43
Complexity is important in the context of river integrity or river health for at least
six reasons.
1. Complexity in the form of habitat diversity can correlate with biodiversity.
Although other factors such as introduced species or limited connectivity can
constrain biodiversity, several studies find that greater habitat diversity in river
corridors correlates with greater biodiversity and biological productivity (Scott
et al. 2003; Luck et al. 2010; Bellmore and Baxter 2014; Greene and Knox
2014).
2. Complexity influences attenuation of downstream fluxes. Diverse forms of complexity—from bedforms and instream wood to extensive floodplains with varying surface topography and vegetation communities—increase hydraulic
resistance, reduce average downstream velocity, enhance hyporheic exchange,
and facilitate at least transient deposition of organic matter and sediment (Jeffries
et al. 2003; Brummer et al. 2006; Ensign and Doyle 2006; Westbrook et al. 2006;
Gooseff et al. 2007; Small et al. 2008; Wohl and Scott 2017). If organic matter is
stored even for a short period of a few minutes to hours, it becomes more available to aquatic and riparian organisms (Battin et al. 2008) and thus helps to support greater biological productivity within the river corridor.
3. Complexity influences the resistance and resilience of river corridors to disturbance. A more complex river corridor can be more resistant and resilient to disturbance, as illustrated by a river corridor in which numerous channel-spanning
logjams promote formation of an anabranching channel (Wohl 2011; Collins
et al. 2012). During peak flows, flood waters spread among the multiple channels
and across the floodplain. Velocity and flow energy are reduced because of the
relatively shallow flow depths and large hydraulic resistance, so the river corridor is more resistant to the flood disturbance than it would be in the absence of
the complexity associated with the logjams. The spatial heterogeneity of the
channel boundaries creates multiple low-velocity refuges for fish and other
organisms during the flood, allowing rapid recolonization of habitats throughout
the river corridor following the flood and thus increasing the resilience of the
river corridor to disturbance.
4. Complexity both reflects and influences processes in rivers. The spatial heterogeneity present in the channel, floodplain, and hyporheic zone each reflect the
combined history of hydraulic force, sediment movement, and biotic colonization (vegetation growth, beaver dams) through time. Habitat diversity, for example, strongly correlates with channel mobility (Choné and Biron 2016).
Complexity influences river processes when bedforms or instream wood increase
hydraulic resistance, which in turn influences hydraulic force and sediment
movement (e.g., Yochum et al. 2012), or when logjams abandoned during channel migration and then buried in the floodplain form erosionally resistant points
that limit subsequent channel migration (Collins et al. 2012).
5. Enhancing complexity is increasingly an explicit goal of river management.
Most of the history of river management has involved making river corridors
simpler and more homogeneous, as discussed in the next chapter. This simplifi2.5 The 4Cs of River Health
Complexity is important in the context of river integrity or river health for at least
six reasons.
1. Complexity in the form of habitat diversity can correlate with biodiversity.
Although other factors such as introduced species or limited connectivity can
constrain biodiversity, several studies find that greater habitat diversity in river
corridors correlates with greater biodiversity and biological productivity (Scott
et al. 2003; Luck et al. 2010; Bellmore and Baxter 2014; Greene and Knox
2014).
2. Complexity influences attenuation of downstream fluxes. Diverse forms of complexity—from bedforms and instream wood to extensive floodplains with varying surface topography and vegetation communities—increase hydraulic
resistance, reduce average downstream velocity, enhance hyporheic exchange,
and facilitate at least transient deposition of organic matter and sediment (Jeffries
et al. 2003; Brummer et al. 2006; Ensign and Doyle 2006; Westbrook et al. 2006;
Gooseff et al. 2007; Small et al. 2008; Wohl and Scott 2017). If organic matter is
stored even for a short period of a few minutes to hours, it becomes more available to aquatic and riparian organisms (Battin et al. 2008) and thus helps to support greater biological productivity within the river corridor.
3. Complexity influences the resistance and resilience of river corridors to disturbance. A more complex river corridor can be more resistant and resilient to disturbance, as illustrated by a river corridor in which numerous channel-spanning
logjams promote formation of an anabranching channel (Wohl 2011; Collins
et al. 2012). During peak flows, flood waters spread among the multiple channels
and across the floodplain. Velocity and flow energy are reduced because of the
relatively shallow flow depths and large hydraulic resistance, so the river corridor is more resistant to the flood disturbance than it would be in the absence of
the complexity associated with the logjams. The spatial heterogeneity of the
channel boundaries creates multiple low-velocity refuges for fish and other
organisms during the flood, allowing rapid recolonization of habitats throughout
the river corridor following the flood and thus increasing the resilience of the
river corridor to disturbance.
4. Complexity both reflects and influences processes in rivers. The spatial heterogeneity present in the channel, floodplain, and hyporheic zone each reflect the
combined history of hydraulic force, sediment movement, and biotic colonization (vegetation growth, beaver dams) through time. Habitat diversity, for example, strongly correlates with channel mobility (Choné and Biron 2016).
Complexity influences river processes when bedforms or instream wood increase
hydraulic resistance, which in turn influences hydraulic force and sediment
movement (e.g., Yochum et al. 2012), or when logjams abandoned during channel migration and then buried in the floodplain form erosionally resistant points
that limit subsequent channel migration (Collins et al. 2012).
5. Enhancing complexity is increasingly an explicit goal of river management.
Most of the history of river management has involved making river corridors
simpler and more homogeneous, as discussed in the next chapter. This simplifi2.5 The 4Cs of River Health