Discharge

Discharge, the flow of water down a river, is the independent variable that largely determines the size of river channels and influences planform through the amplitude and wavelength of meanders (Schumm, 1977). Discharge and sediment is conveyed into the main river channel through tributaries. Natural divisions of river channels often occur at a tributary junction. Tributaries convey water and sediment into the main channel, and the morphological conditions near channel junctions may differ from those in reaches located upstream or downstream (Benda et al., 2004). Wallis et al. (2008) found a step-increase in channel width in post-confluence channels in studies of Australian rivers. In addition to the discrete or natural discontinuities in channel gradient and morphology, bed sediment size and flow properties that may occur at the tributary junctions, strong environmental gradients have also been found to occur up and downstream of confluences (Rice et al., 2006). Despite its influence on channel morphology, hydrology and ecology, discharge has not proved to be an effective criterion for classification, except where river size is a primary consideration (Knighton, 1998). More commonly discharge has been used to derive other classification parameters such as stream power, which has been widely used in the study of channel form, boundaries and thresholds (Chang, 1985, Nanson and Barbettitaylor, 1995, Simpson and Smith, 2001, Brierley and Fryirs, 2005, Marren et al., 2006, Harvey et al., 2008, Jain et al., 2008) and river classification schemes (e.g. Ferguson, 1987, Nanson and Knighton, 1996, Schmitt et al., 2007).

1.2.3.2. Valley setting

Although channel processes are driven by flow and sediment supply, the range of channel adjustments that are possible are often restricted by the valley setting (Charlton, 2008), which

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influences the valley confinement and slope, geology and geomorphology and channel network characteristics. Valley confinement refers to the extent to which lateral migration and channel change is inhibited by contact with the walls of the alluvial valley (Phillips, 2008). Valley confinement acts as a primary control on the differentiation of geomorphic process zones along rivers and has been used as a discriminating factor in a number of geomorphic classifications (e.g. Rosgen, 1994, Montgomery and Buffington, 1997, Brierley and Fryirs, 2000). Changes to valley width may result in significant transitions in in-stream river character and behaviour (Brierley and Fryirs, 2005), and have been demonstrated to be the dominant factor in determining channel pattern in some rivers (Fotherby, 2009). However determining valley confinement requires some degree of expertise in landscape interpretation. Valley slope affects the channel slope, which has been closely associated with changes in substrate (Knighton, 1991, Knighton, 1999). Channel slope, in conjunction with discharge, determines stream-power, which has been widely used to determine thresholds for channel planform (Marren et al., 2006).

Slope, in conjunction with discharge, has been studied to determine thresholds for change in channel planform and substrate (Table 1.1). If not influenced by uplift or variations in bedrock, the gradient of a stream will usually show a downstream decrease (Figure 1.2.) that is associated with an increase of discharge and a decrease in sediment size (Schumm, 1977).

Figure 1.4. Fields of river channel morphological pattern within the domain of slope versus channel-forming discharge (From Church, 2002).

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1.2.3.3. Litholology

Reach morphology may be associated with direct geological controls on channel processes. Geology affects the nature of sediment supply and acts as a constraint on channel adjustment through its influence on bed and bank material composition (Knighton, 1998). Lithology can also effect basin relief (Miller et al., 1990), and has been shown to play a significant role in influencing river structure and function at the reach scale (Harvey et al., 2008). Geological disturbance can disrupt channel networks and long profiles, with channel adjustments at the reach scale including changes in slope, lateral tilting and localised faulting (Charlton, 2008). Inherited geomorphic features such as antecedent fluvial and alluvial topography have also been shown to influence channel form (Phillips and Slattery, 2008, Phillips, 2008).

1.2.3.4. Channel planform

Channel planform has been used in the classification of rivers at least since Leopold and Wolman (1957) attempted to determine discharge and slope thresholds for straight,

meandering and braided channels. While it has been suggested that channel planform may be stage dependant (Kellerhals et al., 1976), many other studies have investigated the variation in form and process between straight, meandering and braided channels (Carson, 1984a, Carson, 1984b, Ferguson, 1987, Bledsoe and Watson, 2001, Burge, 2004, Crosato and Mosselman, 2009), and the types of channels studied has broadened to include anastamosing (Knighton and Nanson, 1993), anabranching (Nanson and Knighton, 1996, Burge, 2006), and multiple channel river patterns (Burge, 2004). Anthropogenic influences on channel planform have also been considered (Marston et al., 1995).

1.2.3.5. Substrate composition

The composition of the substrate is a common parameter in stream classification. The size of material on the bed of a river has a significant influence on sediment transport characteristics, resistance properties and channel form adjustment, with an overall tendency for bed material to decrease downstream (Knighton, 1999). The reach typically possesses a characteristic range of channel bed materials (Frissell et al., 1986) and a large number of studies have compared substrate composition between different types of rivers (Howard, 1987, Smith and Ferguson, 1995, Marren et al., 2006). A number of studies have considered the downstream

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variation in substrate composition in the disturbed Ringarooma Catchments in north-eastern Tasmania (Knighton, 1991, Knighton, 1999).

1.2.3.6. Anthropogenic effects

Over human history rivers and river catchments have become increasingly modified by landuse change within catchments, as well as by direct disturbance to river courses and channels (Brierley and Fryirs, 2005). Most rivers throughout the world have been subjected to some form of human disturbance, and in south-eastern Australian rivers many rivers have become deeper, wider, straighter and more homogenous in the period since European settlement (Brierley et al., 2005). Dams reduce or modify the discharge of rivers, with downstream geomorphic consequences to channel shape and gradient (Knighton, 1998), and water extraction for irrigation reduces summer discharges with potential consequences for aquatic biodiversity (Arthington et al., 2006). Altered floodplain drainage, changed river flow regimes and catchment landuse change combines with riparian vegetation removal to cause bank erosion and increase channel capacity (Brierley and Fryirs, 2005). River channels can also be modified by the addition of sediments by mining waste disposal, such as in the Ringarooma River in north-eastern Tasmania where both direct and indirect changes to the morphology of reaches has occurred as a result of mine tailings (Knighton, 1998).

1.2.3.7. Ecological effects

Geomorphology provides the physical habitat for ecological systems, but ecological systems also influence the hydro-geomorphology of rivers and streams. Stream bank vegetation has been shown to influence channel width and form (e.g. Brown, 1997, Brooks et al., 2003, Anderson et al., 2004, Gurnell et al., 2009), at the point where it occurs or downstream (Wipfli et al., 2007), while in-stream vegetation has been shown to influence channel form, point bar formation, and meander migration (Blanka and Kiss, 2006). Recent studies have emphasised the interdependence between biological and physical forms and processes (Corenblit et al., 2007, Fisher et al., 2007, Viles et al., 2008). Coarse or large woody debris (LWD) has also been shown to influence channel and floodplain morphology in rivers around the world (Gurnell and Petts, 2002, Gurnell et al., 2009), as well as in Australia (Nanson and Barbettitaylor, 1995, Brooks et al., 2003). Interactions between LWD and the stream channel

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stimulate structural heterogeneity (Naiman et al., 2005) and may initiate processes such as channel anabranching (Nanson and Knighton, 1996).