BANK EROSION OF AN INCISED UPLAND CHANNEL BY SUBAERIAL PROCESSES: TASMANIA, AUSTRALIA IAN P. PROSSER 1 *, ANDREW O. HUGHES 1 AND IAN D. RUTHERFURD 2 1 Cooperative Research Centre for Catchment Hydrology, CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia 2 Cooperative Research Centre for Catchment Hydrology, Department of Geography and Environmental Science, The University of Melbourne, Victoria, 3052, Australia Received 10 September 1999; Revised 14 February 2000; Accepted 15 March 2000 ABSTRACT The headwaters of many rivers are characterized by gullies and incised streams that generate significant volumes of sediment and degrade downstream water quality. These systems are characterized by harsh climates, ephemeral flows that do not reach bank top, and bare cohesive banks of clay and weathered bedrock. We investigated the rates and processes of bank erosion in an incised canal that has such characteristics. Detailed measurements of bank position were made over two years with a purpose-built groundprofiler and photo-electronic erosion pins (PEEPs). Stage height and turbidity were also monitored. The bare banks eroded at 13  2 mm a 1 . Erosion is controlled by subaerial processes that loosen bank material. Observations show that needle-ice growth is important in winter and desiccation of clays predominates in summer. Flows are unable to erode firm cohesive clays from the banks, and erosion is generally limited by the availability of loosened material. This produces strong hysteresis in turbidity during events. Peak turbidity is related to the number of days with low flow between events, and not peak stage. Rehabilitation with a moderate cover of grass is able to prevent bank erosion by limiting the subaerial erosion processes. Projections of current erosion suggest that without vegetation cover the banks are unlikely to stabilize for many years. Copyright # 2000 John Wiley & Sons, Ltd. KEY WORDS: bank erosion; erosion rates; needle-ice; desiccation INTRODUCTION Degradation of catchments and streams has led to increased sediment loads (e.g. Meade, 1982; Sutherland and Bryan, 1991; Walling and Webb, 1983; Wasson et al., 1998) with consequent impacts on water quality and marine and freshwater ecosystems (e.g. Lemly, 1982; Galloway et al., 1996; Bunn et al., 1998). Often, the high sediment loads are generated from streambank erosion itself rather than from the surrounding catchments (Lewin et al., 1974; Grimshaw and Lewin, 1980; Simon, 1989; Wasson et al., 1998). This can particularly be the case in humid to semi-arid regions which have experienced historical land-use intensification, and degradation of valley-floor and riparian vegetation. Such degradation has resulted in massive expansion of channel networks in the form of incised streams and gullies often eroded into ancient colluvial and alluvial deposits and saprolite (Graf, 1979; Schumm et al., 1984; Bird, 1985; Prosser et al., 1994). While the main phase of channel expansion is largely complete (Eyles, 1977), extensive areas of bare streambank remain and erosion of incised stream and gully banks can generate up to 90 per cent of the total sediment yield (Olley et al., 1993; Prosser and Winchester, 1996; Wallbrink et al., 1998). Thus, there is considerable interest in sediment generation from streambank erosion in headwater systems. Recently a spatial zonation of bank erosion processes within catchments has been proposed, whereby the relative contributions of particular erosion processes change downstream (Lawler, 1992a, 1995; Abernethy and Rutherfurd, 1998; Lawler et al., 1999). These authors argue that in headwater reaches, subaerial Earth Surface Processes and Landforms Earth Surf. Process. Landforms 25, 1085±1101 (2000) Copyright # 2000 John Wiley & Sons, Ltd. * Correspondence to: Ian Prosser, CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia. E-mail: ian.prosser@cbr.clw.csiro.au Contract/grant sponsor: Tasmanian Hydro-Electric Commission Contract/grant sponsor: Land and Water Resources Research and Development Corporation