Erosion and the sediment conveyor in central Australia

Why are the Neogene sedimentary fills across central Australia generally thin and discontinuous? One long-standing explanation is that sluggish tectonism and intensified aridity have combined to suppress rates of erosion and sediment production yielding a landscape crowded with inherited, preMiocene forms. Quantifying rates of sediment production, residence time and transport is possible with numerous methods, but the recent growth of cosmogenic nuclide (CN) analysis has provided unprecedented quantitative insights to rates of landscape evolution. Measurements of in situ produced cosmogenic 10Be and 26Al integrate rates of surface processes over million-year timescales—the last part of the Neogene in which aridity has strengthened across the continental interior. We present a compilation of ~600 published and unpublished 10Be and 26Al measurements from central Australia with a focus on the Neogene Eyre Basin and its periphery. Outlying and inlying bedrock uplands serve as engines of sediment production via erosion of bedrock. Surrounding the bedrock outcrops are vast sediment conveyors of varying efficiency and tempo: hillslopes, pediments, and alluvial fans are interim storage/burial zones for sediment in transit to the network of low-gradient rivers, dunes, and playas towards base level. Interactions between fluvial and aeolian processes are especially pertinent to sediment flux in the Eyre Basin. Major rivers such as the Cooper and Finke traverse dunefields in their lower reaches where quantities of alluvia are recirculated into dunes and vice versa. Tracking the trajectories of sediment from source-to-sink (including aeolian recirculation) remains a major challenge, but is central to unravelling the sedimentary dynamics of central Australia's Neogene basins. Based on the CN compilation we estimate 1) spatially averaged erosion rates at the scale of a hillslope or river catchment; 2) pointbased erosion rates on bedrock surfaces; 3) residence time of sediment in hillslope regolith and alluvial fans; and 4) cumulative burial history of sediments in transit. Catchment-scale erosion rates (n~100) are consistently low (<10 m/Myr) and include some of the lowest rates ever measured (~0.3 m/Myr); however, a small group of catchments in the Flinders Ras yield higher erosion rates (~30–60 m/Myr). Bedrock hillslopes (n~200) tend to erode even slower (<5 m/Myr), with a subset of Flinders Ras sites again being the exception (~10–30 m/Myr) and suggesting the influence of recent tectonism. Several CN depth-profiles measured on hillslopes and alluvial fans indicate sediment residence times >0.5 Myr, and high-resolution sampling along three hillslopes with differing morphology (linear, convex, and concave) reveals major variations in sediment production and transport rates that hint at the long-term evolution. In the rivers, fluvial sediments show a weak tendency to increase cumulative burial history downstream (1–2 Myr), consistent with the expanding accommodation space for storage and burial. Dune sediments sampled in the Simpson and Tirari dunefields (n~16) contain cumulative burial histories (up to 1.5 Myr) similar to that of the intersecting rivers. This points to an intimate mix of fluvial and aeolian processes in areas approaching base level. Curiously, these sediments occur in the lowest part of the continent and contain the longest histories of cumulative burial, yet do not form part of the thickest sedimentary fills in the Eyre Basin

Classification of river morphology and hydrology to support management and restoration

The work leading to this paper has received funding from the European Union’s FP7 programme under Grant Agreement No. 282656 (REFORM

Floodplain formation and sediment stratigraphy resulting from oblique accretion on the Murrumbidgee River, Australia

Oblique accretion is a significant process of deposition along low-energy, mixed-load and suspended-load Australian rivers. Previously described as accretionary bank deposits sandwiched between well-developed point bars of sand and gravel and muddy overbank deposits, fine-grained oblique-accretion deposits dominate the floodplain stratigraphy of many inland Australian rivers. They contribute more than 65% of floodplain sediments along the Murrumbidgee River and almost all of the floodplain formed by bend migration on the suspended-load channels of the Darling and Cooper basins. Deposits consist mainly of alternating thin beds of sand and mud (inclined heterolithic stratification), with some plant litter, that form as drapes on the prograding bank. These beds dip mostly channelwards and quickly wedge out as they grade up and onto the floodplain. Because oblique accretion traps nearly all of the sediment deposited from suspended load near the channel margin, vertical accretion on distal areas of the floodplain is minimal. Where oblique accretion is associated with scroll formation, the resulting deposits are more complex, sometimes including a component that slopes away from the channel on the distal side of the first floodplain scroll. A model is presented showing how, with point bars or scrolls either present or absent, oblique accretion can make a significant contribution to the preservation of fine-grained within-channel deposits in contemporary floodplains. The examples presented here demonstrate that analogues to ancient point-bar deposits containing alternating sandstone and shale sequences are common in the low-energy fluvial environments of inland Australia

Commentary on a "Conceptual model for complex river responses using an expanded Lane diagram by David Dust and Ellen Wohl", Geomorphology, Volume 139-140, March 2012, Pages 109-121

This paper presents a revision of a river-flow response model by Dust and Wohl (2012). In particular, serious confusion of the relationship between sediment transport capacity and width-to-depth ratio is clarified. As a consequence, we propose a reasonable qualitative model for understanding the complexity of river responses as illustrated in an appropriately modified form of the Lane diagram, which identifies the equilibrium condition based on the balance between channel aggradation and degradation

Modern depositional processes in a confined bedrock setting: benches of the Shoalhaven river

The 8th International Conference on Geomorphology of the International Association of Geomorphologists (IAG/AIG) took place in Paris at the Cité des Sciences de La Villette from August 27 to 31, 2013. The main topic of this 8th Conference was "Geomorphology and Sustainability". Organized by the Groupe Français de Géomorphologie (GFG) and open to all scientists and practitioners, this Conference included 26 scientific sessions, 5 key-note lectures and one Workshop devoted to Young Geomorphologists. YOUNG GEOMORPHOLOGISTS SESSION Convenors: Etienne COSSART, Johnny DOUVINET & Stuart LANE This session welcomed young scientists (PhD students, post-doctorates) who were interested in discussing new approaches and methods in geomorphology (spatial analysis, mathematical modelling, conceptual and quantitative approaches and links between them). Presentations could focus on any component of the discipline and related earth sciences (hydrology, Quaternary geology, soil sciences, etc.), and be either fundamental or applied. Preliminary results and discussions of fieldwork and methodological strategies (observation, data acquisition before modelling) were appropriate, as well as methodological developments, such as geomorphic mapping through GIS. Equally, numerical simulation approaches such as those linked to complexity theory (agent-based modelling, Cellular Automata) or physically-based methods for specific process representations have been welcomed. The participants of this session have been invited to participate to the intensive course for young geomorphologists held at the end of the conference (from September 1st to 3rd).The lower Shoalhaven River provides an opportunity to examine bench processes in a confined setting. Stratigraphic analysis of trenches and augur holes, ground penetrating radar, Hec-RAS modelling and geochronological techniques combine to identify that benches of multiple levels along Bull Reach are composed of coarse material and have been extensively eroded and reworked by modern events. Kermode et al. (2012) established the long-term polycyclical nature of the higher alluvial surfaces (up to 193 ka in age), and this is contrasted with the youth of the lower inset alluvial surfaces, which are shown to be less than 270 years in age. This study evaluates the relative significance of both flood regime and effects of European settlement on the geomorphic effectiveness of high magnitude events and investigates the characteristics of bench formation in this confined setting. It characterises the nature of depositional events and the relationship between facies at an event scale. Using Hec-RAS modelling, events of different recurrence intervals are compared to explore the relative impact of varying flood magnitudes. The results bring into question the theory that inundation frequencies of these surfaces are constant, or associated with formative processes

Kyorhaek mit hohupki chirhwan

National Natural Science Foundation of China 40788001;National Basic Research Program of China 2011CB40330A;Chinese Academy of Sciences;China Postdoctoral Science Foundation 20110490568The Yellow River in China carries an extremely large sediment load. River channel-form and lateral shifting in a dynamic, partly meandering and partly braided reach of the lower Yellow River, have been significantly influenced by construction of Sanmenxia Dam in 1960, Liujiaxia Dam in 1968, Longyangxia Dam in 1985 and Xiaolangdi Dam in 1997. Using observations from Huayuankou Station, 128 km downstream of Xiaolangdi Dam, this study examines changes in the river before and after construction of the dams. The temporal changes in the mean annual flow discharge and mean annual suspended sediment concentration have been strongly influenced by operation of theses dams. Observations of sediment transport coefficient (ratio of sediment concentration to flow discharge), at-a-station hydraulic geometry and bankfull channel form observed from 1951 to 2006 have shown that, although variations in flow and sediment load correspond to different periods of dam operation, changes in channel form are not entirely synchronous with these. The channel has been subject to substantial deposition due to the flushing of sediment from Sanmenxia Dam, resulting in a marked reduction in bankfull cross-sectional area. Flows below bankfull had a greater impact on channel form than higher flows because of very high sediment load. At-a-station hydraulic geometry shows that the variation of channel cross-sectional area below bankfull in this wide and relatively shallow system largely depends on changes in width. Such at-a-station changes are significantly influenced by (1) events below bankfull and (2) overbank floods. Bankfull depth is the main component of channel adjustment in that depth adjusts synchronously with channel area. The channel adjusts its size by relatively uniform changes in depth and width since 1981. Channel morphology is not the product of single channel-forming flow frequency. It is determined by the combination of relatively low flows that play an important role in fine sediment transport and bed configuration as with relatively high flows that are effective at modifying the channel's morphology. The sediment transport coefficient is a useful index for efficiently guiding the operation of the dams in a way that would minimize channel changes downstream. Sedimentation over the nearly 60 years of study period caused the lower Yellow River to aggrade progressively, the only significant exception being the two years following completion of Sanmenxia Dam

Late Quaternary aeolian and fluvial interactions on the Cooper Creek Fan and the association between linear and source-bordering dunes, Strzelecki Desert, Australia.

The Innamincka Dome and associated low-gradient fan in the Strzelecki Desert is the product of Cenozoic crustal warping that has aided formation of an extensive array of palaeochannels, source-bordering transverse dunes and superimposed linear dunes. These dunes have impeded the course of Cooper Creek and provided a repository of evidence for Quaternary climate change as well as the interactive processes between transverse and linear dune formation. At Turra, Gidgealpa and sites nearby are extensive fluvial and aeolian sand bodies that date from marine isotope stages (MIS) 8–3 and the Last Glacial Maximum (LGM) and are now surrounded or buried by overbank mud. The sandy alluvium was deposited on the downstream slope of the dome by large channels transporting abundant bedload, subsequently blown northward to form transverse dunes from what were probably seasonally-exposed bars in a palaeo-Cooper system. Thermoluminescence (TL) and optically stimulated luminescence (OSL) ages demonstrate that the base of the dune complex is at least MIS 7 in age (~250 ka) but that it has been subsequently reworked by wind with additional sand blown from the river. Source-bordering dunes formed during a period of enhanced river flow and sand supply from ~120 to 100 ka, with another short episode of the same at ~85–80 ka and from ~68 to 53. The LGM was associated with enhanced flows and the supply of dune sediment, from 28 to 18 ka. Pronounced river flow and dune activity occurred in the early to mid Holocene, but there is no evidence of dunes being supplied from Cooper Creek since the LGM. The dunes forming the oldest basal sand units appear to be largely transverse in form and are aligned roughly parallel to adjacent east–west trending palaeochannels. Linear dunes have formed from and over these, and yield basal ages ranging from MIS 5 or MIS 4 but continuing to accrete and rework through to the Holocene. The study results in one of the few detailed chronological investigations of the interaction between transverse and linear dunes. It is apparent that long-distance sand transport has played no significant role in dune formation here for the linear dunes show no significant downwind decline in ages. Linear dunes appear to have accreted vertically from underlying transverse dunes. A wind-rift vertical accretion model with only minor lengthwise extension is the dominant mode of linear dune formation in this section of the Strzelecki Desert, the bulk of dune sediment being sourced from adjacent swales since the LGM. © 2010, Elsevier Ltd