Vignettes > Post-European settlement disturbance response of rivers in Bega catchment, South Coast, NSW, Australia

Post-European settlement disturbance response of rivers in Bega catchment, South Coast, NSW, Australia

Kirstie Fryirs
Macquarie University
Author Profile

Shortcut URL: http://serc.carleton.edu/35398

Location

Continent: Australia
Country: Australia
State/Province:New South Wales
City/Town: Bega catchment
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy, Chronology


Figure 1 Evolution of rivers in various parts of Bega catchment since European settlement (modified from Brierley et al. 1999). Details


Figure 2 Post European settlement sediment budget flow diagram for Bega catchment (modified from Fryirs and Brierley, 2001). Details


Figure 3 Variability in geomorphic process zone distribution in Bega catchment prior to European settlement and today. Details



Description

Human activities have altered river systems across most of our planet. Most rivers now operate under fundamentally different conditions to those that existed prior to human disturbance. Habitat loss brought about by human-induced changes to river character and behaviour has eliminated native flora and fauna, altered spatial ranges and interactions, and promoted the incursion of exotic species. In many instances, simplification of river courses has reduced the geomorphic complexity of channels and altered channel-floodplain connectivity, reducing the diversity of habitat and the availability of aquatic refugia. The negative consequences of these actions, have prompted numerous calls for action in efforts to promote an era of river repair (e.g. Wohl, 2004; Brierley and Fryirs, 2008).

Human impacts on biophysical processes in rivers are dramatically exemplified in Australia, where impacts of European settlement since 1788 can be related to a defined pre-disturbance condition. In Bega catchment, on the south coast of New South Wales (NSW), landscape changes since European settlement have fundamentally altered river structure throughout virtually the entire catchment.

Based on analysis of portion plans from the 1850s and 1860s, along with field examination of river geomorphology and associated sediment sequences, a continuous, low-capacity channel characterised the lowland plain (Figure 1c) (Brooks and Brierley, 1997). Metamorphosis of the lower Bega River occurred within a few decades of European disturbance, triggered by clearance of riparian and floodplain vegetation, and drainage of wetlands and backswamps. Comparison of portion plans from the 1850s and 1860s with archival photographs from the 1890s and early twentieth century indicate that the channel immediately upstream from Bega township widened from around 40 m to 140 m (Brooks and Brierley, 1997). As a response to anthropogenic disturbance, pools were infilled, and up to 2 m of sand accumulated on floodplains which were previously dominated by silt. The river had been transformed to a low sinuosity sand bed river style. Detailed field investigations indicate that relatively little change to river structure occurred between 1900-1960 (Brooks and Brierley, 2000). However, since the 1960s, the lowland channel has become choked by willows and other exotic vegetation. A complex pattern of bars and islands has developed within a braided channel planform. These structural changes to river morphology have fundamentally altered physical process interactions with riparian vegetation and coarse woody debris, impacting on the structural and ecological controls that these elements have on aquatic ecosystems.

River metamorphosis in Bega catchment has not been restricted to the lowland plain. At the time of European settlement, base of escarpment sections of various subcatchments comprised continuous valley fills (Figure 1a) (Fryirs and Brierley, 1998; Fryirs, 2002). These cut-and-fill rivers had evolved and accumulated at the base of the escapment over around 6,000 years (Fryirs and Brierley, 1998). In some subcatchments, these extended into mid-catchment floodouts with discontinuous channels (Figure 1b) (Brierley and Fryirs, 1998). Within a few decades of settlement, drainage of upland swamps and a range of indirect responses to early agricultural pursuits triggered headcut incision into these large sediment sources. Incision was quickly followed by extensive channel expansion, supplying over 9 million m3 volumes of sediment to the lower catchment (Fryirs and Brierley, 2001). Incised channels are locally more than 10 m deep and 100 m wide, transforming the river from an intact valley fill to a channelised valley fill river style. Channel floors are functionally detached from their perched valley fills. Many of the ecological values of these former swamps have been lost, and the few tributary swamps which remain have high conservation value.

River reaches that connect the base of the escarpment to the lowland plain are largely bedrock-confined. These parts of the catchment have acted as very efficient sediment transfer or throughput zones. Channel bed elevation has risen and fallen at different stages in the passage of sediment slugs. Only patches of riparian vegetation remain in mid-catchment. Today these reaches are severely degraded in ecological terms (Chessman et al., 2006).

These dramatic changes to river morphology have resulted in a fundamental alteration of bedload sediment flux interactions throughout the catchment (Figure 2). Fryirs and Brierley (2001) document the post-European settlement sediment budget for the catchment. In the period since European settlement in the 1840s, over 14 million m3 of bedload sediment has been released and flushed from the upper catchment (largely from incision of intact valley fills) with a sediment delivery ratio of 68 %. Over 67 % of sediment released has been sourced from just 25 % of the catchment and the slopes are decoupled from the channel network (Fryirs and Brierley, 1999). However, only 16 % of these alluvial sediments have been flushed through to the estuary, as antecedent controls on valley width have resulted in the lowland plain acting as a large sediment sink. Sediment is stored in a large within-channel sediment slug (8 million m3) and as thick floodplain sand sheets (over 4 million m3).

The changing nature of sediment source, transfer and accumulation zones has varied markedly from subcatchment to subcatchment since European settlement (sensu Schumm, 1977) (Figure 3). Prior to European settlement large intact swamps at the base of the escarpment acted as sediment sinks over several thousand years. These have now been transformed into sediment source zones. The lowland plain once acted as a sediment transfer zone and now acts as a sink. The process zone functioning of this catchment has been significantly altered such that sediment exhaustion has effectively occurred and longitudinal, lateral and vertical linkages have been fundamentally altered (Brierley et al. 1999). This has major implications for the geomorphic recovery potential of rivers (see Fryirs and Brierley, 2000, 2001), their ecological health (see Chessman et al., 2006; Brierley et al., 1999) and constrains what can be realistically achieved in terms of river rehabilitation over the short-medium term (see Brierley et al., 2002).

Associated References


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