Form and process in rivers

A fundamental control on the form of a river is the composition of the surface material it passes through. Rivers that are surrounded by a floodplain formed of material transported and deposited by river are able to make large scale changes in channel dimensions and planform, while rivers that are controlled by bedrock are not (Jerie, 2003). Alluvial rivers are rivers in which the channel bed and banks are composed of mobile sediment and soil and in which discharge is the independent variable that largely determines the size and shape of river channels and influences planform through the amplitude and wavelength of meanders

(Schumm, 1977). Because human settlement has often occurred on the floodplains of alluvial rivers, they have long been a subject for human contemplation. Alluvial rivers are shaped by the magnitude and frequency of the floods that they experience (Knighton, 1998), and in recent time they have been a focus for studies attempting to develop an understanding between the form of rivers and the processes operating upon them.

Central to these studies have been the concepts of a channel-forming or dominant discharge, the discharge at which most sediment is transported over a long period of time (Benson, 1966), and bankfull stage, the stage at which the river channel is full and above which flow would exceed the active channel banks (Riley, 1972). In a highly influential work, Wolman and Miller (1960) suggested that while discharge events of varying magnitude, frequency and temporal structure influence channel form, the channel morphology of alluvial rivers appears to be associated with flows at or near the bankfull stage. This correlation between channel shape and bankfull discharge has since been widely debated in Australia (e.g. Pickup and Warner, 1976) as well as elsewhere.

In reality it is a vast simplification to say that only one flood size is responsible for

maintaining channel dimensions and in reality a range of flood sizes contribute (Jerie et al., 2003). Extreme flood events may substantially modify the river channel but occur rarely, in contrast to smaller discharges which, while individually may have little ability to modify the river channel, have a magnified contribution due to their high frequency. The frequency and variability of flows and the flood history also play an important role in alluvial river bank

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morphology. However ideas of a channel forming discharge to which the channel geometry becomes adjusted continue to be central to fluvial geomorphology studies (Doyle et al., 2007), with various models and generalisations realted to variability in channel and form process based on this concept. The magnitude and frequency of the most geomorphically effective flow has also been the subject of considerable debate (Leopold et al., 1964, Armstrong et al., 2012).

A strong theme in the study of alluvial channel form, and of fluvial geomorphology more generally, has been the dichotomy between broad generalisations and narrow specifics when trying to classify rivers. Rivers channels are often highly heterogeneous, with morphology seldom constant and often changing dramatically along a single reach. Standardized

approaches to landscape classification sacrifice heterogeneity in order to generalize, treating landscape units as homogenous entities wherein all members of the same class are considered to be equivalent and interchangeable (Church, 2011). Brierley et al.(2013) suggest new approaches must be developed that move beyond the reductionist approaches to landscape analysis, realistically framing and interpreting specific (local) instances in relation to generalizable understandings of trends and patterns. More recently, large remotely sensed data sets and new GIS techniques are providing some of these new approaches. The ‘riverscape’ framework (Carbonneau et al., 2012) for example, offer new tools which promise to preserve variability and spatial relationships while considering entire river systems.

Brierley et al. (2013) argue that this requires ‘reading the landscape’, where a bottom-up, constructivist approach is applied to identify landforms, assess their morphodynamics, and interpret the interaction and evolution of these features at reach and catchment scales. In this approach contextualized, place-based understandings can be used to detect where local differences matter, thereby addressing concerns for the transferability of insights between locations and the representativeness of sample or reference sites.

Variability also exists in rivers at much large scales. Australian rivers have important differences from those in most other parts of the world, with lower discharge and more variable flow regimes (Finlayson and McMahon, 1988) and a sediment load dominated by fine suspended particles and a reduced coarse sediment load because of the stability of the continent and the almost complete absence of Quaternary glaciations (Tooth and Nanson, 1995). European settlement has resulted in substantial changes to Australian rivers and

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riparian areas, especially in the south-east. Typically, rivers lined by stands of forest, and with channels containing extensive volumes of woody debris before European settlement are today characterized by floodplains that have been cleared for agriculture and channels that have been desnagged for navigation or flood conveyance (Brooks and Brierley, 2002).

There have also been major changes to channel morphology in many south-eastern Australian rivers since European settlement, with many rivers becoming deeper, wider and straighter. Brooks and Brierley, (2002) suggest explanations for this change have focused on the nature of the Australian flood regime, and more specifically the extreme flood variability of

Australian rivers (Finlayson and McMahon, 1988). These views suggest that the changes that have occurred fall within the natural range of variability of river behaviour, and that

European clearance and riparian land use were contributing factors superimposed on this natural instability (Erskine and Warner, 1988). As a result of these differences, Australian rivers have been shown to have relationships between form and process that are different to those in most other parts of the world (Brierley and Fryirs, 2005). Many of the

generalisations that were developed for rivers in other parts of the world are not relevant to Australian conditions.

Tasmanian rivers have significant differences to those in mainland Australia. Tasmanian hydrology is spatially highly variable, and although some rivers have a hydrology similar to mainland Australia, some Tasmanian rivers have been found to have low hydrological variability and relatively predictable flow regimes (DPIPWE, 2010). The highly variable Tasmanian landscape includes areas which have experienced relatively recent tectonic activity and glaciation (Sharples, 1996) and more extensive and intense periglacial processes than found elsewhere in Australia (Jerie et al., 2003). In addition, the large variations in hydrology and lithology have resulted in a wide variety of channel and floodplain forms across Tasmanian catchments (Cohen and Houshold, 2005). Also, the majority of Tasmanian rivers are unregulated, and many have large sections of forested floodplain, intact riparian vegetation and the presence of large woody debris (LWD). The material liberated by glacial and periglacial geomorphic processes in Tasmania has resulted in an increased coarse sediment load.

Rivers also change over time in response to floods or other natural or human induced

disturbance. Some changes, such as the downstream movement of in-channel sand bars have little effect on channel morphology, while other events, such as a river avulsion, may have

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dramatic impacts on channel morphology. Increasingly in fluvial geomorphology, there has been an emphasis on the ‘range of variability’ of a river, in geomorphic, hydrologic and ecologic terms (Brierley and Fryirs, 2005). This has particular implications for river management where a ‘stable’ river has generally been the goal.