44
cation and homogenization has resulted in widespread negative consequences,
however, from loss of flood attenuation and nutrient uptake to sharp declines in
biodiversity. Consequently, river managers and society as a whole are emphasizing
the need to restore physical complexity. Restoration can ta ke the form of engineering channel and floodplain environments to be more spatially heterogeneous,
such as adding sinuosity to channels or introducing large boulders and instream
wood to channels (Nilsson et al. 2005; Helfield et al. 2007; Polvi et al. 2014).
Restoration of physical complexity can also involve restoring processes that create and maintain spatial heterogeneity, such as reducing flow regulation in order
to create more natural water and sediment regimes (Poff et al. 1997; Wohl et al.
2015).
6. Complexity reflects the inherent unevenness through time and space of processes
within a drainage basin and river corridor. This unevenness is reflected in biogeochemical reactions by the existence of hot spots and hot moments. Hot spots
are patches with disproportionately high reaction rates relative to the surrounding matrix. Hot moments refer to short periods with disproportionately high
reaction rates relative to longer intervening time periods (McClain et al. 2003).
Unevenness is reflected in sediment dynamics by numerous examples of drainages in which a small portion of the catchment contributes most of the sediment
(e.g., Meade et al. 1985; Meade 2007) and the great majority of sediment flux
occurs during a very short period of time (e.g., Farnsworth and Milliman 2003;
Kao and Milliman 2008). Unevenness is represented by organic carbon storage
in river networks in which limited portions of the network store the majority of
carbon (e.g., Wohl et al. 2012).
In summary, diverse forms of complexity drive physical processes, biogeochemical processes, habitat diversity, and biodiversity in river ecosystems. Consequently,
understanding the types of complexity present in a river ecosystem, as well as the
factors that create and maintain the complexity, are critical to understanding and
maintaining river health.
2.5.3 Change
Changes through space and time are inherent in natural river corridors. Tributaries
differ from the main channel. Valley geometry and disturbance regime vary longitudinally. Disturbances such as seasonal floods occur at predictable intervals, but
other disturbances such as drought or forest blowdown occur at longer, more irregular intervals. Heredotus’ famous phrase that you can never step in the same river
twice incorporates many layers of meaning.
Both spatial and temporal changes are critical to river integrity because they create and maintain habitat diversity. Human societies tend to regard large floods as
disasters, for example, but these high-energy flows are critical sources of complexity in river corridors. The big floods promote channel avulsion and the formation of
2 Rivers as Ecosystems
cation and homogenization has resulted in widespread negative consequences,
however, from loss of flood attenuation and nutrient uptake to sharp declines in
biodiversity. Consequently, river managers and society as a whole are emphasizing
the need to restore physical complexity. Restoration can ta ke the form of engineering channel and floodplain environments to be more spatially heterogeneous,
such as adding sinuosity to channels or introducing large boulders and instream
wood to channels (Nilsson et al. 2005; Helfield et al. 2007; Polvi et al. 2014).
Restoration of physical complexity can also involve restoring processes that create and maintain spatial heterogeneity, such as reducing flow regulation in order
to create more natural water and sediment regimes (Poff et al. 1997; Wohl et al.
2015).
6. Complexity reflects the inherent unevenness through time and space of processes
within a drainage basin and river corridor. This unevenness is reflected in biogeochemical reactions by the existence of hot spots and hot moments. Hot spots
are patches with disproportionately high reaction rates relative to the surrounding matrix. Hot moments refer to short periods with disproportionately high
reaction rates relative to longer intervening time periods (McClain et al. 2003).
Unevenness is reflected in sediment dynamics by numerous examples of drainages in which a small portion of the catchment contributes most of the sediment
(e.g., Meade et al. 1985; Meade 2007) and the great majority of sediment flux
occurs during a very short period of time (e.g., Farnsworth and Milliman 2003;
Kao and Milliman 2008). Unevenness is represented by organic carbon storage
in river networks in which limited portions of the network store the majority of
carbon (e.g., Wohl et al. 2012).
In summary, diverse forms of complexity drive physical processes, biogeochemical processes, habitat diversity, and biodiversity in river ecosystems. Consequently,
understanding the types of complexity present in a river ecosystem, as well as the
factors that create and maintain the complexity, are critical to understanding and
maintaining river health.
2.5.3 Change
Changes through space and time are inherent in natural river corridors. Tributaries
differ from the main channel. Valley geometry and disturbance regime vary longitudinally. Disturbances such as seasonal floods occur at predictable intervals, but
other disturbances such as drought or forest blowdown occur at longer, more irregular intervals. Heredotus’ famous phrase that you can never step in the same river
twice incorporates many layers of meaning.
Both spatial and temporal changes are critical to river integrity because they create and maintain habitat diversity. Human societies tend to regard large floods as
disasters, for example, but these high-energy flows are critical sources of complexity in river corridors. The big floods promote channel avulsion and the formation of
2 Rivers as Ecosystems