A Comparison of Holocene and Historical Channel Assignment

Assignment Task

Abstract

Changes in channel morphology that occurred along the Macdonald River between 1949 and 1955 are often cited as an example of catastrophic channel change. However, the question of whether these changes represented one component of a cyclical evolutionary pattern, or a systematic and persistent shift to a new morphologic state remains to be clearly defined, as does the significance of these dramatic channel changes when viewed against the river’s longer-term Holocene history. In this paper, new measurements of the Macdonald River’s channel morphology are used to resolve the river’s evolution in the ∼50 years since these major channel-altering floods. By 2002, the Macdonald River’s bed had narrowed considerably from its 1955 post-flood maximum due to the construction of a new floodplain surface within the widened channel. In some locations, the 2002 bed width is comparable to that of the pre-1949 channel. This aspect of the river’s evolution follows models of cyclical channel evolution proposed for the region. However, in light of recent research into the river’s longer term Holocene evolution, it is clear that other channel changes that occurred in response to the 1949–1955 floods, particularly the 2 metres of river bed aggradation and 7 kilometres down-valley shift in the thalweg’s intersection with mean sea level, are less cyclic in nature. The capacity of coastal rivers to prograde into estuaries, which in turn induces river bed aggradation, can be seen in this case to counteract the incisional tendencies associated with post-flood channel contraction, such that persistent river bed aggradation occurs.

Introduction

The Macdonald River in the northern Sydney Basin, Australia, is often cited as an example of a river that experienced ‘catastrophic channel change’ in response to a series of large floods that occurred between 1949 and 1955 (Nanson, 1986; Erskine and Warner, 1988; Kirkup et al., 1998; Rutherfurd, 2000; Young et al., 2001; Brooks et al., 2003; Butzer and Helgren, 2005). Henry (1977) and Erskine (1986) reconstructed a widening and straightening of the channel in response to these floods as well as 2–3 m of river bed aggradation. Contemporaneous and similar channel changes occurred in other nearby streams and rivers (for example, Erskine et al., 1992; Erskine, 1996) affected by the same sequence of floods, which marked the onset of a period of exceptionally high rainfall and enhanced flood activity (Pittock, 1975; Pickup, 1976; Erskine and Bell, 1982; Franks and Kuczera, 2002). The occurrence of alternating decadal-scale wet and dry periods in the New South Wales climatic record, referred to as ‘flood and drought dominated regimes’ by Warner (1987), coupled with evidence for synchronous adjustment in river channel morphology, led Nanson and Erskine (1988) to suggest that channel morphology in many New South Wales rivers was rarely in a singular state of equilibrium, but rather shifted. between distinct morphologic states characterised by a 30–50 year periodicity. Indeed, surveys of the morphology of the Macdonald River in the late 1970s led Erskine (1986) to conclude that the river had shown ‘no tendency for the channel to recover from the 1949–55 floods’, which is consistent with the timescale of river response suggested by Nanson and Erskine (1988).

Whilst the effects on channel morphology of the 1949–1955 and subsequent floods such as those of 1978 are well documented (Erskine and Melville, 1983), the significance of these channel changes, which were large by historical standards, remains to be evaluated against the longer term history of channel evolution. Such long-term perspectives comparing historic (defined as the post-European settlement period) and pre-historic Holocene channel evolution are comparatively rare in Australia, with the study by Brooks et al. (2003) of the pre and post-European settlement histories of the Cann and Thurra Rivers in Victoria being a notable contribution in the literature. They showed that European disturbance to the channel caused a systematic shift in the morphology of the Cann River, from which it is yet to recover. In this study, new measurements of the channel morphology of the Macdonald River are combined with modelling of important characteristics of the river’s hydraulics and hydrology and recent findings pertaining to the river’s Holocene history (drawn from Rustomji et al., 2006), to provide a long-term perspective on the channel changes the Macdonald River experienced in the mid-twentieth century. Furthermore, this paper extends the documented history of channel change along the river (Dyson, 1966; Henry, 1977; Erskine and Melville, 1983; Erskine, 1986) to encompass the full duration of the twentieth century. These new data are used to highlight both cyclical and non-cyclical aspects of channel change along the Macdonald River.

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