Transactions of the Royal Society of South Australia ISSN: 0372-1426 (Print) 2204-0293 (Online) Journal homepage: http://www.tandfonline.com/loi/trss20 Bank instability along a weir pool of the River Murray Martin Thoms To cite this article: Martin Thoms (2017) Bank instability along a weir pool of the River Murray, Transactions of the Royal Society of South Australia, 141:2, 151-168, DOI: 10.1080/03721426.2017.1374158 To link to this article: http://dx.doi.org/10.1080/03721426.2017.1374158 Published online: 08 Sep 2017. Submit your article to this journal Article views: 14 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=trss20 Download by: [Tufts University] Date: 28 October 2017, At: 20:19 TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA, 2017 VOL. 141, NO. 2, 151–168 https://doi.org/10.1080/03721426.2017.1374158 Bank instability along a weir pool of the River Murray Martin Thoms Downloaded by [Tufts University] at 20:19 28 October 2017 Riverine Landscapes Research Laboratory, University of New England, Armidale, Australia ABSTRACT ARTICLE HISTORY River banks are an important transition zone between aquatic and terrestrial environments. This ecotone is highly vulnerable to natural and anthropogenic disturbances. Bank erosion is a common occurrence along the River Murray. Morphological features reﬂecting river bank instability form a near-continuous pattern along large tracts of the river. This study investigates the character and extent of bank instability along the Torrumbarry Weir Pool. Over 90% of the bank length in this weir pool was assessed as being actively eroding. Notch development was the dominant instability mechanism and it promoted other forms of bank instability. Erosion notches result from stable water levels and inherent soil instability of the river banks along the weir pool. The character and extent of river bank erosion recorded along this section of the River Murray is an artefact of ﬂow regulation and not a reﬂection of the natural occurrence of bank erosion in river networks. Water level management in the upstream weir pool has signiﬁcant implications for the management of river banks along regulated reaches of the River Murray. Received 14 June 2017 Accepted 29 August 2017 KEYWORDS Bank erosion; notch development; ﬂow regulation; ecotone; littoral zone Introduction River banks are an important physical component of the riverine landscape and the riparian zone in particular. Their geomorphic complexity plays a fundamental role in various physical and biological processes. In large lowland river systems, the presence of morphological features such as benches and bars located at multiple levels within the bankfull channel contributes to channel roughness and in-channel ﬂow conveyance (Knight & Brown, 2001). The role of river bank complexity in creating diﬀerential physical patterns, trapping organic matter and providing habitats for lower order aquatic organisms in large lowland rivers has been demonstrated by Sheldon and Thoms (2006) and Thoms and Olley (2004). River bank complexity plays a fundamental role in the retention of organic matter, a key component of river ecosystem functioning as suggested in the Riverine Productivity Model of Thorp and Delong (2002). Rivers with high physical complexity retain more organic material than those with comparatively low complexity (Prochazka, Stewart, & Davies, 1991). Reductions in geomorphic complexity could have repercussions for the overall health of river ecosystems. This is especially signiﬁcant for large lowland rivers, such as the River Murray, where river CONTACT Martin Thoms Martin.Thoms@une.edu.au New England, Armidale, Australia © 2017 Royal Society of South Australia Riverine Landscapes Research Laboratory, University of Downloaded by [Tufts University] at 20:19 28 October 2017 152 M. THOMS banks are the main component of river channel complexity that are subject to increasing anthropogenic pressures (cf. Thoms et al., 2000). Bank erosion is a key geomorphological process of meander formation, lateral channel migration and movement of sediment throughout a catchment. Three primary mechanisms are involved in river bank erosion: subaerial processes, ﬂuvial entrainment and mass failure (Couper & Maddock, 2001). The dominance of each mechanism can vary systematically throughout a river network creating distinct bank erosion process zones (cf. the “Process Domain Model” of Lawler, 1992, 1995). In lowland river zones, the Process Domain Model predicts that ﬂuvial entrainment and attrition processes are the dominant mechanisms promoting bank instability. Thus, regardless of the lateral mobility of lowland rivers, bank erosion will be prevalent on the outer bend of meanders (Lawler, 1992, 1995; Thorne, 1992). It has been hypothesised that bank erosion processes are signiﬁcantly inﬂuenced by human activities, a hypothesis that has yet to be fully tested. However, in the Upper Nepean River, New South Wales, which is regulated by 11 weirs, Hubble (2004) showed that the presence of weirs had both positive and negative eﬀects on bank stability. Weir operations promoted bank failure through erosion at the toe of the river banks at many locations within the weir pools. However, proposed weir removal was modelled to potentially increase the magnitude of bank failure. Despite the importance of bank erosion on the riverine landscape, the relative eﬃcacy of bank erosion processes and anthropogenic inﬂuences on these are still poorly understood, especially for large Australian lowland rivers (Saynor & Erskine, 2006). Increased river channel instability is a feature of many rivers subject to ﬂow regulation (Petts, 1984). The ﬂow and sediment regimes of the River Murray are modiﬁed by two headwater dams and a series of low-level weirs, which has a signiﬁcant impact on its channel stability (cf. Thoms & Walker, 1989, 1992, 1993). In particular, the presence of the low-level weirs along the River Murray has seen changes in water level variations (Walker, Thoms, & Sheldon, 1992). In some respects, the studies by Keith Walker and his colleagues have shown that water level variations, rather than discharges, are more appropriate currency for studies of the environmental eﬀects of weirs. Routine weir operations maintain close control over upper pool levels but cause levels below each weir to ﬂuctuate; rapid water level changes in excess of 2 m have been recorded (Maheswahri et al., 1995). This change in the water level regime of the River Murray, imposed by the weirs, has implications for bank stability. Preliminary studies by Thoms and Walker (1989) recorded rates of bank erosion of up to 2 m a year, downstream of two weirs, because of the rapid draw down in water levels. As a consequence, there have been signiﬁcant adjustments in the channel proﬁle and cross-sectional dimensions in these reaches immediately downstream of the weirs (Thoms & Walker, 1993). Repeated surveys of the river channel 0.5 km below 10 weirs in the lower River Murray recorded increases in bankfull cross-sectional areas by 285% in the years following weir construction (Thoms & Walker, 1992). Despite these noticeable eﬀects of weirs on river bank stability and the implications for river management (cf. Thoms et al., 2000), there has been limited research on this aspect of the impact of ﬂow regulation along the River Murray. In this manuscript, bank-instability mechanisms within a weir pool of the lowland River Murray are investigated. It is hypothesised that the character and extent of bank instability along the weir pool will reﬂect the impact of ﬂow regulation, and not the TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA 153 general processes associated with the lateral mobility of this lowland river channel. The research presented here complements that which has focused on general river channel adjustments between a series of weir pools on the lower River Murray by Thoms and Walker (1989, 1992, 1993). Downloaded by [Tufts University] at 20:19 28 October 2017 Study area This study focused on a section of the River Murray regulated by Torrumbarry Weir, an area of extensive ongoing interdisciplinary research. The River Murray drains 420,000 km2 of the western slopes of the South Eastern highlands of Australia. Four river zones have been identiﬁed along the River Murray (Figure 1(a)); these are the Headwater Tract, Riverine Tract, Mallee Tract and Lower Murray Tract (Thoms et al., 2000), each with a unique physical, hydrological and ecological character. The Torrumbarry Weir Pool is located in the Riverine Tract. The River Murray within this zone has a highly sinuous channel (sinuosities = 2.12–3.21; where sinuosity is measured as channel length over valley length), very low river-bed slopes (average gradient = 6.4 cm km−1) thus low stream power and contains river-bed sediments composed of medium-size sands and highly cohesive river bank soils. Extensive ﬂoodplains are a feature of the River Murray in the Riverine Tract, with ﬂoodplain widths exceeding 30 km. These ﬂoodplains contain an array of physical features including contemporary and relict meanders, distributary and anabranch channels along with numerous billabongs, deﬂation basins and shallow lakes. Thus, this section of the River Murray has a relatively high geomorphic diversity. Several tributaries enter immediately upstream of the Torrumbarry Weir Pool, including the Goulburn River, which enters the River Murray several kilometres upstream of Echuca, and the Campaspe River, which joins the River Murray further upstream. Approximately 25,000 years ago, tectonic activity in the southern Murray–Darling Basin resulted in a block uplift creating what is now known as the Cadell Fault (Brown & Stephenson, 1991). The fault has a north/south orientation and is most notable in the Echuca to Deniliquin region. The presence of the Cadell Fault resulted in the formation of two large-scale alluvial fan complexes – the Barmah and Gunbower Fans (Figure 1(b)). The apex of the Gunbower Fan is located between the Torrumbarry Weir and Echuca. This reach of the River Murray between Echuca and Torrumbarry has a conﬁned ﬂoodplain that is often less than 300 m in width but becomes unconﬁned as it ﬂows further downstream of the Gunbower Fan (Rutherfurd, 1991). In the conﬁned sections of the Murray, the river channels are typically wider and shallower, have sandier bank sediments and follow a more sinuous path compared to the unconﬁned sections of the river. In general, the planform of the River Murray has been relatively stable over the last 200 years (Thoms et al., 2000). This lateral stability is considered to result from low stream powers and the abundance of silts and clays in the channel perimeter (Rutherfurd, 1991). River red gums (Eucalyptus camaldulensis) are the dominant riparian vegetation of the River Murray in the Riverine Tract. These riparian trees are common along most Australian inland water ways and occur in stands along the River Murray. The understorey vegetation of the riparian zone is generally dominated by herbaceous species such as grasses, daisies, sedges and peas. Large stands of Phragmities australis, common before European settlement, now occur in isolated stands along the River Murray (Walker, Boulton, Thoms, & Sheldon, 1994). M. THOMS Downloaded by [Tufts University] at 20:19 28 October 2017 154 Figure 1. The Torrumbarry Weir Pool study reach. (a) The location of the weir pool and the four geomorphological tracts of the River Murray and (b) the Riverine Tract with the location of Barmah and Gunbower fans. Downloaded by [Tufts University] at 20:19 28 October 2017 TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA 155 Flows in the River Murray within the Torrumbarry region are a combination of unregulated ﬂows, including ﬂows from the upper River Murray through spills from Hume Dam, the Kiewa, Ovens and Goulburn Rivers, as well as regulated releases from Hume Dam. There are also regulated releases from Eildon Reservoir on the Goulburn River. Unregulated ﬂows are generally prominent during the winter – spring months (June–October) with regulated ﬂows occurring between September and April, depending on downstream water requirements. Flows in the River Murray have become increasingly regulated since the 1920s. Hume Dam became operational in 1934, and in 1961, the capacity of the reservoir was doubled to its present capacity of 3.07 million ML by the addition of an earthen weir. Other important structures that regulate ﬂows along the River Murray are the Yarrawonga Weir, impounding Lake Mulwala (0.116 million ML) and Dartmouth Dam, a large headwater reservoir on the Mitta Mitta River that impounds 4.0 million ML. Despite the presence of these dams and weirs, ﬂows along the River Murray are variable (Maheshwari et al., 1995). At Tocumwal, upstream of Echuca, the coeﬃcient of variation of long-term annual ﬂows is 125%, compared with an average 33% for similar-sized rivers elsewhere in the world (Finlayson & McMahon, 1988). At Barham, 360 river-km downstream of Tocumwal, annual ﬂows vary from 25% to 214% of the mean, 150 m3 s−1 (69 years of record), while at Blanchetown, 274 river-km from the Murray Mouth in South Australia, ﬂows have varied from 6% to 491% of the average daily ﬂow of 318.2 m3 s−1 (average of 88 years). In the Torrumbarry region, annual ﬂows exceed channel capacity approximately once in every 10 years, on average. Major ﬂoods (>2000 m3 s−1) occurred along the River Murray in 1870, 1917, 1931, 1956 and 1974–75. Long-term patterns of runoﬀ have not varied markedly, although signiﬁcant changes have occurred in other parts of the Murray–Darling Basin (Riley 1988). In general, ﬂow management along the River Murray has resulted in reductions in ﬂow variability as well as overall decreases in the magnitude and frequency of certain ﬂood events. Originally, the construction of weirs on the River Murray (including Torrumbarry Weir) was designed to promote riverboat navigation but has come to assume important roles as ﬂow regulators. They are operated to maintain a steady upper pool level (surface water level variations range between 50 and 200 mm), except when ﬂows exceed storage capacity. Stable water levels are thought to have been associated with increased river bank instability, thereby causing subsequent reductions in channel complexity and loss of channel habitat due to increased erosion and sedimentation (Thoms et al., 2000). Torrumbarry Weir is located ~83 km downstream of Echuca on the River Murray. Completed in 1924, it was built for the River Murray Commission by the State Rivers and Water Supply Commission of Victoria. This weir operated until 1992 when damage to its foundations required the construction of a new weir. The current weir was commissioned in 1997 and is 73.5 m wide, 13 m high, incorporates six radial gates and has a capacity of 35,000 ML. At full supply, water levels are maintained at a height of 86.05 m Australian Height Datum, primarily for irrigation purposes. The installation of the new radial gates for water level management has reduced the range of water level movements in the upstream pool. The weir pool is also used for recreation, including tourism activities such as paddle steamers and house boats, ﬁshing, water skiing and wakeboarding. The current weir also has a ﬁsh ladder to allow migratory ﬁsh to pass the structure. 156 M. THOMS Methods Downloaded by [Tufts University] at 20:19 28 October 2017 Physical survey The physical character of river banks along an 85-km reach immediately upstream of the Torrumbarry Weir (known as the Torrumbarry Weir Pool) was determined in 1 km intervals during a period of low water levels. At ﬁve equidistant locations within each 1km interval, the height of the exposed river bank was recorded with a Vertex Laser VL400. All bank heights were measured at bankfull capacity. Between one and four soil samples were collected from each of these locations at sites near the low water level, and at mid-bank and upper bank sites, with the number of samples collected being dependant on the height of the river bank. Each soil sample was classiﬁed as per the International Soil Standards protocol and its mechanical stability determined via the Emerson Aggregate Test, which assesses aggregate soil stability (cf. Emerson, 1964). In addition, the particle-size composition of each soil sample was measured by determining the clay, silt and sand fractions using an ASTM 152H soil hydrometer (ASTM, 1985). A survey of the extent of each bank-instability mechanism identiﬁed (Table 1) within the 1-km interval was also undertaken along the Torrumbarry Weir Pool. The linear extent of each bank-instability mechanism was surveyed on both the left and right banks with a theodolite. In addition, the severity of notch development was recorded according to the classes of “none” (no notch evident), “low” (0–20 cm), “moderate” (20-40 cm) and “severe” (>40 cm) as recommended in the stream reconnaissance surveys of Thorne (1998) and Parsons, Thoms and Norris (2004). Vegetation survey The abundance of emergence aquatic and bank-side vegetation was classiﬁed into ﬁve groups. These being dominant, >70% of bank covered by vegetation; abundant, 50–70% of bank covered by vegetation; frequent, 20–50% of bank covered by vegetation; occasional, 0–20% of bank covered by vegetation and absent, 0% of bank covered by vegetation. Data analysis Data collected in the ﬁeld were digitised into ARC GIS and the corresponding bank heights and linear lengths of the various instability mechanisms and vegetation classes on both river banks were calculated for the individual 1-km intervals of the Torrumbarry Weir Pool. These data were used to assess the extent of bank exposure and bank instability as well as the diversity of instability mechanisms along the Torrumbarry Weir Pool. The diversity of bank-instability mechanisms was calculated via the Margalef Richness Index (DMg) as recommended by Magurran (2004). A breakpoint analysis was undertaken on both bank height and the extent of bank erosion in each 1-km interval to determine if systematic changes in bank instability occurred along the weir pool. Signiﬁcant zero crossings (SiZer) models were applied to both datasets for each 1-km interval. For these SiZer models, a non-parametric, locally weighted polynomial smoother was applied to each interval (predictor) and the primary and secondary derivatives of the smoothed curves were used to identify possible thresholds, and thus the locations in bank exposure and instability along the weir Description Mass failure by slumping of bank materials occurs as the result of reduced bank material strength induced by saturation. Banks are most vulnerable to slumping failures following draw down of water levels in the river channel. Draw down can leave the bank saturated but without the support provided by the weight of water against the bank The direct removal of material from the face of a river bank by the action of ﬂowing water. Rates of attrition can be inﬂuenced by the coherence of the bank material (in water), velocity of ﬂow adjacent to the bank, as well as the binding, protection and ﬂow retarding eﬀects of vegetation. This is a common form of bank erosion in lowland rivers, like the River Murray Notch development is material which is preferentially entrained at the water surface. Notch development is usually related to wave action. This relationship may be the direct result of ﬂuctuations in water velocity against the bank or it may result from weakening of bank material by repeated wetting and drying. Bank erosion notches may form within the bank proﬁle and lead to subsequent block failure and collapse Instability mechanism Mass failure by bank slumping Attrition Notch development Table 1. Common river bank-instability mechanisms occurring along the River Murray. Downloaded by [Tufts University] at 20:19 28 October 2017 Schematic (Continued ) TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA 157 Rill erosion Rill erosion is the result of rainfall or surface ﬂow running down the face of river banks and cutting small gullies or rills into the bank surface. Generally, rill erosion occurs more frequently on river banks that are devoid of vegetation Instability mechanism Description Block failure Banks are also eroded by undermining and block collapse. Undermining may result from the preferential removal of a weak layer of materials in the bank, deepening of the channel causing basal undercutting or diverging secondary currents in a bend attacking and removing the bank toe Table 1. (Continued). Downloaded by [Tufts University] at 20:19 28 October 2017 Schematic 158 M. THOMS Downloaded by [Tufts University] at 20:19 28 October 2017 TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA 159 pool. SiZer deﬁnes a threshold as a point where the ﬁrst derivative changes signiﬁcantly (Clements, Vieira, & Sonderegger, 2010). Four bandwidths (h = 2, 4, 8 and 15 km), equivalent to levels of smoothing, were applied to the model as recommended by Sonderegger, Wang, Clements and Noon (2009). Advantages of SiZer include its ability to handle multiple thresholds in one dataset and the examination of potential thresholds at diﬀerent of levels resolution. Delong and Thoms (2016) applied the SiZer model to a long-term isotope dataset for ﬁve rivers in the Mississippi River Basin subjected to various levels of ﬂow regulation and land use. SiZer analysis identiﬁed a signiﬁcant change in food web character in all rivers in the 5 years following ﬂow regulation. The character of bank instability along the Torrumbarry Weir Pool was further examined through a range of multivariate statistical analyses. Initially, a similarity matrix, using the Gower similarity coeﬃcient, was calculated using the number and linear extent of instability mechanisms identiﬁed in each 1-km unit. Diﬀerences in bank instability between sub-reach diﬀerences identiﬁed by the SiZer analysis were undertaken using the analysis of similarity (ANOSIM; Clarke, 1993). This was followed by a Semi-Strong-Hybrid Multidimensional Scaling ordination (cf. Belbin, 1993) in order to represent the similarity matrix graphically. A stress level <0.2 indicated that this ordination solution was not random. Relationships between the location of the 1-km intervals in the ordination and the diﬀerent bank-instability mechanisms were determined using Principal Axis Correlation (PCC; Belbin, 1993). For this, only those variables with an R2 greater than 0.8 were considered signiﬁcant. Results Physical bank character River bank heights ranged from 0.1 to 10.2 m above the low weir pool water level (Figure 2). There is a general trend for bank heights to increase with distance upstream from Torrumbarry Weir; this is consistent for both banks. At approximately 37 river- Figure 2. River bank heights along the Torrumbarry Weir Pool. Average heights above lower water weir pool levels are given along with a moving average (as determined by a moving window analysis, where the window gap = 2 km). Downloaded by [Tufts University] at 20:19 28 October 2017 160 M. THOMS km upstream of Torrumbarry Weir, bank heights increased signiﬁcantly, from an average of 1.82 m above the water level to 4.96 m. A SiZer model for bank heights along the weir pool conﬁrmed that this location represents a signiﬁcant change in the character of bank heights along the weir pool. Thus, based on bank heights, the weir pool can be separated into two zones, a lower zone from 0 to 37 km (mean bank heights range: 0.1–3.8 m) and an upper zone from 37.1 to 83 km (mean bank heights range: 3.6–6.2 m) (Figure 2). River bank sediments along the weir pool can be classed as either Silty Clay Loam (41%) or Silty Loam (25%) (Figure 3), based on the international soil classiﬁcation (ASTM, 1985). The character of the river bank sediments diﬀers between the upper and lower weir pool zones. Riverbank sediments in the lower weir pool zone are predominantly Silty Clay Loams (64%) and Silty Loams (25%) whilst in the upper weir pool zone, they are composed of Silty Loam (24%), Silty Clay Loam (22%) and Loam (20%) (Figure 3). In addition, soil character varied vertically from the water level to the top of the river bank and this vertical variation diﬀered between the lower and upper weir pool zones (Figure 3). The character of the river bank soil in the lower weir pool zone displayed relatively minor vertical variations compared to those in the upper weir pool zone. Soils in the lower weir zone are predominantly Silty Clay Loams or Silty Loams and are present mainly at mid-river bank levels and at the bank top. In upper weir pool zone, soils vary from predominantly Loams, closer to the water level to Loamy Sands in the mid-river bank levels, and then Sandy Loams at the top of the river bank. These variations in river bank sediments reﬂect the coarser soils of the Gunbower Fan sequence that inﬂuence the upper weir pool zone, while ﬁner soils of the lower weir pool zone are part of the general Riverine Tract ﬂoodplain sediment complex. The stability of river bank sediments, as assessed by the Emerson aggregate stability test, also varied along the weir pool and vertically on the river bank. Stable sediment aggregates do not slake, disperse or swell, whereas unstable soil aggregates do slake and disperse when added to water. In the Torrumbarry Weir Pool, 56% of the soil samples can be classed as being unstable or have the potential to become unstable when saturated at ﬁeld capacity (Figure 4). Only 19% of soils tested were classed as being stable. In the lower weir pool zone, soils are either stable or moderately stable in comparison to those in the upper weir pool zone, which are either unstable or unstable when wet. Vertically, soil stability on the lower river bank sections is generally moderately stable and this grades to unstable soils further up the river bank. This vertical transition in soil stability occurs in both weir pool zones. Thus, the river bank soils in the Torrumbarry Weir Pool are inherently unstable although the degree of instability varies along the weir pool. River bank vegetation The majority of the river banks along the Torrumbarry Weir Pool have no vegetation. Only 40% of the river bank recorded a presence of vegetation. In addition, 16% of the 1km intervals recorded an abundance class of dominant or abundant reeds and sedges or grasses. Therefore, approximately 60% of the banks are bare of reeds, sedges or grasses. The general depauperate nature of the bank vegetation suggests that it would not have 161 Downloaded by [Tufts University] at 20:19 28 October 2017 TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA Figure 3. The character of the river bank sediment along the Torrumbarry Weir Pool. (a) The entire reach, (b) the lower and upper Weir Pool sub-reaches and (c) examples of the longitudinal and vertical variations in sediment character. 162 M. THOMS Downloaded by [Tufts University] at 20:19 28 October 2017 Figure 4. Soil stability of the river bank sediments in the Torrumbarry Weir Pool. an inﬂuence on the character or extent of river bank stability along the Torrumbarry Weir Pool (Abernethy & Rutherfurd, 1998). River bank stability Instability is a feature of the river banks along the Torrumbarry Weir Pool with 90% of the total bank length displaying some form of erosion (Figure 5(a)). The spatial distribution of bank-instability mechanisms was not uniform along the weir pool. Bank instability was relatively more continuous along both banks with increasing distance upstream from Figure 5. River bank instability along the Torrumbarry Weir Pool. (a) Diﬀerent forms of river bank instability and (b) the richness of instability mechanisms (as determined via the Margalef Richness Index). Downloaded by [Tufts University] at 20:19 28 October 2017 TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA 163 the weir (Figure 5(a)). Notch development is the dominant instability mechanism along the Torrumbarry Weir Pool. It accounted for 57% of bank instability or 102 km of the river bank in the weir pool (Figure 5(a)). Moderate notch development was the most common form of notch development in the weir pool aﬀecting 28% of the bank, followed by low notch development (26%) and then severe notching (17%). Other bank-instability mechanisms observed along the weir pool included block failure, cantilever failure, rotational slipping and bank slump (Figure 5(a)) accounting for a total of 29% of instability mechanisms inﬂuencing 31.3 km of river bank. General attrition accounted for 4% or 7 km of the river bank, whilst the instability mechanisms of rill and gully erosion accounted for 10% (17.5 km of river bank). Multiple instability mechanisms were present on the river banks along the Torrumbarry Weir Pool. Over 90% of the river banks displayed more than one instability mechanism (Figure 5(a)). The composition of bank-instability mechanisms changed along the weir pool. A SiZer model for bank instability detected a change of 37 km upstream of the weir (Figure 5(a)) – the identiﬁed boundary of the lower and upper weir pool zones. There is a notable increase in the number of instability mechanisms within each 1-km bank unit, in the upper weir pool zone compared to that in the lower weir pool zone. The increase in the occurrence of diﬀerent bankinstability mechanisms is associated with an increase in the mechanisms of mass failure, attrition and rill gully erosion. Although the occurrence of notch development was relatively constant along the weir pool, notching in the lower weir pool zone was observed to be less severe. In the lower weir pool zone, notch development was dominated by low and moderate notching compared to the moderate-to-severe notching of the upper weir pool zone (Figure 5(a)). The change in the composition of bankinstability mechanisms along the weir pool is reinforced by the spatial pattern of the richness or diversity of the diﬀerent bank-instability mechanisms (Figure 5(b)). Margalef Richness Index (DMg) values, which is a measure of diversity and calculated for each 1-km bank interval, range from 0.14 (10 km upstream of the weir) to 1.02 (71 km upstream of the weir). Overall, the median Margalef Richness value was 80% higher in the upper weir pool zone compared to the lower weir pool zone; i.e., there is a greater diversity of the types of bank erosion mechanisms in the upper weir pool. Bank instability is signiﬁcantly diﬀerent between the upper and lower zones of the Torrumbarry Weir Pool. Results of a pair-wise ANOSIM indicate statistical diﬀerences between the lower and upper sections of Torrumbarry Weir Pool, in terms of the extent and composition of bank-instability mechanisms (Global R = 0.691, p < 0.001). A PCC analysis reveals that three bank-instability mechanisms, notch development, bank failure and rill/gully erosion, had R2 > 0.60 and their positions in ordination space were strongly associated with the upper weir pool zone. Thus, the increased incidence of notch development, bank failure and rill/gully erosion in the upper weir pool was responsible for diﬀerences in bank-instability between the upper and lower weir pool zones. River bank-instability mechanisms also varied between diﬀerent morphological subreaches of the weir pool. In highly meandering reaches, bank erosion and in particular general bank scour or attrition are more prevalent on the outer bends (Figure 6). The outer banks of meander bends in the weir pool have a higher proportion of their length eroding compared to inner banks and straight sections of river channel, all of the outer banks displayed instability compared to 52% of the inner meander bends and 63% on 164 M. THOMS Downloaded by [Tufts University] at 20:19 28 October 2017 Figure 6. The composition of river-instability mechanisms between the inner, outer and straight river sections. straight river sections. Overall, notch development is the predominant instability mechanism in all three sections of river bank accounting for 62%, 47% and 65% of the various mechanisms observed along the outer, inner and straights sections of river bank, respectively. Discussion The “Process Domain Model” (cf. Lawler, 1992, 1995) predicts the dominant processes promoting bank instability within a river network. Accordingly, the upper reaches of the river, where stream power is lower, subaerial processes tend to dominate. If the rate of erosion increases downstream and the contribution of subaerial erosion remains constant, the relative signiﬁcance of these processes decreases. Maximum stream powers commonly occur in the mid-reaches of a river network resulting in the dominance of ﬂuvial erosion or attrition processes. With increasing channel depth downstream, Lawler (1992) suggests that there will be a point where the maximum (“critical”) bank height for stability with respect to mass failure is exceeded. Mass failure processes therefore tend to dominate bank erosion in the lower reaches of river systems. Field evidence of the Process Domain Model along Australian river systems is provided by Abernethy and Rutherfurd (1998) in the Latrobe River, Victoria and Saynor and Erskine (2006) in Ngarradj River catchment, Northern Territory. The character and extent of bank-instability mechanisms recorded in the Torrumbarry Weir Pool does not support the Process Domain Model of Lawler (1992, 1995) for the River Murray. According to this model, ﬂuvial erosion or attrition processes should dominate along the Torrumbarry Weir Pool. While attrition was identiﬁed, it accounted for only 4% of the bank-instability mechanisms. Notch development was the dominant mechanism promoting bank instability in this mid-reach section of the River Murray. Moreover, most of the river banks (90%) displayed signs of instability along the Torrumbarry Weir Pool. In addition, erosion notches were present on both outer and inner bends of meanders as well as along straighter sections of the river. These results diﬀer markedly from other studies of channel morphologies and bank erosion in large lowland meandering river systems (cf. Habersack, Haspel, & Schober, 2014). In a study of the channel morphology of the Brahmaptura–Jamuna River, Sarker, Thorne, Aktar and Ferdous (2014) established a Downloaded by [Tufts University] at 20:19 28 October 2017 TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA 165 spatio-temporal sequence of river instability over a 200-year period. This highlighted bank instability was signiﬁcantly greater in the outer sections of meander bends compared to other morphological sections of the river and that general scour or attrition was the dominant erosional process. The generality of these results has been conﬁrmed through a systematic analysis of channel patterns for many large rivers across multiple continents by Lewin and Ashworth (2014). The near-continuous spatial distribution of bank instability is a geomorphological feature along this section of the River Murray and an artefact of ﬂow regulation. River channel erosion and bank instability are natural processes of riverine landscapes. River channels adjust their morphology primarily in response to changes in sediment and water regimes. However, the rate and type of river channel adjustment is inﬂuenced by many factors, and human activities, such as ﬂow regulation, can exacerbate these processes. The predominance of well-developed erosion notches on the river bank along the regulated section of the Torrumbarry Weir Pool can be attributed to two factors, the inherent instability of river bank soils and the relative stability of water levels in the weir pool. The majority of the river bank soils (56%) along Torrumbarry Weir Pool become unstable after periods of wetting; they become saturated, soften, lose strength and become easily erodible. This inherent instability is associated with the presence of well-developed notches at multiple bank elevations along the length of the Torrumbarry Weir Pool. Stable water levels also exacerbate the eﬀects of wave erosion on river banks. Waves created by either wind or boating activities erode material from riverbanks, and rates of this wave-induced erosion are enhanced if wave activity is focused on one area of the river bank (Nanson, Von Krusenstierna, Bryant, & Renilson, 1994). Thus, stable water levels in the Torrumbarry Weir Pool narrow the areal extent of wave-induced erosion on riverbanks. In a preliminary study of wave action along the Torrumbarry Weir Pool, Southwell and Thoms (2004) recorded 3 cm increases in notch depth, over a 24-h period, associated with boat-generated waves. The presence of well-developed notches promotes other forms of river bank instability. Increases in the diversity of bank-instability mechanisms were recorded along the Torrumbarry Weir Pool. This increase in diversity was the result of increases in the presence of mass failure mechanisms. The inﬂuence of the presence of erosion notches on the overall instability of riverbanks is dependent on the height of the river bank above the eroded notch. With suﬃcient bank height and the exceedance of a critical bank height, bank collapse through cantilever erosion and or block collapse will occur. Diﬀerences in the extent, severity and diversity of instability mechanisms were recorded between the upper and lower weir pool zones and this is directly associated with a change in the bank height between these zones. River banks in the lower weir pool zone (average 1.82 m) are not as high as those in the upper weir pool (average 4.76 m). The prominence of erosion notches and the associated increases in other mass failure mechanisms suggest that river bank instability is an artefact of the presence of Torrumbarry Weir and its water level management regime in the upstream weir pool. Conclusions River banks or the aquatic river-edge environment are important ecotones within the riverine landscape. As ecotones, they encompass sharp gradients of environmental factors, 166 M. THOMS physical and ecological processes as well as displaying enhanced species diversity, elevated rates of matter and energy exchange and providing important refuges for species from these areas. The research of Keith Walker, in the River Murray, established the signiﬁcance of this littoral zone to be an outstanding example of an ecological boundary. In addition, he demonstrated this ecotone to be an indicator of environmental change and one especially sensitive to changes in water levels (cf. Walker et al., 1992). The research presented in this study of the Torrumbarry Weir Pool further highlights the vulnerability of the aquatic river-edge environment to water management. It reinforces the need for sympathetic weir operations in the River Murray and the relaxation of the current 50-mm water level tolerance in weir pools; a call made by Keith Walker over 30 years ago. Downloaded by [Tufts University] at 20:19 28 October 2017 Acknowledgements The assistance of Erin Lenon and Mark Southwell in collecting much of the ﬁeld data is gratefully acknowledged. 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