20
Friedman 2009); and ultimately change the relative importance of vertical versus
lateral accretion (Nanson and Croke 1992). The relative dominance of lateral and
vertical accretion in turn has implications for river management. Agricultural lands
in the floodplain may be enriched by vertical accretion of organic-rich sediment, for
example, but largely removed where lateral channel erosion is followed by accretion of coarse-grained channel-bed sediments.
Floodplains along some rivers undergo repeated episodes of aggradation and
erosion, as described for high-energy, laterally stable channels in southeastern
Australia. Overbank deposition along these channels gradually builds a floodplain
of finer sediment during hundreds to thousands of years before a single large flood
strips the accumulated sediment to a basal layer of coarser sediment (Nanson 1986).
The sequence of floodplain sediment accumulation via vertical accretion then
begins again (Nanson 1986).
The final example of the effects of changing water and sediment inputs comes
from the hyporheic zone. The location and rate of downwelling from the streambed
into the hyporheic zone, and upwelling from the hyporheic zone into the active
channel, reflect pressure gradients within the surface and subsurface flow, as well as
the porosity and permeability of near-surface bed sediments (Tonina and Buffington
2009; Gooseff 2010). Interactions between river flow stage and ground water pressure dynamics govern hyporheic exchange flows at broad scales. These interactions
change with hydrologic conditions and may reverse during dry and wet periods. At
the local scale, downwelling occurs at obstacles to flow such as instream wood or
beaver dams and bedforms such as riffles and bars, with upwelling downstream
from the obstacle or in pools (Buffington and Tonina 2009; Wondzell et al. 2009).
Consequently, changes in the configuration of the active channel resulting from
altered water and sediment inputs also change the rate and location of hyporheic
exchange and this can influence water quality in the channel.
In summary, a healthy river continually adjusts to changing inputs of water,
sediment and, in forested rivers, large wood. Figure 2.4 visually summarizes the
conceptualization of river forms and physical processes discussed in this book.
Understanding the controls on physical form and process is vital for at least two
reasons. First, these underlie and support the biotic communities present in river
ecosystems. Second, river management can target either the inputs to a river
corridor (top row in Fig. 2.4) or the resulting forms and processes within the
river corridor (bottom row in Fig. 2.4), but the interactions between inputs and
resulting form and process must be recognized if management is to achieve
desired outcomes.
2.3 Rivers as Ecosystems
As noted in the introductory chapter, a river is most appropriately conceptualized as
an ecosystem because of the close coupling among water and sediment inputs;
channel configuration and substrate erosional resistance; biotic communities; water
2 Rivers as Ecosystems
Friedman 2009); and ultimately change the relative importance of vertical versus
lateral accretion (Nanson and Croke 1992). The relative dominance of lateral and
vertical accretion in turn has implications for river management. Agricultural lands
in the floodplain may be enriched by vertical accretion of organic-rich sediment, for
example, but largely removed where lateral channel erosion is followed by accretion of coarse-grained channel-bed sediments.
Floodplains along some rivers undergo repeated episodes of aggradation and
erosion, as described for high-energy, laterally stable channels in southeastern
Australia. Overbank deposition along these channels gradually builds a floodplain
of finer sediment during hundreds to thousands of years before a single large flood
strips the accumulated sediment to a basal layer of coarser sediment (Nanson 1986).
The sequence of floodplain sediment accumulation via vertical accretion then
begins again (Nanson 1986).
The final example of the effects of changing water and sediment inputs comes
from the hyporheic zone. The location and rate of downwelling from the streambed
into the hyporheic zone, and upwelling from the hyporheic zone into the active
channel, reflect pressure gradients within the surface and subsurface flow, as well as
the porosity and permeability of near-surface bed sediments (Tonina and Buffington
2009; Gooseff 2010). Interactions between river flow stage and ground water pressure dynamics govern hyporheic exchange flows at broad scales. These interactions
change with hydrologic conditions and may reverse during dry and wet periods. At
the local scale, downwelling occurs at obstacles to flow such as instream wood or
beaver dams and bedforms such as riffles and bars, with upwelling downstream
from the obstacle or in pools (Buffington and Tonina 2009; Wondzell et al. 2009).
Consequently, changes in the configuration of the active channel resulting from
altered water and sediment inputs also change the rate and location of hyporheic
exchange and this can influence water quality in the channel.
In summary, a healthy river continually adjusts to changing inputs of water,
sediment and, in forested rivers, large wood. Figure 2.4 visually summarizes the
conceptualization of river forms and physical processes discussed in this book.
Understanding the controls on physical form and process is vital for at least two
reasons. First, these underlie and support the biotic communities present in river
ecosystems. Second, river management can target either the inputs to a river
corridor (top row in Fig. 2.4) or the resulting forms and processes within the
river corridor (bottom row in Fig. 2.4), but the interactions between inputs and
resulting form and process must be recognized if management is to achieve
desired outcomes.
2.3 Rivers as Ecosystems
As noted in the introductory chapter, a river is most appropriately conceptualized as
an ecosystem because of the close coupling among water and sediment inputs;
channel configuration and substrate erosional resistance; biotic communities; water
2 Rivers as Ecosystems