62
undertaken within the river corridor. Indirect alterations such as changes in upland
vegetation, however, can strongly influence inputs of water, sediment, and nutrients to the river corridor and thus strongly influence river ecosystem function.
This section reviews the different types of indirect alterations and their effects on
river ecosystems.
The basic categories of indirect alterations are climate change; changes in
land cover; changes in topography; introduced upland species; water diversions
from other catchments; and urbanization (Table 3.1), which commonly includes
changes in land cover, topography, and species. The details of the magnitude and
duration of changes in water, sediment, nutrients, and large wood yields to river
corridors vary between specific sites, but the general direction (increase or
decrease) of changes in response to indirect alterations is consistent for many of
these categories. Deforestation and agriculture, for example, typically increase
sediment yields to river corridors, whereas urbanization increases water yields.
Climate change is an exception in that its effects are difficult to generalize
between regions. Although global average temperatures are increasing, the
effects of this increase on specific meteorological parameters such as mean
annual precipitation or precipitation intensity and seasonality vary significantly
between different watersheds around the world, as illustrated by two specific
examples (Fig. 3.1).
A confounding effect of climate change is that other human alterations of landscapes and river networks can limit the ability of species to migrate to or colonize
more suitable habitats if climate change renders a particular river segment unsuitable for the species (Palmer et al. 2009). Endangered bull trout (Salvelinus confluentus) in the Northern Rockies of the United States, for example, require
relatively cold water temperatures that typically occur at higher elevations within
mountainous river networks. As warming air temperatures reduce the abundance
of thermally suitable portions of river networks, human alterations such as flow
regulation, dams, and diversions that limit longitudinal connectivity or change
water temperature can prevent bull trout from migrating between suitable habitats.
This causes isolated populations to become genetically inbred and makes these
populations more susceptible to local extinction associated with a severe natural
or human-induced disturbance (Davies 2010; Wenger et al. 2011; Isaak et al.
2012). Similar scenarios have been described for freshwater fauna in global biodiversity hot spots such as southwestern Australia (Davies 2010). Restoration
options that focus on restoring connectivity and natural flow regimes are more
likely to ameliorate climate-related changes in flow and temperature and to
increase population resilience than are restoration activities focused on withinchannel rehabilitation (Beechie et al. 2013). Rehabilitation within a channel can
increase habitat abundance and diversity, but this may be of limited use if species
cannot migrate to other portions of a river network as climate change makes a river
segment unsuitable for the species.
3 Human Alterations of Rivers
undertaken within the river corridor. Indirect alterations such as changes in upland
vegetation, however, can strongly influence inputs of water, sediment, and nutrients to the river corridor and thus strongly influence river ecosystem function.
This section reviews the different types of indirect alterations and their effects on
river ecosystems.
The basic categories of indirect alterations are climate change; changes in
land cover; changes in topography; introduced upland species; water diversions
from other catchments; and urbanization (Table 3.1), which commonly includes
changes in land cover, topography, and species. The details of the magnitude and
duration of changes in water, sediment, nutrients, and large wood yields to river
corridors vary between specific sites, but the general direction (increase or
decrease) of changes in response to indirect alterations is consistent for many of
these categories. Deforestation and agriculture, for example, typically increase
sediment yields to river corridors, whereas urbanization increases water yields.
Climate change is an exception in that its effects are difficult to generalize
between regions. Although global average temperatures are increasing, the
effects of this increase on specific meteorological parameters such as mean
annual precipitation or precipitation intensity and seasonality vary significantly
between different watersheds around the world, as illustrated by two specific
examples (Fig. 3.1).
A confounding effect of climate change is that other human alterations of landscapes and river networks can limit the ability of species to migrate to or colonize
more suitable habitats if climate change renders a particular river segment unsuitable for the species (Palmer et al. 2009). Endangered bull trout (Salvelinus confluentus) in the Northern Rockies of the United States, for example, require
relatively cold water temperatures that typically occur at higher elevations within
mountainous river networks. As warming air temperatures reduce the abundance
of thermally suitable portions of river networks, human alterations such as flow
regulation, dams, and diversions that limit longitudinal connectivity or change
water temperature can prevent bull trout from migrating between suitable habitats.
This causes isolated populations to become genetically inbred and makes these
populations more susceptible to local extinction associated with a severe natural
or human-induced disturbance (Davies 2010; Wenger et al. 2011; Isaak et al.
2012). Similar scenarios have been described for freshwater fauna in global biodiversity hot spots such as southwestern Australia (Davies 2010). Restoration
options that focus on restoring connectivity and natural flow regimes are more
likely to ameliorate climate-related changes in flow and temperature and to
increase population resilience than are restoration activities focused on withinchannel rehabilitation (Beechie et al. 2013). Rehabilitation within a channel can
increase habitat abundance and diversity, but this may be of limited use if species
cannot migrate to other portions of a river network as climate change makes a river
segment unsuitable for the species.
3 Human Alterations of Rivers