77
Changes to a dam’s operating regime commonly involve one of three modifications. The first is releasing flows that mimic the natural hydrograph. This can involve
experimental floods to mimic naturally occurring peak flows in order to restore
particular channel forms or facilitate reproduction and dispersal of riverine organisms (Collier et al. 1997; Ortlepp and Mürle 2003; Wilcox and Shafroth 2013).
Changes to a dam’s operating regime can also focus on slowing the rate of change
in discharge released by a hydropower dam so that aquatic organisms are able to
move in response to changing flows (Meile et al. 2011). In some cases, a dam’s
operating regime is changed to create more fluctuations in base flow rather than
maintaining a static discharge throughout the year (Auble et al. 1994).
A second common modification to dam operations is to increase sediment supply
below the dam. Designing a dam to pass sediment is difficult and few dams are
designed to pass sediment (Shen 1999). Instead, sediment can be bypassed around
the dam or removed from the reservoir or elsewhere and dumped into the river
downstream from the dam (Sumi and Kantoush 2010). Sediment bypassing or direct
introduction below a dam tends to be expensive and it can be difficult to create a
balance between altered water and sediment yields below a dam that results in
desired physical and ecological effects.
A third basic type of modification to an existing dam involves modifying the dam
to release water from different depths within the reservoir (Patten et al. 2001;
Arthington and Pusey 2003; Shafroth et al. 2010). This improves the ability to regulate water temperature and chemistry below the dam.
3.2.3 Alterations of Riverine Organisms
This section reviews beaver trapping, introduced riverine species, commercial
extinction of species, floodplain deforestation, and riparian grazing as five particularly common examples of human alterations of biotic communities in river ecosystems. Recognition of alterations of riverine biota is important in the context of river
management for at least two reasons. First, removal of native species or introduction
of non-native species can substantially alter the physical form of rivers, as when
removal of beaver decreases floodplain wetlands or the introduction of non-native
riparian trees changes bank stability and channel width. Second, the presence of
non-native species may be the primary factor limiting recovery of native species that
are the target of river management. An example discussed in more detail later in this
chapter comes from the Colorado River in the United States, where the presence of
introduced fish species severely limits the recovery of endemic fish species, even
though habitat for endemic species is being restored.
Historical beaver trapping resulted from the fur trade, which caused nearextinction of Castor fiber in Eurasia and Castor canadensis in North America.
Beaver continue to be killed, largely to prevent the backwaters associated with their
dams or the mobilization of wood from their dams that can block downstream river
infrastructure such as road culverts. The primary emphasis now throughout Europe
3.2 Direct Alterations of River Networks and River Corridors
Changes to a dam’s operating regime commonly involve one of three modifications. The first is releasing flows that mimic the natural hydrograph. This can involve
experimental floods to mimic naturally occurring peak flows in order to restore
particular channel forms or facilitate reproduction and dispersal of riverine organisms (Collier et al. 1997; Ortlepp and Mürle 2003; Wilcox and Shafroth 2013).
Changes to a dam’s operating regime can also focus on slowing the rate of change
in discharge released by a hydropower dam so that aquatic organisms are able to
move in response to changing flows (Meile et al. 2011). In some cases, a dam’s
operating regime is changed to create more fluctuations in base flow rather than
maintaining a static discharge throughout the year (Auble et al. 1994).
A second common modification to dam operations is to increase sediment supply
below the dam. Designing a dam to pass sediment is difficult and few dams are
designed to pass sediment (Shen 1999). Instead, sediment can be bypassed around
the dam or removed from the reservoir or elsewhere and dumped into the river
downstream from the dam (Sumi and Kantoush 2010). Sediment bypassing or direct
introduction below a dam tends to be expensive and it can be difficult to create a
balance between altered water and sediment yields below a dam that results in
desired physical and ecological effects.
A third basic type of modification to an existing dam involves modifying the dam
to release water from different depths within the reservoir (Patten et al. 2001;
Arthington and Pusey 2003; Shafroth et al. 2010). This improves the ability to regulate water temperature and chemistry below the dam.
3.2.3 Alterations of Riverine Organisms
This section reviews beaver trapping, introduced riverine species, commercial
extinction of species, floodplain deforestation, and riparian grazing as five particularly common examples of human alterations of biotic communities in river ecosystems. Recognition of alterations of riverine biota is important in the context of river
management for at least two reasons. First, removal of native species or introduction
of non-native species can substantially alter the physical form of rivers, as when
removal of beaver decreases floodplain wetlands or the introduction of non-native
riparian trees changes bank stability and channel width. Second, the presence of
non-native species may be the primary factor limiting recovery of native species that
are the target of river management. An example discussed in more detail later in this
chapter comes from the Colorado River in the United States, where the presence of
introduced fish species severely limits the recovery of endemic fish species, even
though habitat for endemic species is being restored.
Historical beaver trapping resulted from the fur trade, which caused nearextinction of Castor fiber in Eurasia and Castor canadensis in North America.
Beaver continue to be killed, largely to prevent the backwaters associated with their
dams or the mobilization of wood from their dams that can block downstream river
infrastructure such as road culverts. The primary emphasis now throughout Europe
3.2 Direct Alterations of River Networks and River Corridors