Gary Brierley

Variability in sediment delivery and storage along river courses in Bega catchment, NSW, Australia : implications for geomorphic river recovery

In many catchments in southeastern Australia, alluvial stores have been the dominant source of sediments mobilised in the period since European settlement. In Bega catchment, on the South Coast of New South Wales (NSW), this has been reflected by dramatic changes to river morphology. Extensive volumes of material have been released and efficiently flushed to the lowland plain, with a sediment delivery ratio of almost 70%. 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. The changing nature of sediment source, transfer and accumulation zones has varied markedly from subcatchment to subcatchment. The volume of material supplied to the lowland plain from differing subcatchments is not related to subcatchment area. Rather, the pattern of river types dictates the spatial variability in storage and transfer. Over 67% of sediment released has been sourced from just 25% of the catchment, from subcatchments characterised by large valley fills (cut and fill River Style) that previously stored extensive volumes of material at the base of the escarpment. These parts of Bega catchment were especially sensitive to disturbance. Sediment exhaustion from these parts of the catchment, and from river courses elsewhere, has major implications for the geomorphic recovery potential of rivers, constraining what can be realistically achieved in terms of river rehabilitation.

Geomorphic responses of lower Bega River to catchment disturbance, 1851–1926

Abstract Prior to significant European settlement of the area in the 1850s, lower Bega River on the South Coast of NSW had a narrow, relatively deep channel lined by river oaks. The river had a suspended or mixed load, with platypus habitat available in pools. Banks were fine-grained and relatively cohesive (silts and clays), as was the floodplain, which graded to a series of valley-marginal swamps and lakes. Extensive evidence from maps and portion plans, archival photographs, bridge surveys, and anecdotal sources, complemented by field analysis of floodplain sedimentology (including radiocarbon-dated samples) and vegetation remnants are used to document the dramatic metamorphosis in the character and behaviour of lower Bega River in the latter half of the nineteenth century. By 1926 the channel had widened extensively (up to 340%) and shallowed in association with bed aggradation by coarse sandy bedload. Floodplain accretion was dominated by fine to medium sands, with some coarse sand splays. In contrast with most other studies of channel metamorphosis in Australia, which have emphasised river responses to climatically-induced flood histories, relegating human impacts to a secondary role, the profound changes to the geomorphic condition and behaviour of Bega River reflect indirect human disturbance of Bega catchment, and direct but non point source disturbance to the channel. Extensive clearance of catchment, floodplain, and channel-marginal vegetation occurred within a few decades of European settlement, altering the hydrologic and sediment regime of the river, and transforming the geomorphic effectiveness of floods. Although this study is situated in a relatively sensitive, granitic catchment, catchment clearance is likely to have induced equally significant responses in many other river systems in eastern Australia. In some instances the diffuse aspects of human disturbance on landscapes induce impacts on river character that are just as profound as major direct disturbances of river channels. This may have profound implications in understanding, and hence managing, the geomorphic consequences of river behaviour in Australia and elsewhere.

Inside the "Black Box" of River Restoration: Using Catchment History to Identify Disturbance and Response Mechanisms to Set Targets for Process-Based Restoration

Many river restoration projects fail. Inadequate project planning underpins many of the reasons given for failure (such as setting overly ambitious goals; selecting inappropriate sites and techniques; losing stakeholder motivation; and neglecting to monitor, assess, and document projects). Another major problem is the lack of an agreed guiding image to direct the activities aimed at restoring the necessary biophysical and ecological processes within the logistic constraints of on-ground works. Despite a rich literature defining the components of restoration project planning, restoration ecology currently lacks an explicit and logical means of moving from the initial project vision through to on-ground strategies. Yet this process is fundamental because it directly links the ecological goals of the project to the on-ground strategies used to achieve them. We present a planning process that explicitly uses an interdisciplinary mechanistic model of disturbance drivers and system responses to build from the initial project vision to the implementation of on-ground works. A worked example on the Upper Hunter River in southeastern Australia shows how understanding catchment history can reveal disturbance and response mechanisms, thus facilitating process-based restoration.

Application of the River Styles framework as a basis for river management in New South Wales, Australia

Abstract If strategies in natural resource management are to ‘work with nature’, reliable biophysical baseline data on ecosystem structure and function are required. The River Styles framework provides a geomorphic template upon which spatial and temporal linkages of biophysical processes are assessed within a catchment context. River Styles record river character and behaviour. As the capacity for a river reach to adjust varies for each style, so too do management issues and associated rehabilitation programmes. The framework also provides a basis for assessing geomorphic river condition and recovery potential, framed in terms of the evolutionary pathways of differing River Styles in the period since the European settlement of Australia. Within a catchment context, the River Styles framework provides a unified baseline upon which an array of additional information can be applied, thereby providing a consistent framework for management decision-making. The framework was developed as a research tool by geomorphologists working in collaboration with the New South Wales Department of Land and Water Conservation, which has used it for a range of river management applications. Target conditions for rehabilitation programmes are framed within a catchment vision that integrates understanding of the character, behaviour, condition and recovery potential of each reach. A prioritization procedure determines the most cost-effective and efficient strategies that should be implemented to work towards the catchment vision. In addition, the River Styles framework is being used to identify rare or unusual geomorphic features that should be preserved, assess riparian vegetation patterns and habitat availability along river courses, and derive water licensing, environmental flow and water quality policies that are relevant to river needs in each valley. Based on these principles, representative biomonitoring, benchmarking and auditing procedures are being developed to evaluate river health.

What is a fluvial levee

Abstract Fluvial levees are elevated partitions between channels and floodplains. Because of their character and position, levees may provide critical controls on, and insights into, geomorphic processes that determine the distribution of water and sediment within river systems. Few studies have analysed the character, distribution, sedimentology and processes that form levees in modern depositional environments. Characterisation of levee deposits from the Mississippi River continues to form the basis for most levee interpretations from the rock record. This discussion paper assesses the reliability of interpretations of levee deposits in numerous examples from the rock record, and their associated inferences for river style. This uncertainty reflects the lack of definitive sedimentological attributes for levee deposits, their limited preservation potential, and the fact that levee identification in the rock record is inhibited by the reliance on geometric descriptors or indirect associations between channel and floodplain facies. Given these concerns, it is suggested that geomorphologists and sedimentologists need to recognise the limitations of our present knowledge of levees, and work towards a more systematic understanding of these significant fluvial landforms in the full spectrum of modern (and ancient) river settings.

Reading the landscape: Integrating the theory and practice of geomorphology to develop place-based understandings of river systems

Assertions of a ‘naughty world’ (Kennedy, 1979) point to the importance of place-based knowledge in informing landscape interpretations and management applications. Building upon conceptual and theoretical insights into the geomorphic character, behaviour and evolution of rivers, this paper outlines an approach to the practice of fluvial geomorphology: ‘reading the landscape’. This scaffolded framework of field-based interpretations explicitly recognizes the contingent nature of biophysical interactions within any given landscape. 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. Reading the landscape is framed as an open-ended and generic set of questions that inform process-form interpretations of river landscapes. Rather than relying unduly on conceptual or theoretical representations of landscapes that suggest how the world ‘should’ ideally look and behave, appropriately 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. The approach provides a basis for scientifically informed management efforts that respect and work with the inherent diversity and dynamics of any given river system.

Slope–channel decoupling in Wolumla catchment, New South Wales, Australia: the changing nature of sediment sources following European settlement

Within a few decades of European settlement, channel incision transformed discontinuous river courses throughout Wolumla catchment, on the south coast of New South Wales, Australia. The development of continuous channels greatly increased sediment delivery from the catchment. This paper documents the character, timing and proportion of sediment sourced from upland valley fills, channel expansion sites, and gully networks. Volumes of material transferred from these sources are compared with estimates of sediment eroded from hillslopes, and the movement of sediment off the slopes to the valley floor is assessed. Although disturbance of slopes resulted in significant movement of materials, most of this material has been stored on-slope, in trapped tributary fills and along lower order drainage lines. The slopes are effectively decoupled from the channel. Sediment accumulation in farm dams over the past few decades has been negligible. Around 75% of the total volume of material released from creeks in Wolumla catchment since 1865, i.e., 5500×103 m3, has been derived from channel incision into valley fills at the base of the escarpment. Sediment flushing occurred within a few decades of catchment disturbance. Bedrock confinement in the middle and lower catchment resulted in very efficient downstream transfer of materials. Although gully networks and channel expansion sites have released a relatively small volume of material, these sources are the greatest contemporary source of sediment in Wolumla catchment.

Don't fight the site: three geomorphic considerations in catchment-scale river rehabilitation planning.

Three geomorphic considerations that underpin the design and implementation of realistic and strategic river conservation and rehabilitation programs that work with the nature are outlined. First, the importance of appreciating the inherent diversity of river forms and processes is discussed. Second, river dynamics are appraised, framing the contemporary behavioral regime of a reach in relation to system evolution to explain changes to river character and behavior over time. Third, the trajectory of a reach is framed in relation to downstream patterns of river types, analyzing landscape connectivity at the catchment scale to interpret geomorphic river recovery potential. The application of these principles is demonstrated using extensive catchment-scale analyses of geomorphic river responses to human disturbance in the Bega and Upper Hunter catchments in southeastern Australia. Differing implications for reach- and catchment-scale rehabilitation planning prompt the imperative that management practices work with nature rather than strive to ‘fight the site.’

A critical review of catchment-scale stream rehabilitation programmes

The development of catchment-scale stream rehabilitation programmes in many parts of the world marks a shift from the application of reach-based engineering principles towards an adoption of ecosystem-centred, adaptive and participatory approaches to river management. From a biophysical viewpoint, this represents recognition of the importance of the inherent geodiversity of aquatic ecosystems and the benefits that are gained through enhancing natural recovery mechanisms. As this approach to river management matures, it is important that its key elements and assumptions are subjected to critical appraisal. In this paper, the main features of contemporary catchment-wide programmes are examined through a review of pertinent literature and through examination of various case studies from North America, Europe, Asia and Australia. Emerging challenges and tensions include those of generating an authentic and functional biophysical vision at the catchment scale, of developing a proactive adaptive management approach, of achieving genuine community participation and of integrating biophysical and social factors in a transdisciplinary framework. Issues of scale, natural variability and complexity must be addressed in meeting these challenges.

The character and age structure of valley fills in upper Wolumla Creek catchment, south coast, New South Wales, Australia

Extensive valley fills at the base of the escarpment in upper Wolumla Creek, on the south coast of New South Wales, Australia, have formed from a combination of ‘cut and fill’ processes. The valley fills comprise series of alternating, horizontally bedded sand and mud units, reflecting reworking of detritus from deeply weathered granites of the Bega Batholith. Sand units are deposited as sand sheets or splays on floodplain surfaces or in floodouts that form atop intact valley fill surfaces downstream of discontinuous gullies. Alternatively, sands are deposited from bedload and form bars or part of the valley floor within channel fills. Organic-rich mud units are deposited from suspension in swamps or in seepage zones at the distal margin of floodouts. Within 5 km of the escarpment, valley deposits grade downstream from sand sheet and splay deposition in floodouts, to mud deposition in swamp and seepage zones. Radiocarbon dates indicate that virtually the entire valley fill of upper Wolumla Creek was excavated prior to 6000 years BP. Remnant terraces are evident at valley margins. The valley subsequently filled between 6000 years BP and 1000 years BP producing valley fills around 12 m deep, but no greater than 300 m wide. Reincision into the valley fill, on a scale smaller than the present incision phase, is indicated at around 1000 years BP, following which the channel refilled. Portion plans dated from 1865 refer to the study area as ‘Wolumla Big Flat’, and show large areas of swampy terrain, suggesting that the valley fill had re-established by this time. Within a few decades of European settlement the valley fill incised once more. Upper Wolumla Creek now has a channel over 10 m deep and 100 m wide in places, draining a catchment area of less than 20 km2.