P u b l i s h i n g
Marine Freshwater Research Volume 52, 2001 © CSIRO 2001
A journal for the publication of original contributions in physical oceanography, marine chemistry, marine and estuarine biology and limnology
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Large-scale patterns of erosion
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Corrigendum Volume 52, Number 1 Large-scale patterns of erosion and sediment transport in river networks, with examples from Australia. Ian P. Prosser, Ian D. Rutherfurd, Jon M. Olley, William J. Young, Peter J. Wallbrink and Chris J. Moran p. 91. Equation 12 is given as: vA f − Qf Dx = (Tx + I x ) 1 − e
(12)
However, the correct form is vA f − ∑Q f Qf Dx = Tx + I x ) 1 − e ( ∑Q
where the additional terms Q and Qf are already defined in the paper.
(12)
Mar. Freshwater Res., 2001, 52, 81–99
Large-scale patterns of erosion and sediment transport in river networks, with examples from Australia Ian P. ProsserA,B, Ian D. RutherfurdB,C, Jon M. OlleyA,B, William J. YoungA,B, Peter J. WallbrinkA,B, and Chris J. MoranA ACSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia; email:
[email protected] BCooperative Research Centre for Catchment Hydrology CDepartment of Geography and Environmental Science, The University of Melbourne, Vic. 3010, Australia
Abstract. This paper examines the patterns of sediment transport in rivers in terms of the sources of sediment and its transport and deposition through the river network. The analysis is in the context of dramatic human influences on river sediment transport and how they might influence freshwater ecosystems. The review of Australian work shows that erosion of hillslopes and stream banks has greatly increased in historical times, supplying vast quantities of sediment to rivers, much of which is still stored within the river system. The stored sediment will continue to effect in-stream and estuarine ecosystems for many decades. In most Australian catchments the dominant source of sediment is streambank erosion. An analysis of historical channel widening suggests that a conceptual framework of relative stream power can explain the diversity of behaviour observed in the numerous case studies. Sediment delivery through catchments is considered first in a generic whole network sense, which emphasizes the crucial role played by riverine deposition in determining catchment sediment budgets. A method is then presented for analysing the diverse spatial patterns of sediment storage in any river network. Finally, the paper considers the temporal changes to channel morphology in response to a humaninduced pulse of sediment.
Introduction Concern over sediment and erosion has shifted recently from on-site effects on productivity and engineering stability to downstream influences on in-stream and estuarine ecosystems. Human activities have greatly increased the supply of sediment in historical times from agricultural hillslopes (e.g. Edwards 1993), from rapid extension of gully networks (e.g. Prosser and Winchester 1996), and from catastrophic widening of river channels (e.g. Rutherfurd 2000). In Australia, changes to sediment supply have occurred within the last 200 years, and have not been matched by concomitant increase to river sediment yields (Wasson et al. 1996). Thus, residence times of sediment in river systems are long with much intermediate storage of eroded material, as is common to most large river systems (Trimble 1981; Meade 1982). Increased storage of sediment can result in substantial changes to river physical form, chemical processes and ecological health. This storage of sediment is despite historical channel changes which have increased the sediment transport capacity of many streams (Brooks 1999), and there is no doubt that within river systems there is a great diversity of impacts of the changed sediment regime. High energy bedrock reaches can propagate the increased load downstream, and much of the impact is © CSIRO 2001
absorbed by the lower energy reaches (Montgomery et al. 1996). Thus, we need analyses that capture the diversity of response within river networks. A further consequence of long residence times in rivers is that the major historical changes will continue to influence river behaviour for many decades to come. In many cases, it will not be possible to return rivers to their natural state. This makes it crucial for us to predict the trajectory of response to historical change and to assess whether it is desirable and possible to influence that trajectory in the interests of ecological health. The temporal scale of analysis thus becomes decades or longer, rather than individual events. Although sediment moves in individual floods, one can question the need to predict the response to any individual event. Of more relevance is the net response of sediment to the distribution of floods over many years. The interest in freshwater and estuarine ecosystems means that we need to understand the patterns, rates and processes of sediment transport at the scale of regional catchments (>1000 km2). It raises questions of the sources of sediment in complex landscapes, the potential for that sediment to be conveyed through the river network, and the patterns of deposition in the landscape. Sediment has its greatest potential effect on aquatic ecology in areas of 10.1071/MF00033_CO 1323-1650/01/010081
I. P. Prosser et al.
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deposition. These are areas where benthic habitat is smothered (Bartley and Rutherfurd 1999), where resuspension of fines increases turbidity, and where sediment and water column interactions influence nutrient concentrations. There is increasing awareness that river management needs to protect the currently intact and restore recoverable ecosystems (Rutherfurd et al. 2000), but to do this we need to know the potential for downstream recovery. Considerable resources are allocated each year to river management, much of them targeted toward erosion control and sediment management (White et al. 1999). Yet these resources are small relative to the scale of the problem. Consequently, there is a need to target resources effectively by making an explicit link between the management of sediment sources or stores and the pattern of downstream benefits from that management. The highly variable patterns of sediment delivery in a river network may mean that to halve sediment yield to an estuary requires treatment of just a small fraction of the upstream catchment as sediment from other areas of the catchment is deposited before reaching the estuary. Furthermore, in developing countries such as Australia there is considerable prospect for future land use change, ranging from further agricultural expansion to extensive afforestation. Again, there is a need to identify areas where those changes will influence downstream ecosystems through the response of sediment transport through the whole river network. Our attempts to predict sediment transport at regional scales need to recognise the limited spatial information that is available as a basis for prediction. For many catchments the information may be limited to a coarse resolution digital elevation model; satellite imagery; maps of geology, soils, vegetation and land use; climate data and scattered gauging stations; and channel cross-sections. Any additional catchment properties required for prediction, such as regional flood behaviour, channel geometry, or sediment particle size, need to be inferred, or measured spatially, which is often impractical or subject to uncertainty. Thus, prediction of sediment transport at large scales is by necessity based upon relatively simple process representations. The challenge is to ensure that the choice of parsimonious process rules best reflects the controls on sediment transport at the large scale. In this paper we first summarize the ecological significance of sediment transport through rivers. We then review the history of erosion and our current understanding of the patterns of sediment sources to rivers. This leads to consideration of the whole network of patterns of sediment delivery and an exploration of processes that might control spatial patterns of sediment storage in river systems. Finally, we examine the rehabilitation potential of streams affected by sediment. The anlyses are generic but we present examples from Australia, where as a result of its
relatively short history, there has been a focus on the human-induced changes to erosion and sediment transport. Significance of sediment transport The diversity and extent of physical habitat influence the structure and health of ecosystems. In this context, physical habitat includes the size and shape of the channel, the form of the bed sediments, the quality of the water and the supply of nutrients. These are all influenced by the supply and characteristics of the sediments, and the interactions between channel and floodplain. Significant fractions of phosphorus (P), carbon (C) and nitrogen (N) are transported in particulate form. Most of the phosphorus is associated with surface coating on the mineral grains, and a large proportion of this is ultimately bioavailable (Cullen 1995). Nitrogen and carbon are transported as particulate organic matter in association with the mineral sediment. The supply of C, N and P to downstream reaches is intimately linked to the transport and transformations of the organic and inorganic sediments (Drever 1988). Similarly, many agricultural chemicals and heavy metals are transported into aquatic systems in association with sediments (e.g. Peterson and Batley 1993; Thoms et al. 2000). In-channel chemical transformation and bacterial processes change the bioavailability of both the nutrients and contaminants in the rivers. Suspended and bed sediments can act both as sinks and sources of the nutrients and contaminants, depending on the prevailing environmental conditions. The concentration of fine sediment suspended in the water column alters the light regime and this affects the phytoplankton habitat and benthic biofilm production. A decrease in light and heat transmission associated with increases in the concentration of suspended sediments reduces the rate of photosynthesis, and thus the production of new algal material (Davies-Colley et al. 1992). In effect, low-flow turbidity limits the volume of stream habitat that is actually available for primary production. Increases in suspended sediment concentrations affect macroinvertebrate communities, causing morbidity (Newcombe and MacDonald 1991), and decreased abundance and diversity (Quinn et al. 1992; Metzeling et al. 1995). Fish populations are also adversely affected by high levels of fine sediment (Koehn and O’Connor 1990). These effects include reduced feeding efficiency, decreased growth rates and increased disease. Changing the load or type of sediment delivered to a channel changes the channel and bed morphology (Schumm 1977; Milhous 1998). For example, increasing the volume of coarse sediment can alter the gross morphology of the channel, decreasing the amount of geomorphic complexity by filling pools, and burying large woody debris (Rutherfurd 2000). As there is a strong correlation in streams between habitat complexity and species diversity
Patterns of river sediment transport
(Giller et al . 1994; Huston 1994; Palmer et al . 1997), decreasing the amount of geomorphic complexity decreases the biotic diversity. The channel form and the sediment type determine the physical nature of river edge habitats. The size and variability of the bed sediments, and the small and large bed-forms (ripples, dunes, riffles and pools) determine the physical nature and extent of different benthic habitats. These factors determine boundary roughness and hence the near-bed flow regime, and determine the nature and extent of surfaces on which biofilms can grow, and invertebrates can dwell. Sources of sediment Hillslope erosion Wasson et al. (1996) using the results of Rosewell (1997) predicted that the rate of hillslope erosion across Australia has increased 100-fold in historical times as a result of catchment clearing and agricultural land use. Whether there is a matching increase in sediment delivery from hillslopes to streams is uncertain. Sediment yields of catchments of the scale of 1 km 2 or so are approximately an order of magnitude lower than those recorded on subpaddock scale plots (Edwards 1993). Plot scale data were used to develop the Universal Soil Loss Equation (USLE; Wischmeier 1978) and its Australian variants upon which the predictions of Rosewell (1997) were based. Much of the sediment recorded, or predicted, from subpaddock scale plots may have been stored on the hillslopes or in riparian lands and not have reached the streams. The mechanics of soil detachment and its transport by rainfall and overland flow are well described by several models, at least for low cover agricultural surfaces (Hairsine and Rose 1991; Foster et al. 1995; Morgan et al. 1998). These models have a stronger physical foundation than the widely used empirical approach of USLE. They are useful for engineering design purposes where detailed information is available, but for application to catchment planning and regional or larger scale assessment there is still widespread use of the USLE or its Australian derivatives (PERFECT, Littleboy et al. 1992; SOIlLOSS, Rosewell 1993). This is largely because of the gap between small scale process understanding and the data available to parameterise the more detailed models. These detailed models might require information on spatial patterns of hydraulic roughness or the proportion of stream power used to transport sediment. These themselves are empirical constructs for which there are only limited available data. Thus, empirical relationships of the influence of land use, soil type and topography on erosion are all that can be applied at present. To understand the downstream impacts of land use requires information on rates of sediment delivery from hillslopes to streams rather than the rate of erosion at the
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scale of hillslope plots. Plot-scale measurements of erosion or sediment transport give results on sediment passing a point in the landscape but reveal little information on the travel distance of that sediment before it is redeposited. Studies of travel distance in flumes suggest that most entrained sediment is only transported a few metres before being redeposited (e.g. Parsons and Stromberg 1998; Bryan and Brun 1999). This helps explain the results of field monitoring at nested scales on hillslopes, which have shown little tendency for sediment yield to increase down a hillslope. Bonell and Williams (1987) found complex patterns of erosion and redeposition on a grazed semi-arid hillslope (Fig. 1) such that no one plot was a reliable guide to sediment yield from the whole slope. Overall there was no systematic growth in sediment yield with distance downslope. Similarly, Prosser and Williams (1998) found that the mass of sediment recorded moving into four 2-m-wide troughs on a burnt forested hillslope was greater than the sediment mass passing from the 5.4-ha subcatchment in which they were located.
Fig. 1. (a) Local runoff, infiltration, erosion and deposition down a semi-arid hillslope over a two-year period, inferred by Bonell and Williams (1987) from (b) the material trapped in a catena of five troughs.
Rainfall simulation experiments of erosion following forestry operations showed that although log extraction tracks and the general harvest area were significant sources of sediment (~10 t ha–1),
