19
South Carolina, USA (Braccia and Batzer 2001); and small mammals in southeastern
Australia (MacNally et al. 2001).
Forested river corridors were much more abundant prior to alteration of land
cover, flow regime, and channel-floodplain connectivity. Even ephemeral rivers in
deserts of the southwestern United States or central Australia included a forested
corridor of trees able to send their roots deep enough to access the water table
(Minckley and Rinne 1985; Dunkerley 2014). Wood loads, commonly expressed as
volume of wood per unit surface area of active channel or river corridor, were likely
orders of magnitude greater in natural forests than in contemporary managed forests
and river corridors from which large wood has been continuously removed for centuries (Wohl 2014a). Lower wood loads translate to lower levels of complexity and
connectivity within river corridors (Livers and Wohl 2016). Brierley et al. (2005)
describe how rapid, systematic clearance of riparian vegetation and removal of
instream wood in colonial societies (the Americas, Australia, New Zealand) caused
widespread alteration of river form and function in these regions.
A key point in understanding interactions in natural rivers is recognizing that the
balance among flow energy, sediment supply, large wood, and channel erosional
resistance changes across space and through time. Changes in form and process are
inherent in natural river corridors because water, sediment, and wood inputs change
across space and through time. These changes result in altered form and function
within the river corridor.
The implications of changing water and sediment inputs, in particular, can be
explored using three simple examples, one for each of the primary components of
the river corridor. The dimensions of the active channel reflect the volumes of water
and sediment supplied to the channel. In a channel with an erosionally resistant
boundary such as bedrock, only the largest flows will be capable of eroding the
channel boundaries. Cross-sectional channel area is likely to reflect the volume of
these large flows, even if they only occur once every few decades or centuries (Baker
1988; Wohl 2002). In a more readily eroded channel, the channel cross section may
enlarge during high discharges, but deposition of sediment and regrowth of riparian
vegetation during subsequent smaller flows may allow the channel cross-sectional
area to decrease with time (e.g., Friedman and Lee 2002). The shape and size of the
active channel can thus fluctuate over time spans of decades to centuries, but these
fluctuations may not result in any net change or trend in active channel geometry
over longer time spans.
A second example of the implications of changing water and sediment inputs
involves floodplains. The characteristics of a floodplain reflect both overbank flows
and lateral movements of the active channel through time. Floodplains are built via
the combined effects of vertical accretion, or settling of sediment from suspension
during overbank flows, and lateral accretion when the active channel migrates laterally across the valley bottom, leaving channel lag and bar deposits in its wake
(Nanson and Croke 1992; Dunne and Aalto 2013). Changing water and sediment
inputs to the river corridor can accelerate or reduce lateral channel migration rates
(Shields et al. 2000; Constantine et al. 2014); increase or decrease overbank flooding and sedimentation (Gomez et al. 1998; Owens and Walling 2002; Miller and
2.2 Controls on Physical Form and Process in River Corridors
South Carolina, USA (Braccia and Batzer 2001); and small mammals in southeastern
Australia (MacNally et al. 2001).
Forested river corridors were much more abundant prior to alteration of land
cover, flow regime, and channel-floodplain connectivity. Even ephemeral rivers in
deserts of the southwestern United States or central Australia included a forested
corridor of trees able to send their roots deep enough to access the water table
(Minckley and Rinne 1985; Dunkerley 2014). Wood loads, commonly expressed as
volume of wood per unit surface area of active channel or river corridor, were likely
orders of magnitude greater in natural forests than in contemporary managed forests
and river corridors from which large wood has been continuously removed for centuries (Wohl 2014a). Lower wood loads translate to lower levels of complexity and
connectivity within river corridors (Livers and Wohl 2016). Brierley et al. (2005)
describe how rapid, systematic clearance of riparian vegetation and removal of
instream wood in colonial societies (the Americas, Australia, New Zealand) caused
widespread alteration of river form and function in these regions.
A key point in understanding interactions in natural rivers is recognizing that the
balance among flow energy, sediment supply, large wood, and channel erosional
resistance changes across space and through time. Changes in form and process are
inherent in natural river corridors because water, sediment, and wood inputs change
across space and through time. These changes result in altered form and function
within the river corridor.
The implications of changing water and sediment inputs, in particular, can be
explored using three simple examples, one for each of the primary components of
the river corridor. The dimensions of the active channel reflect the volumes of water
and sediment supplied to the channel. In a channel with an erosionally resistant
boundary such as bedrock, only the largest flows will be capable of eroding the
channel boundaries. Cross-sectional channel area is likely to reflect the volume of
these large flows, even if they only occur once every few decades or centuries (Baker
1988; Wohl 2002). In a more readily eroded channel, the channel cross section may
enlarge during high discharges, but deposition of sediment and regrowth of riparian
vegetation during subsequent smaller flows may allow the channel cross-sectional
area to decrease with time (e.g., Friedman and Lee 2002). The shape and size of the
active channel can thus fluctuate over time spans of decades to centuries, but these
fluctuations may not result in any net change or trend in active channel geometry
over longer time spans.
A second example of the implications of changing water and sediment inputs
involves floodplains. The characteristics of a floodplain reflect both overbank flows
and lateral movements of the active channel through time. Floodplains are built via
the combined effects of vertical accretion, or settling of sediment from suspension
during overbank flows, and lateral accretion when the active channel migrates laterally across the valley bottom, leaving channel lag and bar deposits in its wake
(Nanson and Croke 1992; Dunne and Aalto 2013). Changing water and sediment
inputs to the river corridor can accelerate or reduce lateral channel migration rates
(Shields et al. 2000; Constantine et al. 2014); increase or decrease overbank flooding and sedimentation (Gomez et al. 1998; Owens and Walling 2002; Miller and
2.2 Controls on Physical Form and Process in River Corridors