Sediment transport (dis)connectivity in the upper Hunter catchment, NSW, Australia part of Vignettes:Vignette Collection
Sediment flux within river systems has been described as a 'jerky conveyor belt' (Ferguson, 1981). The distribution of riverine sediment stores and sinks, and the frequency with which sediment is added or removed from these stores, reflects the degree to which a river system is (dis)connected or (de)coupled (Harvey, 2002; Hooke, 2003). (Dis)connectivity along the conveyor belt affects the extent and rate of transfer of energy and matter through catchments. Any factor that impedes sediment conveyance constrains sediment delivery to the catchment outlet. These impediments have been termed buffers, barriers and blankets (Fryirs et al. 2007 a, b). Buffers disrupt lateral linkages within catchments (Figure 1a). They may include alluvial pockets of floodplain, fans or piedmont zones that occur at breaks in slope along valley margins, disconnecting lateral connectivity in catchments. Other buffers include features such as intact valley fills and floodouts (either terminal or intermediate) that have discontinuous or absent water courses or low slope alluvial floodplains. Elevated floodplain or terraces along trunk streams may block tributary confluences, disconnecting material supply from lower order drainage lines to the primary channel network. Barriers most commonly disrupt longitudinal linkages through their effect on the base level or bed profile of a channel (Figure 1b). For example, bedrock steps or woody debris may locally reduce slopes by introducing a local base level control. Sediments are trapped as they backfill areas immediately upstream of the step, inducing local discontinuity in sediment transfer. Sediment slugs may act as plugs to sediment movement along channels. Over-widened channels may not have the competence to carry sediments made available to them, acting as barriers to downstream sediment conveyance. Human modification to rivers may induce considerable barriers to sediment movement. For example, dams may trap all but suspended load materials from moving through the system. Blankets disrupt vertical linkages through their effect on surface-subsurface interactions and the entrainment of sediment (Figure 1c). They are sediments that smother other landforms, and can occur instream or on floodplains and may include features such as floodplain sand sheets or fine grained materials that infill the interstices of gravel bars. Bed armour acts as a blanket which inhibits the reworking of subsurface sediments. These impediments to sediment movement within a catchment restrict the rate of sediment transfer from the area upstream of that point. Hence, the position of buffers, barriers and blankets determines the area or proportion of a catchment that has the potential to directly contribute sediment to, or transport sediment along, the channel network under given flow conditions. This is referred to as the effective catchment area (Fryirs et al., 2007a). Landscape (dis)connectivity, in turn, determines how geomorphic instability is manifest throughout a river system, shaping the direction and rate of geomorphic change, and lagged and off-site (propagatory) responses. However, the degree of (dis)connectivity in a catchment will change over time as different types of impediments are breached (reworked) or formed. The form, composition, scale and position of impediments, and the ease with which these features are reworked and breached allows the measurement of the effective timescale of (dis)connectivity (c.f., Harvey, 2002). The breaching capacity (c.f., Brunsden, 2001) can be considered as the magnitude-frequency characteristics of perturbations required to breach the buffer, barrier or blanket, or as the residence time of these landforms. Switches in catchments: The importance of magnitude-frequency relationships on sediment transport Changing landscape connectivity and effective catchment area through breaching of certain types of buffers, barriers and blankets can be considered analogous to the operation of a series of switches which determine which parts of the landscape contribute to the sedimentary cascade over different time intervals. Figure 2 conveys a conceptual framework by which these interactions may be assessed. The spatial pattern of buffers, barriers and blankets influences the timeframe over which sediments are reworked in different landscape compartments. At low flow stages associated with frequent, low magnitude energy inputs, the capacity for slope erosion and fluvial sediment reworking is limited. Landscape disconnectivity is significant and the effective catchment area is low. At this stage, none of the buffers, barriers or blankets is breached and sediment cascading in the channel network is limited. As flow stage increases within the channel network, more readily reworked instream barriers and blankets are breached. There is sufficient energy to initiate reworking of interstitial fines that form instream blankets, and sediments are conveyed through the valley constriction, connecting the upstream channel network to the lowland plain. As connectivity increases, the effective catchment area increases. In a high magnitude-low frequency event, other buffers and barriers are breached. Reworking of alluvial fans connects the hillslopes to the channel network. The sediment slug along the lowland plain is reworked, contributing sediment to the river mouth. However, unless an extreme event occurs, the floodplains and terraces along the lowland plain are not reworked and maintain their buffering capacity. Indeed, sediments derived from upstream may be deposited on the floodplain surface forming a blanket. During high magnitude events connectivity within the system is at its highest and the effective catchment area the largest. The degree to which certain parts of catchments are switched on, and the timeframe over which this occurs dictates how the effects of geomorphic changes are propagated through a catchment. In highly connected landscapes, the cumulative effects of alterations to forms of (dis)connectivity in upper parts of a catchment are manifest relatively quickly. In contrast, in highly disconnected landscapes, changes to the nature of (dis)connectivity in one part of a catchment are absorbed or suppressed in the system and geomorphic changes are not propagated through the catchment. Application of the buffers, barriers and blankets framework An application of how buffers and barriers restrict bedload supply in the upper Hunter catchment will now be presented. The upper Hunter catchment drains 4200km2 in the northern part of the greater Hunter catchment which is located just north of Sydney in coastal NSW, Australia. The catchment is composed of metasediments in the east and sedimentary rocks in the west with dissected Tertiary basalt occurring in headwater areas. These two provenances are separated by the Hunter-Mooki fault. Topography in the eastern part of the catchment is rugged and steep with little available accommodation space in valleys. The western part of the catchment is characterized by relatively subdued topography and wider, more open valleys where sediments are stored. Under simulated conditions, effective catchment area, which is a measure of the proportion of a catchment that has the potential to contribute bedload sediment to the channel network, varies from 73 % to just 3 % of the total catchment area for differing subcatchments in the upper Hunter (Figure 3). In total, around 52 % of the catchment is disconnected and does not contribute to the overall sediment cascade as measured at Muswellbrook. This calculation reflects the role of buffers that determine lateral connectivity and does not take account of longitudinal disconnectivity induced by barriers. When the combined effect of buffers and barriers is examined, the effective catchment area of the entire upper Hunter catchment at Muswellbrook drops significantly to 334 km2 or 9 %. These results suggest that available sediments are likely to be efficiently contributed to, and conveyed through, Rouchel subcatchment (located in the east; Figure 4) while the Dart and Lower Hunter subcatchments (located in the west; Figure 4) are extremely inefficient at contributing and conveying sediment. This variability can be explained by the spatial distribution and assemblage of buffers and barriers in each subcatchment (Figure 5). Multiple forms of disconnectivity are evident in some subcatchments (e.g. Dart Brook), such that when one buffer or barrier is breached, other features still impede sediment transfer within the system (Fryirs et al. 2007b). The importance of the position of buffers and barriers within any given subcatchment is emphasised. Spatial variability in valley width exerts a critical control on catchment connectivity, with more efficient sediment conveyance in narrow valleys relative to wider valleys characterised by piedmonts, terraces, fans and extensive floodplains in which conveyance is impeded. This variability reflects the landscape history and geological setting of each subcatchment. Landscape change, whether natural or human-induced, may alter the extent and pattern of physical (dis)connectivity. Similarly, changes to the magnitude, frequency, duration or sequencing of flow events affect the sediment transport regime. Formation or breaching of a buffer, barrier or blanket alters the pattern and rate of biophysical linkages in a catchment (Fryirs et al. 2007a). Effective description and explanation of landscape (dis)connectivity throughout a catchment provides a basis to predict the future trajectory of geomorphic change (Brierley and Fryirs 2005). Hence, management applications must emphasize explicitly whether (dis)connectivity is 'natural' in the system and make informed decisions about whether they strive to connect, disconnect or re-disconnect biophysical relationships within any given river system (Hillman et al. 2008).
Post-European settlement disturbance response of rivers in Bega catchment, South Coast, NSW, Australia part of Vignettes:Vignette Collection
Human activities have altered river systems across most of our planet. Most rivers now operate under fundamentally different conditions to those that existed prior to human disturbance. Habitat loss brought about by human-induced changes to river character and behaviour has eliminated native flora and fauna, altered spatial ranges and interactions, and promoted the incursion of exotic species. In many instances, simplification of river courses has reduced the geomorphic complexity of channels and altered channel-floodplain connectivity, reducing the diversity of habitat and the availability of aquatic refugia. The negative consequences of these actions, have prompted numerous calls for action in efforts to promote an era of river repair (e.g. Wohl, 2004; Brierley and Fryirs, 2008). Human impacts on biophysical processes in rivers are dramatically exemplified in Australia, where impacts of European settlement since 1788 can be related to a defined pre-disturbance condition. In Bega catchment, on the south coast of New South Wales (NSW), landscape changes since European settlement have fundamentally altered river structure throughout virtually the entire catchment. Based on analysis of portion plans from the 1850s and 1860s, along with field examination of river geomorphology and associated sediment sequences, a continuous, low-capacity channel characterised the lowland plain (Figure 1c) (Brooks and Brierley, 1997). Metamorphosis of the lower Bega River occurred within a few decades of European disturbance, triggered by clearance of riparian and floodplain vegetation, and drainage of wetlands and backswamps. Comparison of portion plans from the 1850s and 1860s with archival photographs from the 1890s and early twentieth century indicate that the channel immediately upstream from Bega township widened from around 40 m to 140 m (Brooks and Brierley, 1997). As a response to anthropogenic disturbance, pools were infilled, and up to 2 m of sand accumulated on floodplains which were previously dominated by silt. The river had been transformed to a low sinuosity sand bed river style. Detailed field investigations indicate that relatively little change to river structure occurred between 1900-1960 (Brooks and Brierley, 2000). However, since the 1960s, the lowland channel has become choked by willows and other exotic vegetation. A complex pattern of bars and islands has developed within a braided channel planform. These structural changes to river morphology have fundamentally altered physical process interactions with riparian vegetation and coarse woody debris, impacting on the structural and ecological controls that these elements have on aquatic ecosystems. River metamorphosis in Bega catchment has not been restricted to the lowland plain. At the time of European settlement, base of escarpment sections of various subcatchments comprised continuous valley fills (Figure 1a) (Fryirs and Brierley, 1998; Fryirs, 2002). These cut-and-fill rivers had evolved and accumulated at the base of the escapment over around 6,000 years (Fryirs and Brierley, 1998). In some subcatchments, these extended into mid-catchment floodouts with discontinuous channels (Figure 1b) (Brierley and Fryirs, 1998). Within a few decades of settlement, drainage of upland swamps and a range of indirect responses to early agricultural pursuits triggered headcut incision into these large sediment sources. Incision was quickly followed by extensive channel expansion, supplying over 9 million m3 volumes of sediment to the lower catchment (Fryirs and Brierley, 2001). Incised channels are locally more than 10 m deep and 100 m wide, transforming the river from an intact valley fill to a channelised valley fill river style. Channel floors are functionally detached from their perched valley fills. Many of the ecological values of these former swamps have been lost, and the few tributary swamps which remain have high conservation value. River reaches that connect the base of the escarpment to the lowland plain are largely bedrock-confined. These parts of the catchment have acted as very efficient sediment transfer or throughput zones. Channel bed elevation has risen and fallen at different stages in the passage of sediment slugs. Only patches of riparian vegetation remain in mid-catchment. Today these reaches are severely degraded in ecological terms (Chessman et al., 2006). These dramatic changes to river morphology have resulted in a fundamental alteration of bedload sediment flux interactions throughout the catchment (Figure 2). Fryirs and Brierley (2001) document the post-European settlement sediment budget for the catchment. In the period since European settlement in the 1840s, over 14 million m3 of bedload sediment has been released and flushed from the upper catchment (largely from incision of intact valley fills) with a sediment delivery ratio of 68 %. Over 67 % of sediment released has been sourced from just 25 % of the catchment and the slopes are decoupled from the channel network (Fryirs and Brierley, 1999). 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. Sediment is stored in a large within-channel sediment slug (8 million m3) and as thick floodplain sand sheets (over 4 million m3). The changing nature of sediment source, transfer and accumulation zones has varied markedly from subcatchment to subcatchment since European settlement (sensu Schumm, 1977) (Figure 3). Prior to European settlement large intact swamps at the base of the escarpment acted as sediment sinks over several thousand years. These have now been transformed into sediment source zones. The lowland plain once acted as a sediment transfer zone and now acts as a sink. The process zone functioning of this catchment has been significantly altered such that sediment exhaustion has effectively occurred and longitudinal, lateral and vertical linkages have been fundamentally altered (Brierley et al. 1999). This has major implications for the geomorphic recovery potential of rivers (see Fryirs and Brierley, 2000, 2001), their ecological health (see Chessman et al., 2006; Brierley et al., 1999) and constrains what can be realistically achieved in terms of river rehabilitation over the short-medium term (see Brierley et al., 2002).