Spatial variability in the timing, nature and extent of channel response to typical human disturbance along the Upper Hunter River, New South Wales, Australia

Abstract

Prior to European settlement, the Upper Hunter River near Muswellbrook, New SouthWales, was a passively meandering gravel-bed river of moderate sinuosity and relatively uniform channel width. Analyses of floodplain sedimentology, archival records, parish maps and aerial photographs document marked spatial variability in the pattern of channel change since European settlement in the 1820s. Different types, rates and extents of change are reported for seven zones of adjustment along an 8 km study reach. This variable adjustment reflects imposed antecedent controls (buried terrace material and bedrock), which have significantly influenced local variability in river sensitivity to change, as well as contemporary morphodynamics and geomorphic complexity. Local variability in system responses to disturbance has important implications for future river management and rehabilitation.Lagged response and variability in the timing and magnitude of channel adjustment to imposed disturbances are persistent themes in fluvial geomorphology. Lagged response to disturbance is often explained in terms of the recurrence of floods of a given magnitude and duration that exceed the threshold conditions lowered by a disturbance (see, e.g., Schumm, 1979). It can also be examined in the context of a sequence of floods that magnifies the response of individual floods (Costa and O'Connor, 1995;Nanson, 1986;Newson, 1980). In south-eastern Australia multi-decadal cycles in the flood regime, known as flood and drought dominated regimes (FDRs and DDRs), have been invoked to explain channel change in some rivers (Erskine and Warner, 1988;Warner, 1987). While it has been demonstrated that these secular climatic phases do not represent the ultimate cause of channel metamorphosis in south-eastern Australia over the last 200 years Brierley, 1997, 2000;Brooks et al., 2003), they may account for lagged responses, particularly in circumstances where the disturbance occurs within a DDR and several decades pass before floods of an appropriate magnitude cause the necessary threshold exceedance and geomorphic response. Furthermore, there is little doubt that in the post-disturbance period such flood and drought phases exert a strong bearing on channel morphodynamics.It is now clear that European settlement of Australia in the late 18th and early 19th centuries significantly altered fluvial dynamics in many rivers (Brooks and Brierley, 1997;Olley and Wasson, 2003;Prosser et al., 2001). For 2000 years prior to European settlement rivers in coastal south-eastern Australia were characterized by relative geomorphic stability (Nanson and Doyle, 1999). Large scale channel adjustments such as lateral migration were rare and overbank deposition of fine material dominated (Rustomji et al., 2006). Hydraulic roughness was high as a result of well vegetated riparian zones and instream large woody debris (Brooks et al., 2003). Following European settlement, unprecedented channel change resulted from a combination of riparian vegetation and wood removal Brooks and Brier...

Antecedent controls on river character and behaviour in partly confined valley settings: Upper Hunter catchment, NSW, Australia

“…7). Given their more downstream location where sediment supply is highest, the floodplains tend to bury these features (see Hoyle et al, 2008). Fans also occur in the upper parts of Dart Brook and act as a significant control along the low sinuosity planform-controlled floodplain pockets.…”

“…While these drapes tend to be very thin (between 10s of centimetres to 2 m), the surface expression of the confining features is almost nonexistent. While planview perspectives may give the impression that these are alluvial rivers, field evidence indicates that these partly confined rivers have limited capacity to adjust (Hoyle et al, 2008).…”

From the myth of a lost paradise to targeted river restoration: forget natural references and focus on human benefits

“…From a practical point of view, a simple scheme should be promoted to make sure that all aspects are developed Step one: diagnosis, problem statement and potential functioning appraisal. First, we need to know the historical trajectory of the system to be restored, to understand its properties, to identify both natural and human influences on the past landscape (Antrop, 2005;Brierley and Fryirs, 2005;Wohl, 2005;Hoyle et al, 2008). What are the critical past and present processes?…”

The relationship between geomorphic river adjustment and management actions over the last 50 years in the Upper Hunter Catchment, NSW, Australia

“…Based on archival maps including the first Parish maps of the early 1880s and aerial photographic evidence, verified through field investigations, three major phases of river adjustment have been identified in the upper Hunter catchment since European settlement (Hoyle et al, 2008;Fryirs et al, in press). Initial channel incision and expansion began between 1881 and 1938 in response to human disturbance associated with European settlement.…”