45
secondary or cutoff channels, as well as creating new germination sites for riparian
vegetation and redistributing nutrients across the river corridor. Big floods recruit
large wood to the channel and floodplain and redistribute that wood into jams that
provide critical habitat for aquatic and terrestrial species. Big floods mobilize the
bed in coarse-grained channels, flushing interstitial fine sediment and maintaining
hyporheic exchange and fish spawning habitat. From a river ecosystem perspective,
big floods are vitally important to river health.
A critical corollary of change is the capacity of the river to adjust to disturbances.
Returning to the definition of physical integrity, the location, dimensions, and configuration of a river channel must be able to adjust to changing water and sediment
inputs. The organisms within the river corridor must be able to seek refuge during
disturbance and to migrate and recolonize following disturbance. Consequently, the
continuing occurrence of changes and the capacity to adjust to these changes characterize river integrity. Understanding the processes that drive change, as well as the
magnitude and frequency of these processes, is thus critical to understanding and
maintaining river health.
2.5.4 Context
The final C of river health is context, which here refers to the environmental setting
of the river corridor: the climate and precipitation regime, geology, topography,
biome, position in the river network, land use, and land use history. The physics and
chemistry underlying river process and form are consistent around the world, but
the specific details of local history as expressed through climate, geology, evolution,
and human history can differ enormously and these details matter to river integrity
and river management. Numerous classification systems have been proposed for
rivers in recognition of these differences. This section focuses on three that are
applied at relatively broad scales within or between river networks: ecoregions, process domains, and river styles.
Ecologists designate terrestrial and freshwater ecoregions based on characteristic
combinations of physical factors (topography, soils) and biotic communities
(Omernik 2004). An ecoregion is a relatively large area of land or water that contains a geographically distinct assemblage of natural communities (WWF 2013).
Bailey (1995), for example, describes 25 divisions of terrestrial ecoregions that
encompass 54 provinces within the United States. The World Wildlife Fund designates 76 freshwater ecoregions within North America (Fig. 2.17), about 45 of which
are within the United States (WWF 2013). Either terrestrial or freshwater ecoregions provide a broad-scale differentiation of common characteristics and a starting
point when seeking reference sites for river ecosystems. Unfortunately, only 17 of
the freshwater ecoregions in North America were rated as being relatively stable or
relatively intact, in terms of overall conservation status, in 2013 (WWF 2013).
These ecoregions in good condition are concentrated in Alaska and northern Canada.
The remainder of the ecoregions were rated as being critical, endangered, or vulner2.5 The 4Cs of River Health
secondary or cutoff channels, as well as creating new germination sites for riparian
vegetation and redistributing nutrients across the river corridor. Big floods recruit
large wood to the channel and floodplain and redistribute that wood into jams that
provide critical habitat for aquatic and terrestrial species. Big floods mobilize the
bed in coarse-grained channels, flushing interstitial fine sediment and maintaining
hyporheic exchange and fish spawning habitat. From a river ecosystem perspective,
big floods are vitally important to river health.
A critical corollary of change is the capacity of the river to adjust to disturbances.
Returning to the definition of physical integrity, the location, dimensions, and configuration of a river channel must be able to adjust to changing water and sediment
inputs. The organisms within the river corridor must be able to seek refuge during
disturbance and to migrate and recolonize following disturbance. Consequently, the
continuing occurrence of changes and the capacity to adjust to these changes characterize river integrity. Understanding the processes that drive change, as well as the
magnitude and frequency of these processes, is thus critical to understanding and
maintaining river health.
2.5.4 Context
The final C of river health is context, which here refers to the environmental setting
of the river corridor: the climate and precipitation regime, geology, topography,
biome, position in the river network, land use, and land use history. The physics and
chemistry underlying river process and form are consistent around the world, but
the specific details of local history as expressed through climate, geology, evolution,
and human history can differ enormously and these details matter to river integrity
and river management. Numerous classification systems have been proposed for
rivers in recognition of these differences. This section focuses on three that are
applied at relatively broad scales within or between river networks: ecoregions, process domains, and river styles.
Ecologists designate terrestrial and freshwater ecoregions based on characteristic
combinations of physical factors (topography, soils) and biotic communities
(Omernik 2004). An ecoregion is a relatively large area of land or water that contains a geographically distinct assemblage of natural communities (WWF 2013).
Bailey (1995), for example, describes 25 divisions of terrestrial ecoregions that
encompass 54 provinces within the United States. The World Wildlife Fund designates 76 freshwater ecoregions within North America (Fig. 2.17), about 45 of which
are within the United States (WWF 2013). Either terrestrial or freshwater ecoregions provide a broad-scale differentiation of common characteristics and a starting
point when seeking reference sites for river ecosystems. Unfortunately, only 17 of
the freshwater ecoregions in North America were rated as being relatively stable or
relatively intact, in terms of overall conservation status, in 2013 (WWF 2013).
These ecoregions in good condition are concentrated in Alaska and northern Canada.
The remainder of the ecoregions were rated as being critical, endangered, or vulner2.5 The 4Cs of River Health