km below each dam, but multiple dams with closer downstream spacing interrupt this spatial sequence.
A secondary effect of dams is that, by destabilizing the river corridor downstream, the dam can result in additional channel engineering. Sediment storage
behind a dam can result in accelerated channel erosion, for example, leading to bed
and bank stabilization as mitigation measures (Gendaszek et al. 2012).
Dam removal or changes to the operating regime of a dam can mitigate the negative effects created by the dam. Dam removal has been undertaken at many sites in
the United States, with varying degrees of success in restoring river ecosystems
(Bednarek 2001; Stanley and Doyle 2003). Although more than 1200 dams had
been removed in the USA as of 2016, fewer than 10% of these removals have been
scientifically evaluated and most evaluations involve only short-term (1–2 years)
monitoring (Bellmore et al. 2016). Although restoration of longitudinal connectivity
and a more natural flow regime may be immediate following dam removal, adjustments in sediment transport and channel and floodplain form both upstream and
downstream from the former dam may continue for decades (Pizzuto 2002).
3 Human Alterations of Rivers