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Linking contemporary geomorphology with catchment history on the Scamander River (northeast Tasmania)

Fearman, LP ORCID: 0000-0002-8807-4287 2021 , 'Linking contemporary geomorphology with catchment history on the Scamander River (northeast Tasmania)', Research Master thesis, University of Tasmania.

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Abstract

This study investigates influences on change in the Scamander River in northeast Tasmania. It focuses on the interaction between boundary conditions set by geology and significant post European settlement human disturbance, in determining contemporary channel condition. It takes its lead from the Aboriginal principle of ‘listening to country’ to learn the stories hidden within the landscape from long ago. Aboriginal knowledge is based on 40,000 years of intimate connection with the landscape, passed on by oral traditions. This study does not have access to this knowledge and is not attempting to replace it. Rather, it seeks to derive understanding of the Scamander River and its country from a range of modern information sources. It respectfully takes its lead from the stories in the landscape, rather than devising hypotheses to test. Scientists describe this as an idiographic approach, which describes the landscape and infers meaning from observations.
Historical research, spatial analysis in GIS, field assessment based on the Reading the Landscape approach, and community knowledge are combined in a multiple-evidence-based approach to create an enriched picture of factors influencing change in this river catchment. My original contribution to research is the use of Innovative methods, based on readily available information sources, to derive understanding of change at different scales of time and space from the stories in the landscape.
The Scamander is a medium sized coastal catchment in northeast Tasmania, with rugged terrain, and a semi-arid climate, prone to extreme rainfall events. Previous studies focus on investigation of geology relating to mineral resources. The Scamander River has been omitted from hydrological studies, despite being a middle-sized river in the region. It has not previously been studied from a geomorphological perspective. It has a single flow meter which began operating on 10/04/1968, with a gap from 22/03/1974 to 13/03/1975 and from 25/09/1996 to 05/12/2001, with several additional brief interruptions. This makes a total of 46 years and 11 months of record over a period of 53 years and 7 months.
To investigate catchment history (Ch 2), newspaper reports available online from the Trove National Library Archive have been used to create a coherent timeline for the post European settlement period (1860 to 1935). This reveals that a series of six bridges were constructed at a vulnerable site near the Scamander’s estuarine opening over a period of sixty-five years, and their destruction by natural forces influenced the morphology and behaviour of the River. A stable barrier typical of Australia’s wave dominated south-eastern coastal rivers, existed at the river opening in 1844. Such barriers restrict tidal movements and create a low energy environment in estuarine channels. Over the period investigated, the opening was converted to an unstable sand delta, and highly energetic channels were scoured out in estuarine reaches. This change resulted from accumulation of large wood behind bridges during floods, until they collapsed, releasing large volumes of water confined in the estuarine basin, which scoured out the opening. Changes to sediment budgets and the opening regime followed. This part of the study shows that lost catchment history can be recovered from newspaper reports available online to provide insights into human impact on rivers.
Eyewitness reports of floods and changes to river morphology that followed them allowed processes contributing to change on the Scamander to be identified. Their role has been elucidated based on contemporary scientific literature. Processes involved include: the role of supercritical flow in destabilising bridge piers; the destabilisation of sediment budgets due to changes in tidal flows through the opening; and the recruitment of increasing volumes of wood into flood flows over time.
Site specific influences are recognised as significant in forming the character of many rivers. In this catchment, they include growth form and wood density characteristics of Eucalyptus sieberi, which is endemic to south-eastern Australia, and prominent on dry hillslopes of this catchment. Its wood density ranges from greater than seawater when green, to less than freshwater when dry. This allows the wood to be transported at all levels of the water column, complicating its interaction with bridges. Along with its long stem and small crown, this characteristic increases the likelihood of log jams forming. Investigation of the literature also showed that limitations remain in contemporary understanding of important processes, including the behaviour of wood of extreme size, when transported in large volumes by floods. Much of the wood involved in the Scamander floods was an order of magnitude larger than the contemporary definition of large wood.
This unconventional approach to understanding historical human impacts on the Scamander River provides a perspective from which contemporary condition of this river can be considered. It also shows that bridges constructed at vulnerable sites can interact directly with channels and flood flows to influence river condition in the long term, a different mechanism from the more well recognised impact of land use change.
The influence of boundary conditions on the Scamander’s susceptibility to disturbance was investigated (Ch 3) through spatial analysis in geographic information systems (GIS) supported by field observations. The River has two main sub-catchments, the Scamander headwaters and its main tributary the Avenue River, which are similar in size but differ in lithology distribution and channel form. To understand the influence of lithology on their channel character, these two sub-catchments were compared using spatial analysis techniques, applied to a 2m resolution digital elevation model using ArcGIS software. Several methods were applied.
Stream networks were developed and classified to Strahler orders. A buffer system based on Strahler orders was used to extract slope data from near stream zones. Slope distribution within these zones was graphically compared for the two sub-catchments, after being sorted to lithology types and Strahler orders within each. Stream cross sections were also extracted from first order streams of each sub-catchment’s headwaters and compared for channel form. The area of granite and Mathinna Group sedimentary rock lithologies was measured within each sub-catchment and along their main channel trunks.
The results showed differences in lithology distribution. Granite makes up 45% of the area and 39% of the main channel length of the Scamander’s headwaters, but less than 5% of the Avenue’s catchment area and only 1% of its channel length. Mathinna Group sedimentary rocks form 60% of the Avenue’s catchment area, with 35% younger sedimentary rocks, and 99% of its channel length. Mathinna Group rocks make up 54% of the Scamander’s catchment area and 61% of its channel length.
Weathering processes in these two lithologies differ greatly. Perpendicular planes of weakness in Mathinna Group rocks cause them to dislodge rectangular blocks to form regular, steep sided troughlike channels. Blocks break down to cobbles and gravel which travel as saltating bedload during energetic flows and settle as sheets on the channel bed. Abrasion by saltating bedload is the chief mechanism for channel bed incision in rivers. Its effects are evident in the steep, low roughness channels, which promote high energy flows, that field observations show dominate the Avenue. Channels of the Avenue are further characterised by a series of stable, ingrained meanders formed by interaction of flow with vertical layers of metamorphosed rock which may be unique to Mathinna Group lithology. Granite breaks down to boulders, then to coarse quartz sand. Granite bedrock armours channel beds against incision, boulders increase channel roughness, and sand deposits hydraulically within channels to provide anchorage for vegetation. Field observations show these influences in the shallow, rough, well vegetated channels prevalent in the Scamander, conditions which promote low energy flows.
Channel cross sections show that headwaters of the Avenue are steeply incised, but those of the Scamander are shallow and irregular. Graphs of slope distribution confirm differences in channel form between the two catchments, which are most evident in first order streams. On Mathinna Group lithology in the Avenue most slopes are between 30 to 50 degrees, with those on granite in the Scamander less than 10 degrees. These differences in channel form suggest that channel bed incision into tributaries has been a long-term influence on the development of the Avenue catchment, while the Scamander’s headwater catchment has been protected by its lithology from upstream transmission of incision. At least 5 glacial cycles have occurred in Tasmania over the past 2.4MY. Rising and falling sea levels that accompany each cycle alternately raise and lower base level in river systems. Repeated cycles of changes to base level, and the contrasting response of granite and Mathinna Group lithology is a likely mechanism to explain the contrasting channel character that has developed in the Scamander and Avenue Rivers. Channel incision triggered by human disturbance, such as occurred post European settlement on the Scamander, is likely to affect the two sub-catchments differently.
Contemporary channel character has been assessed using the Reading the Landscape approach (Ch 4), including assessment of position on trajectory of change for reaches where sufficient evidence is available. This approach considers site-specific peculiarities related to catchment history as well as general principles common to many rivers. Community knowledge (Ch 5) from an online survey completed by 33 participants and historical interviews of long-term residents has contributed further insights.
The assessment shows that landscape memory has a strong influence on channel form and response to disturbance in the lower reaches of the Scamander’s main trunk. This is typical for partially confined rivers in south-eastern Australia. Examples include confined meanders formed from Mathinna Group rocks which restrain floodplain formation to stable pockets, and limit planform change. Response to disturbance is expressed mainly through vertical incision and aggradation. This is particularly evident in Prices Reach where community input from water skiers indicates the reach has been deeply incised in the past and has been aggrading over several decades. Significant features of the lower channels are not consistent with the sea level high stand existing for 5,000 years in the region. These include the unstable tidal delta at the opening, high energy incised channels through the basin to the outlet. These changes are consistent with channel incision in response to base level fall triggered by historical bridge collapses.
Reaches differ in position on trajectory of degradation and recovery. Prices Reach is now widening by block failure, depositing sediments at base of banks, and revegetating. This reach appears to be approaching a new equilibrium condition that could be promoted by sensitive management. Significant channel bed aggradation over decades is also reported in some other reaches. In channel banks and vegetation are further signs of recovery processes. Ongoing incision into tributaries continues in the Avenue, and ongoing disturbance is evident at Ryans Bridge and the water supply. This is typical of rivers undergoing a lagged response to incision, which can continue for hundreds of years following disturbance to base level.
The findings suggest that morphological change at the Scamander River opening effectively lowered base level at the River’s downstream limit and initiated the upstream transmission of channel bed incision into the catchment. The long-term influence of differing distribution of granite and Mathinna Group lithology made the two main sub-catchments, the Scamander headwaters and the Avenue differentially susceptible to incision transmitted from downstream. The influence of landscape memory on lower channels, and changes at the river opening further complicate the ongoing, lagged response. Other disturbances such as forest clearing, bushfire, and the more recent collapse of bridges at other sites in the catchment are further contributing factors, whose influence is beyond the scope of this study.
Community input shows that the Scamander River is highly valued as a resource. Concern over the ongoing instability of the river opening and its impact on natural values and infrastructure is considerable. Views differ about whether this is a natural phenomenon, or a problem that needs to be addressed, and what type of actions should be taken.
The results of this study suggest historical disturbance has had a strong influence on the river opening, and the overall condition of the Scamander River. The complexity of forces acting at the opening, and the time frame over which river response to incision can play out, suggest that any effort to intervene would require thorough investigation based on appropriate expertise.

Item Type: Thesis - Research Master
Authors/Creators:Fearman, LP
Keywords: Catchment history, river geomorphology, spatial analysis, lithology, channel incision, community knowledge.
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Copyright 2021 the author

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