Stephen Tooth

Process, form and change in dryland rivers: a review of recent research

Abstract Many of the worlds extensive warm dryland regions support numerous, albeit often infrequently flowing, rivers. Dryland rivers are increasingly a focus of scientific and applied interest but empirical research and fluvial theory for drylands need to be strengthened. Recent research in arid central Australia indicates greater diversity in dryland river process, form and change than has hitherto been appreciated, and highlights the need for a global review assessing the present state of knowledge. This review outlines the distinctive characteristics of dryland fluvial environments (hillslope and channel hydrological and sediment transport processes, river pattern and geometry, temporal and spatial aspects of channel change, sedimentary structures and bedforms), many of which contrast with more humid fluvial environments. Although features common to many dryland fluvial environments can be identified (extreme temporal and spatial variability of rainfall, runoff and sediment transport, poor integration between tributary and trunk channels, importance of large floods as a control on channel morphology, lack of equilibrium between process and form), the fluvial diversity that exists within drylands requires recognition of the limitations to these generalisations. In particular, research in central Australia illustrates the need to understand the rivers of this region using empirical relationships, terms, and concepts additional to those defined by earlier work in drylands. Key deficiencies in dryland fluvial research are identified, and relate to three main areas: limited study of some aspects of modern dryland rivers (floodplain characteristics, influence of vegetation, downstream changes, importance of scale); limited understanding of dryland river behaviour over longer (Cenozoic) timescales; and lack of integration between the results from short-term, process-form studies and studies of the longer term histories of river behaviour. Linking knowledge of past hydrological and channel changes to present-day changes in dryland rivers is suggested as a key research priority. This will help develop a sound theoretical basis for the assessment of future developments in dryland river systems which will contribute to their improved scientific understanding and environmentally sensitive management.

The role of vegetation in the formation of anabranching channels in an ephemeral river, Northern plains, arid central Australia

As the distribution and abundance of vegetation in drylands is often controlled by the greater availability of water along river channels, riparian vegetation has the potential to influence significantly dryland river form, process and behaviour. This paper demonstrates how a small indigenous shrub, the inland teatree (Melaleuca glomerata), influences the formation and maintenance of anabranching channels in a reach of the ephemeral Marshall River, Northern Plains, arid central Australia. Here, the Marshall is characterized by ridge-form anabranching, where water and sediment are routed through subparallel, multiple channels of variable size which occur within a typically straight channel-train. Channels are separated by channel-train ridges — narrow, flow-aligned, vegetated features — or by wider islands. By providing a substantial element of boundary roughness, dense stands of teatrees growing on channel beds or atop the ridges and islands influence flow velocities, flow depths and sediment transport, resulting in flow diversion, bank and floodplain erosion, and especially sediment deposition. Ridges and islands represent a continuum of forms, and their formation and development can be divided into a three-stage sequence involving teatree growth and alluvial sedimentation. 1 Teatrees colonize a flat, sandy channel bed, initiating the formation of ridges by lee-side accretion. Individual ridges grow laterally, vertically and longitudinally and maintain a geometrically similar streamlined (lemniscate) form that presents minimum drag. 2 Individual ridges grow in size, and interact with neighbouring ridges, causing the lemniscate forms to become distorted. Ridges in the lee of other ridges tend to be protected from the erosive effects of floods and survive, whereas individual teatrees or small ridges exposed to flow concentrated between larger ridges, tend to be removed. 3 Ridges lengthen, and coalesce with downstream ridges, eventually subdividing the channel-train into well-defined anabranches. This sequence turns a channel, initially obstructed with dense and chaotic stands of teatrees, into a well-organized system of ridge-form anabranches. In the moderate- to low-gradient Marshall River, which is colonized by an abundance of within-channel vegetation and subject to declining downstream discharges, this helps to minimize flow resistance, thereby maintaining an efficient water and sediment flux. Copyright

Wetlands in drylands: geomorphological and sedimentological characteristics, with emphasis on examples from southern Africa

Wetlands are poorly documented features of many landscapes, and there is often little understanding of the geomorphological controls on their origin, development and characteristics. This paper addresses the apparent paradox of wetlands in drylands, focusing particularly on the geomorphology and sedimentology of wetlands in southern Africa. Drylands are characterized by high (but variable) levels of aridity, reflecting low ratios between precipitation and potential evapotranspiration, so wetlands can only exist where there are locally positive surface water balances for all or part of the year. Most moderate to large wetlands in drylands are thus maintained by river inflows that combine with other factors that serve to impede drainage or reduce infiltration, including faulting, rock outcrops, swelling soils, and ponding by tributary or aeolian sediments. Together with variations in sediment supply, vegetation communities, and levels of animal activity, this promotes a diverse range of wetlands that span a continuum from permanently inundated, to seasonally inundated, to ephemerally inundated. In detail, every wetland has a unique range of geomorphological and sedimentological characteristics but, at a general level, the dryland setting can be shown to impart some distinctive features. By comparison with humid region (tropical and temperate) wetlands, we propose that many wetlands in drylands are characterized by: 1) more frequent and/or longer periods of desiccation; 2) channels that commonly decrease in size and even disappear downstream; 3) higher levels of chemical sedimentation; 4) more frequent fires that reduce the potential for thick organic accumulations and promote aeolian activity; and 5) longer timescales of development that may extend far back into the Pleistocene. Additional studies of wetlands in different drylands may reveal other distinctive characteristics. Correct identification of the factors giving rise to wetlands, and improved understanding of the geomorphological and sedimentological processes governing their development, is vital for the design of sustainable management guidelines for these diverse yet fragile habitats.

Anabranching rivers on the Northern Plains of arid central Australia

Anabranching rivers are a widespread feature of the Northern Plains in the Alice Springs region of central Australia but their unusual characteristics previously have not been described. On the Northern Plains, anabranching occurs on rivers transporting bedloads of coarse sand and gravel and is characterised by channels of variable size and shape which occur within a broader, typically well-defined, channel-train. Channels are separated by channel-train ridges—narrow, flow-aligned, vegetated features—or by wider islands. Ridges and islands are either depositional features (formed in situ by accretionary processes) or erosional features (formed by excision from once-continuous areas of floodplain). Vegetation plays a key role in the initiation, survival and growth of depositional forms through its influence on flow, sediment transport and ridge and island stability. Anabranching is also related to the influence of tributaries, for some large rivers alternate from single-thread to anabranching along their length in response to tributary inputs of water and sediment. Tributary inputs occur during flow events that are either independent from, or in concert with, floods in the trunk channel. Ridges and islands form in association with tributaries as a result of various hydrological, depositional and erosional processes, including irrigation of enhanced numbers of in-channel trees and resulting lee-side sediment accretion, floodplain scour, and the formation and maintenance of deferred-junction tributaries. The change from single-thread to anabranching downstream of tributary junctions occurs in the absence of any significant change in channel gradient or degree of channel confinement. On the Northern Plains, anabranching appears to be a stable river pattern that helps to maintain the throughput of relatively coarse sediment in low-gradient (typically 0.0005–0.002) channels characterised by an abundance of within-channel vegetation and subject to declining downstream discharges.

Downstream changes in dryland river channels: the Northern Plains of arid central Australia

Abstract Many dryland rivers undergo marked downstream changes owing to factors such as infrequent floods, flow transmission losses, and typically few tributary inflows beyond the headwaters. Along the Sandover, Bundey (Sandover–Bundey) and Woodforde Rivers on the Northern Plains of arid central Australia, downstream channel changes are broadly similar. In upland zones, small, rocky channels transporting sand and gravel gradually increase in size before entering piedmont zones, where channels and narrow floodplains are confined by bedrock, alluvial terraces, or aeolian dunes. In lower gradient lowland zones, channels and floodplains remain confined and, in the absence of tributary inflows, channel cross-sectional areas and discharges decrease downstream. Confining landforms are not present in floodout zones, which results in splay formation, increased floodplain widths, and marked overall downstream decreases in cross-sectional areas. Eventually, channelised flow and bedload transport terminate, although occasional large floods continue across extensive unchanneled alluvial surfaces termed “floodouts”. These broad similarities apart, downstream changes along the three rivers differ in detail. The Sandover is largely a single-thread channel, whereas many reaches of the Sandover–Bundey and Woodforde are anabranched. On the small Woodforde River, downstream decreases in parameters such as cross-sectional area and width are roughly linear. On the larger Sandover and Sandover–Bundey, downstream changes are more irregular, particularly through the floodout zones where there are marked fluctuations in widths, depths and bed slopes. The irregular downstream changes typical of the lower reaches of these large rivers may be due to the reduced influence of vegetation on bankline stability and width adjustment relative to that of smaller rivers. On the Northern Plains, as in other drylands, complex interactions between discharge, sediment transport, slope, patterns of tributary drainage, bank sediment type and vegetation result in variable patterns of downstream channel change.

Spatial distribution of coarse woody debris dams in the Lymington Basin, Hampshire, UK

Abstract Debris dams of coarse woody debris have a significant influence on channel processes in forested areas but few detailed studies have been made of variations within a single basin. Results from previous research are standardised and show that average variations throughout basins include densities of debris dams up to 40 per 100 metres of channel and can involve a loading value of up to 225 kg per m2 of channel. Variations have been ascribed to distance downstream, to channel width, to land-use effects, to felling, and to the management of coarse woody debris in streams. This study of the Lymington Basin, 110.4 km2 in drainage area, shows that the input of storm debris resulting from blowdown accounts for 45% of the gross load. The remaining 55% net load varies to distance downstream, to land use with the greatest loads in deciduous woodland areas, and according to management removal of debris from streams and multiple regression equations are provided. It is deduced that as a consequence of long-term management the present channel debris may be as little as 7% of the total net load that could have been present if no management had occurred.

Anabranching in mixed bedrock-alluvial rivers: the example of the Orange River above Augrabies Falls, Northern Cape Province, South Africa

Abstract Anabranching is characteristic of a number of rivers in diverse environmental settings worldwide, but has only infrequently been described from bedrock-influenced rivers. A prime example of a mixed bedrock-alluvial anabranching river is provided by a ∼150-km long reach of the Orange River above Augrabies Falls, Northern Cape Province, South Africa. Here, the perennial Orange flows through arid terrain consisting mainly of Precambrian granites and gneisses, and the river has preferentially eroded bedrock joints, fractures and foliations to form multiple channels which divide around numerous, large (up to ∼15 km long and ∼2 km wide), stable islands formed of alluvium and/or bedrock. Significant local variations in channel-bed gradient occur along the river, which strongly control anabranching style through an influence on local sediment budgets. In relatively long (>10 km), lower gradient reaches ( 0.0013) within the anabranching reach, local transport capacity exceeds sediment supply, bedrock crops out extensively, and channels flow over an irregular bedrock pavement or divide around rocky islands. Channel incision into bedrock probably occurs mainly by abrasion, with the general absence of boulder bedforms suggesting that hydraulic plucking is relatively unimportant in this setting. Mixed bedrock-alluvial anabranching also occurs in a number of other rivers worldwide, and appears to be a stable and often long-lived river pattern adjusted to a number of factors commonly acting in combination: (1) jointed/fractured granitoid rock outcrop; (2) erosion-resistant banks and islands; (3) locally variable channel-bed gradients; (4) variable flow regimes.

EQUILIBRIUM AND NONEQUILIBRIUM CONDITIONS IN DRYLAND RIVERS

Rivers in drylands typically are characterized by extreme flow variability, with long periods of little or no flow interspersed with occasional large, sometimes extreme, floods. Complete adjustment of river form and process is sometimes inhibited, resulting in a common assumption that equilibrium conditions may rarely, if ever, exist in dryland rivers, and that transient and unstable (nonequilibrium) behavior is the norm. Examples from the Channel Country and the Northern Plains in central Australia challenge that notion. Along the middle reaches of these intermediate and large, low-gradient rivers, where long duration floods generate moderate to low unit stream powers and boundary resistance is high as a result of indurated alluvial terraces, cohesive muds or riparian vegetation, there is evidence that: (1) channels have remained essentially stable despite large floods; (2) sediment transport discontinuities, while present at a catchment scale, are largely insignificant for channel form and process in individual reaches; (3) there are strong correlations between many channel form and process variables; and (4) many rivers appear to be adjusted to maximum sediment transport efficiency under conditions of low gradient, abundant within-channel vegetation and declining downstream discharge. In these middle reaches, rivers are characterized by equilibrium conditions. However, in the aggradational lower reaches of rivers on the Northern Plains, where upstream terraces are buried by younger sediments and channels are less confined, nonequilibrium conditions prevail. Here, channels sometimes undergo sudden and substantial changes in form during large floods, sediment transport discontinuities are readily apparent, and landforms such as splays remain out-of-balance with normal flows. Hence, dryland rivers can exhibit both equilibrium and nonequilibrium conditions, depending on factors such as catchment size, channel gradient, flood duration, unit stream power, channel confinement, sediment cohesion, and bank strength. [Key words: dryland rivers, floods, equilibrium, nonequilibrium, central Australia.]

Forms and processes of two highly contrasting rivers in arid central Australia, and the implications for channel-pattern discrimination and prediction

Tooth, S., Nanson, G. C. (2004). Forms and processes of two highly contrasting rivers in arid central Australia, and the implications for channel-pattern discrimination and prediction. Geological Society of America Bulletin, 116 (7-8), 802-816 RAE2008

Riparian vegetation and the late Holocene development of an anabranching river: Magela Creek, northern Australia

Many anabranching rivers are characterized by dynamic interactions between fluvial processes and riparian vegetation, but uncertainties surround the processes and time scales of anabranch development. We use geomorphological investigations and optically stimulated luminescence (OSL) dating to determine spatial and temporal trends in the development of anabranching along a 6.5-km-long reach of Magela Creek in the seasonal tropics of northern Australia. Many trees and shrubs that survive the wet-season floods establish on the sandy beds and lower banks, such that anabranches divide and rejoin around numerous ridges and islands that are formed mainly by accretion in the lee of in-channel vegetation and, less commonly, by excision from formerly continuous island or flood plain surfaces. Once ridges and islands form, colonizing vegetation maintains their stability by increasing sediment cohesion and decreasing flow erosivity. Over the Holocene, Magela Creek has vertically aggraded and extended in length by delta progradation into Madjinbardi Billabong, resulting in a time sequence of anabranches and associated ridges and islands from older (upstream) to younger (downstream). OSL ages for islands in the upstream and middle reaches are ca. 1.6 ka and older, and the narrow, deep anabranches (width/depth [w/d] typically ~10–30) have few in-channel obstructions. Farther downstream, island OSL ages are ca. 0.7 ka and younger, anabranches tend to be wider and shallower (w/d >30) with more obstructions, and splays and locally scoured island and floodplain surfaces are more common. Based on these findings, previous flow and sediment-transport measurements, and theoretical analyses, we posit that there is a decline in anabranch efficiency from an upstream equilibrium system in mass-flux balance to a downstream disequilibrium system characterized by bed aggradation and localized island and floodplain erosion. In the downstream reaches, inefficient (high w/d and obstructed) anabranches do not persist because they either aggrade and are abandoned, or they are subdivided into more efficient (lower w/d and less obstructed) anabranches as a result of the interactions between in-channel vegetation growth and ridge and island accretion or local excision. Consequently, a more efficient anabranching system gradually develops with characteristics similar to those in the upstream reaches. This enhances downstream sediment transfer, which enables ongoing delta progradation and provides fresh sediment surfaces for vegetation to colonize and initiate new anabranches. The OSL ages from Magela Creek demonstrate that a recognizable but relatively inefficient anabranching system can develop within a few centuries, while adjustment to a more efficient system occurs over a few millennia.