Slope–channel decoupling in Wolumla catchment, New South Wales, Australia: the changing nature of sediment sources following European settlement

Catena 35 Ž1999. 41–63

Slope–channel decoupling in Wolumla catchment, New South Wales, Australia: the changing nature of sediment sources following European settlement Kirstie Fryirs ) , Gary J. Brierley School of Earth Sciences, Macquarie UniÕersity, North Ryde, NSW 2109, Australia Received 11 February 1998; revised 17 September 1998; accepted 27 October 1998

Abstract Within a few decades of European settlement, channel incision transformed discontinuous river courses throughout Wolumla catchment, on the south coast of New South Wales, Australia. The development of continuous channels greatly increased sediment delivery from the catchment. This paper documents the character, timing and proportion of sediment sourced from upland valley fills, channel expansion sites, and gully networks. Volumes of material transferred from these sources are compared with estimates of sediment eroded from hillslopes, and the movement of sediment off the slopes to the valley floor is assessed. Although disturbance of slopes resulted in significant movement of materials, most of this material has been stored on-slope, in trapped tributary fills and along lower order drainage lines. The slopes are effectively decoupled from the channel. Sediment accumulation in farm dams over the past few decades has been negligible. Around 75% of the total volume of material released from creeks in Wolumla catchment since 1865, i.e., 5500 = 10 3 m3, has been derived from channel incision into valley fills at the base of the escarpment. Sediment flushing occurred within a few decades of catchment disturbance. Bedrock confinement in the middle and lower catchment resulted in very efficient downstream transfer of materials. Although gully networks and channel expansion sites have released a relatively small volume of material, these sources are the greatest contemporary source of sediment in Wolumla catchment. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Human impact; Slope–channel coupling; Sediment sources; Channel incision; NSW

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1. Introduction Various studies have documented the transformation of river landscapes following European settlement of southeastern Australia ŽBird, 1982, 1985; Erskine, 1992, 1993, 1994a,b, 1996; Brizga and Finlayson, 1994; Prosser et al., 1994; Eyles, 1977a,b; Brierley and Murn, 1997; Brooks and Brierley, 1997, Brooks and Brierley, in press; Fryirs and Brierley, 1998; Brierley and Fryirs, 1998.. However, to date, there have been few systematic assessments of the relative timing and volume of materials eroded and transferred from differing sediment sources within these catchments. To construct a meaningful sediment budget, catchment-based inventories of sediment sources, yield, and delivery must incorporate an assessment of material moving off slopes, along with analysis of materials reworked from the drainage system. Of critical concern is the linkage between erosion of materials from hillslopes and its transfer to the valley floor. In a coupled system, sediments are supplied directly from the slopes to the channel network, where they are reworked and transported downstream. In general, such a mechanism occurs primarily in headwater settings where the channel and slopes are directly connected, or where the channel abuts the valley margin. In a decoupled system, where sediment transfer to the drainage system is inefficient, sediments are stored on-slope, in intact lower order drainage lines, or on floodplain surfaces. In these cases, colluvial sediment contributions to the alluvial sediment budget are insignificant. Hence, the nature of slope-channel coupling is a key determinant of the effectiveness of downstream sediment transfer within a catchment ŽPhillips, 1989, 1995; Harvey, 1992, 1994, 1996.. Dramatic changes to river morphology occurred along the lower Bega River, on the far south coast of New South Wales ŽNSW., in the second half of the 19th century ŽBrooks and Brierley, 1997, Brooks and Brierley, in press.. This process was aided by highly efficient downstream transfer of materials eroded from tributary catchments in the few decades following European settlement of the region ŽBrierley and Fryirs, 1998.. Extensive volumes of sediment were reworked from valley fill deposits that had accumulated over thousands of years at the base of the escarpment ŽFryirs and Brierley, 1998.. Wolumla catchment, which drains an area of 131 km2 , joining the Bega River just south of Bega township, is one of the primary tributary subcatchments in Bega Valley ŽFig. 1.. This paper documents the spatial and temporal pattern of sediment storage and release from the three primary sources in Wolumla catchment Žvalley fills, gullies, and channel expansion sites.. These volumes are compared with estimates of the amount of sediment released from hillslopes. A schematic framework showing the changing relationship between hillslope and channel sediment sources in the post-settlement period is proposed.

2. Methods used to analyse sediment sources in Wolumla catchment Portion plans from the 1860s, along with air photographs from 1944, 1962, 1971 and 1994, were used to map, at a scale of 1:12,500, the distribution of sediment sources

K. Fryirs, G.J. Brierleyr Catena 35 (1999) 41–63 43

Fig. 1. Sediment sources within Wolumla catchment. ŽA. Location of 25 farm dams used to determine slope contributions. ŽB. Valley fill sediment sources. ŽC. Location of the three main gully systems. ŽD. Channel expansion sites.

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throughout Wolumla catchment ŽFig. 1.. Detailed historical air photograph and archival portion plans were used to determine the timeframe of valley fill incision and channel adjustment, the rate of headward extension of gullies, and the timeframe of channel

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Fig. 2. Examples of the four primary sediment sources in Wolumla catchment. ŽA. Valley fill exposure in upper Wolumla Creek. The exposure is 12 m deep and the channel at this site is 105 m wide. ŽB. Ayrdale Gully, looking downstream to cross-section 2 in Fig. 6. Note the extensive sidewall fluting. ŽC. Greendale channel expansion site. The channel is around 100 m wide and 3–4 m deep. Note the extensive sand accumulation on the channel bed. ŽD. Sediment storage was analysed along a representative hillslope transect in Pulpit Park subcatchment. The transect was located in the middle ground of this photograph. Note the abrupt break in slope at the valley margin.

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expansion throughout Wolumla catchment. Representative photographs showing the four types of sediment source analysed in this paper are presented in Fig. 2. Throughout this study, reference is made to storage and transfer of sand-sized materials which comprise the bedload fraction of all river courses in Wolumla catchment. It is recognised implicitly that while a significant proportion of the fine grained fraction Ži.e., silts and clays. may have been stored along slopes and valley floors, much of this material has likely been flushed through the river system. 2.1. Contemporary slope contributions Following procedures outlined by Neil and Fogarty Ž1991., contemporary sediment contributions from hillslopes were calculated by analysing sediment accumulation in 25 farm installed after 1950 throughout Wolumla catchment ŽFig. 1A.. In addition, an anecdotal survey of sedimentation in farm dams was conducted in collaboration with the NSW Department of Land and Water Conservation. Data on topsoil thickness and composition were derived from the recent soil survey completed by Tulau Ž1997.. Pits and auger holes were used to analyse sediment thickness and composition along a hillslope transect ŽFig. 2D., and in trapped tributary fills. These analyses were used to determine the link between the hillslopes and the valley floor, and where in the catchment slope-derived materials were stored. 2.2. Valley fill deposits at the base of the escarpment On the 1865 portion plans of the catchment, valley fills along Wolumla Creek were noted as ‘Wolumla Big Flat’ and South Wolumla fills were called ‘Lithgow Flat’. The area was described as open, containing swamps which were well grassed and subject to inundation Ž1865 Portion plan nos. 5951572 , 196 1438 and 18 1438 .. Portion plans and stratigraphic evidence from Greendale and Frogs Hollow subcatchments show similar features along the length of the valley floor. Headcut retreat and subsequent channel expansion has formed channels up to 10 m deep and 100 m wide within these valley fills ŽFig. 1B; Fig. 2A.. The valley fills themselves extend up to 12 m deep and 300 m wide. Frogs Hollow subcatchment retains a discontinuous watercourse, with an intact swamp at the base of the escarpment, and a floodout in mid-catchment ŽMelville and Erskine, 1986; Brierley and Fryirs, 1998.. These sections of the subcatchment have not been incised and store significant volumes of material. The pre-disturbance swamp and valley fill surfaces Ži.e., the 1865 surface. were used as a bench mark to calculate the volumes of material removed since European settlement. Forty-eight valley cross-sections were surveyed to determine the volume of sediment stored in valley fills versus that which has been removed. Areas of each valley fill surface were determined using a planimeter. In estimating the volumes of material stored at valley margins, extrapolations were based on adjacent slope angles. 2.3. Gully networks in Wolumla catchment Only 12 of the 70 lower order drainage lines which join trunk streams in Wolumla catchment have developed continuously incised channels ŽBrierley and Fryirs, in press..

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Of these, three significant gully networks have developed in tributary valley fill and colluvial materials. These are located in the mid-catchments of Wolumla and Frogs Hollow ŽFig. 1C.. On the 1865 portion plans, these sites were unincised and noted as swampy. Ayrdale and Nancy’s Place gullies extend up to 13 m deep and 60 m wide ŽFig. 2B.. A number of lobes have radiated from the trunk in the upper third of these gully networks, with active sidewall and headward erosion. Sidewall erosion is less active in the lower two-thirds of the gullies, which have meandering planforms and wide, stepped cross sections. A number of bedrock steps have acted as local base levels. Bailey’s gully is much smaller than the other two gully networks. The volume of material released from each gully network was measured using field-derived planform maps and up to 55 closely tied cross sections in each gully, along with analysis of air photographs dating from 1944 to 1994. 2.4. Channel expansion sites in Wolumla catchment On portion plans from 1865, numerous sections of creek lines in the middle and lower part of Wolumla catchment were noted as swamps with discontinuous ponds and

Table 1 Sediment sources in Wolumla catchment Ž1865–1997. Source

Location

Removed volume Ž=1000 m3 .

Remaining volume Ž=1000 m3 .

Within channel stores Ž=1000 m3 .

Valley fill

Wolumla South Wolumla Frogs Hollow

3481 585 nra

910 64 nra

Greendale Total

80 4146

1540 1300 swamps950 floodouts 480 18 4288

Channel expansion

Greendale Wolumla South Wolumla Frogs Hollow Total

378 229 140 239 986

750 260 188 59 1257

151 58 66 52 176

Gullies

Wolumla

Ayrdales 230 Nancy’s Places152 nra Bailey’ss 0.12 nra 382

a

Trivial

nra 4 nra 4

nra Trivial nra Trivial

Trivial being actively supplied 5549

nra

South Wolumla Frogs Hollow Greendale Total Hillslopes

3708 Grand total

a

9222

13 987

1314

Estimating volumes of material that are available to be removed from Ayrdale and Nancy’s Place gully networks is constrained by the fact that these systems are actively reworking colluvial deposits as well as extending along drainage lines.

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drainage lines. In comparison to upland valley fills, there is limited storage of sediment in this part of the catchment. Following incision, the channel has adopted a meandering planform through these former swamp deposits, with significant channel expansion and erosion of concave banks. Four channel expansion sites have been identified in Wolumla catchment ŽFig. 1D.. Of these, the largest and most actively eroding site is in Greendale subcatchment, where up to 100 m of channel widening has occurred to date, and channel banks are 3 to 4 m deep ŽFig. 2C.. At each of the channel expansion sites, cross-sections were surveyed, and detailed maps compiled using portion plans from 1865 and air photographs from 1944, 1962 and 1994, to determine the timing and volume of sediment released from these sites. Data pertaining to volumes of material removed Žand remaining. from all sediment sources are summarised in Table 1. 3. Historical changes to sediment sources in Wolumla catchment Four phases have been differentiated in the history of sediment transfer following European settlement of Wolumla catchment ŽFig. 3.. 3.1. Phase 1 pre-disturbance (i.e., prior to 1865) There is no available evidence to suggest that there was significant movement of materials from vegetated slopes in the pre-disturbance period. Given that valley fills were intact, there were no deeply incised channels, no channel expansion sites and no gully networks in the catchment at this time. This indicates that sediment transfer in Wolumla catchment was extremely low at the time of European settlement of the region. Materials were stored within-catchment in extensive valley fills. Basal age estimates from valley fills in upper Wolumla Creek of around 5600 years BP ŽFryirs and Brierley, 1998. indicate that average rates of valley fill accumulation were less than 2 mmryear over this period, although this estimate is constrained by limited knowledge of the extent of sediment reworking in this cut-and-fill landscape. 3.2. Phase 2 1865–1900 The area surrounding Wolumla township was settled in 1851. Between 1851 and the 1890s, extensive vegetation clearance from the valley flats and hillslopes occurred ŽBrooks and Brierley, in press.. By 1860, many of the hillslopes and valley flats in Bega catchment had been extensively ploughed and cultivated with potatoes, oats, sorghum, corn and wheat ŽLunney and Leary, 1988.. Upland swamps had been drained by 1865, and were sustaining crops of lucerne and corn ŽFryirs, 1995.. Around 1890, most of these agricultural practices ceased, and dairying became the dominant land use. Subsequently, most of the catchment has become dominated by pasture, with scattered pockets of forest. These land use changes triggered major changes to sediment delivery. In the early decades post-disturbance, sediment loss from hillslopes, as a result of vegetation clearance, ploughing and cultivation, was likely the dominant initial sediment

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Fig. 3. The timing of sediment contributions from Wolumla catchment, 1865–present. Note that sediment thickness on the hillslopes are greatly exaggerated. In phase 1, little if any bedload material was released from the catchment. Alluvial materials were stored in intact valley fills which were characterised by discontinuous watercourses. Slopes were well vegetated and colluvial contributions were low. Prior to incision of valley fills Žc. 1900., it is inferred that extensive slope erosion occurred. This is shown as Phase 2. Most of the slope-derived material was not supplied to the valley floor, and remains stored on slopes or in trapped tributary fills. In phase 3, incision into valley fills formed deep channels which subsequently expanded to over 100 m wide. This released massive volumes of material. In all subcatchments other than Frogs Hollow, channels became continuous along trunk streams. Finally, in phase 4, rates of sediment removal have been at least an order of magnitude lower than in phase 3.

source in the catchment Žcf., Wasson et al., 1996, 1998; Groth, 1997.. Sediment accumulations up to 20 cm thick behind fences installed late last century attest to material movement in this phase. However, a large proportion of the material derived from slopes has remained stored on-slope, with small volumes accumulating in trapped tributary fills and along intact lower order drainage lines. Remarkably little colluvial material reached the drainage network.

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Assessment of sediment release from slopes is hindered by limited knowledge of the baseline Žpre-European settlement. condition of the catchment. It has been estimated that topsoils in Bega catchment were a maximum of 35 cm thick at the time of European settlement ŽTulau, 1997, pers. comm.., whereas today they average 25 cm thick ŽTulau, 1997.. However, field analysis and evidence from road cuttings suggest that many parts of the catchment probably had very thin topsoils in the pre-disturbance period, and that these have been stripped to - 15 cm thick today. Between 1865 and 1900 continuous valley fill incision did not occur, so there was no associated tributary activity and gully development Žc.f., Brierley and Fryirs, in press.. Discontinuous gullies likely developed in mid-catchment locations, with materials retained as floodouts within-catchment Že.g., Brierley and Fryirs, 1998.. However, channel expansion in these sections of the catchment had not yet begun. 3.3. Phase 3 1900–1944 The major phase of sediment release in Wolumla catchment occurred between 1900 and 1944. This was the most active phase of geomorphic change and sediment movement in the period following European settlement. It was characterised by incision and lateral expansion of channels, with extensive material removal from the fluvial system. Anecdotal evidence suggests that incision into ‘Wolumla Big Flat’ began around 1900 ŽFig. 4.. It is likely that incision and lateral expansion of channels in South Wolumla, Frogs Hollow and Greendale subcatchments occurred at a similar time, and certainly prior to the first set of air photographs, in 1944. Since 1944 there have been negligible changes to channel dimensions. Hence, the initial flush of sediment likely took the form of a slug Žcf., Erskine, 1994b; Nicholas et al., 1995; Rutherfurd, 1996., which passed through Wolumla catchment in the early decades of the twentieth century. In total, over 4150 = 10 3 m3 of material has been removed from valley fills in the upper parts of Wolumla, South Wolumla and Greendale subcatchments ŽTable 1.. Upper Frogs Hollow subcatchment has retained a discontinuous channel, and is characterised by an intact swamp and floodout, which store almost 1500 = 10 3 m3 of material. As noted for upland valley fills, adjustments in channel morphology in the middle and lower parts of the subcatchments have been negligible since 1944, with the exception of the channel expansion site along Greendale Creek ŽFig. 5.. Hence, release of materials associated with incision and lateral expansion of channels in these sections of the catchment were at a maximum in the early decades of the twentieth century. An estimated 990 = 10 3 m3 of material was eroded from channel expansion sites between 1900 and 1944. Incision of the trunk streams promoted gully initiation along tributary streams, such as Ayrdale and Nancy’s Place gullies, which were well developed by 1944. The 1944 air photographs indicate that riparian vegetation cover was non-existent at this time, and the gullies had extensive sidewall flutes. However, numerous lobes that radiate from the trunk in headward sections of the gullies have not reached their present extent ŽFig. 6.. Based on an intact surface at the time of European settlement, an estimated 380 = 10 3 m3 of material was removed from gullies between 1865 and 1944.

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Fig. 4. Incisional history of upper Wolumla valley fill since 1865. ŽA. Intact valley fill around 1865. ŽB. Incision of the valley fills commenced around 1900 forming a continuous channel throughout this section of the catchment. ŽC. Lateral expansion of channels produced a wide channel. Reworking and accumulation of deposits within the incised channel formed inset features at channel margins and bars on the channel bed. ŽD. The present geomorphic character of this section of the catchment was attained by 1944. Only slight increases in sedimentation have occurred on the channel bed.

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Working on these figures, over 95% of the total sediment released from Wolumla catchment since 1865 occurred between 1900 to 1944 Ž5240 = 10 3 m3 out of a total volume removed of 5510 = 10 3 m3 .. This is summarised in Fig. 7. Although extensive downstream transfer of materials occurred along river courses in Wolumla catchment between 1865 and 1944, the connectivity between slopes and the valley floorrincised channels was poor and the majority of slope derived materials remained stored on-slope. 3.4. Phase 4 post-1944 The air photograph record from 1944 to 1994 shows little change in the geomorphic character of Wolumla catchment. In this phase, the most significant sediment contributors have been tributary gullies and channel expansion sites. However, contemporary rates of sediment release from these sites are minor compared to the volume of sediment released between 1900 and 1944 ŽFig. 7.. Today, hillslope surfaces throughout the catchment are generally well covered with pasture grasses. There are no sites of significant hillslope erosion. Extrapolating derived sediment accumulation rates in farm dams across the catchment, an estimated 2100 = 10 3 m3 of material has been reworked on slopes since 1950, at an average rate of 3 m3 hay1 yeary1 . This rate is higher than that of other studies which have quantified the movement of materials on hillslopes in southeastern Australia ŽOlive and Rieger, 1986; Neil and Fogarty, 1991; Wasson, 1994.. However, it is not the rate of material movement on hillslopes that is the critical component in the contemporary sediment budget. Rather, it is the efficiency of transfer of this material to the valley floor that determines the release of this material from the catchment. Since 1944 there have been minor changes to channel dimensions in upland valley fills. At numerous sites, up to 3 m of sands line the channel bed. Local widening of the channel has released an estimated 50 = 10 3 m3 of material. This represents - 5% of the total volume of sediment released from valley fills. While the primary incised channels have been relatively stable since 1944, headward extension of tributary gully networks has continued in this period, and these sediment sources have become more significant Žin volumetric terms. than the incised channel source. An estimated 120 = 10 3 m3 of material has been released from gully networks since 1944. Since 1962, Ayrdale Gully has expanded upstream by around 200 m ŽFig. 6.. A similar picture is evident in Nancy’s Place Gully, where 150 m of headward extension and a new lobe have developed since 1962. Channel expansion has effectively ceased in the lower sections of the gullies, where riparian vegetation cover has greatly increased. Morphologic changes have been more pronounced at Bailey’s Gully. Gullying Fig. 5. Temporal development of the channel expansion site in Greendale subcatchment. ŽA. An intact valley fill characterised by chains of ponds is noted on the 1865 portion plans. ŽB. By 1944 a continuous channel had incised through the valley fill. The channel was around 40 m wide with a sinuosity of 1.17. ŽC. By 1962 the incised channel had widened to around 50 m and sinuosity had increased to 1.19. The channel was choked with sand. ŽD. Today, the channel is around 100 m wide, 3–4 m deep and has a sinuosity of 1.39. Numerous bars and inset features store significant volumes of material in the within-channel zone. Perpendicular gully development is releasing further valley fill material.

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Fig. 7. Temporal changes in alluvial sediment transfer in Wolumla catchment. Note the passage of the sediment slug between 1865 and 1944.

at this site occurred after 1962, and has subsequently released around 10 = 10 3 m3 of material. Given that numerous unincised tributary fills could incise as a lagged response to base level lowering of the trunk streams ŽBrierley and Fryirs, in press., gully erosion within the catchment represents a significant potential future sediment source. Finally, channel expansion sites have released an estimated 100 = 10 3 m3 of material since 1944. By 1962, channel dimensions at the Greendale expansion site had increased to around 50 m wide Žcompared to 40 m in 1944. and extensive erosion of concave banks was occurring ŽFig. 5.. The channel bed at this time was choked with sand. By 1997, continued concave bank erosion had produced a channel over 100 m wide and 3 to 4 m deep, with an increased channel sinuosity Žnow 1.39, compared to 1.19 in 1962 and 1.17 in 1944.. In Wolumla catchment as a whole, around 1250 = 10 3 m3 of valley fill material remains stored at sites subjected to ongoing channel expansion. These materials are Fig. 6. ŽA. Gully initiation and development at Ayrdale. Ži. Intact swampy surfaces are noted on the 1865 portion plans. Žii. By 1944, the trunk of the gully was well developed and lateral expansion had occurred. Žiii. By 1962, numerous lobes were extending headward. Živ. By 1997, the lobes at the head of the network had extended further. Active sidewall erosion is still evident. ŽB. Contemporary morphology of Ayrdale Gully. Cross-section locations are indicated on Fig. 6aŽiv.. Channel geometry shallows and widens in a downstream direction. A significant proportion of material released from upstream cross-sections Ž1–5. has been restored along downstream cross-sections Ž6–10..

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protected behind bedrock spurs and steps in Wolumla, South Wolumla and Frogs Hollow subcatchments. However, the Greendale channel expansion site, which has contributed almost 40% of the total volume removed by channel expansion, still stores an estimated 750 = 10 3 m3 of material Ži.e., almost 60% of that remaining at potential channel expansion sites.. Given the accessibility of this material, and the actively eroding nature of this site, significant further sediment release is likely. A large proportion of this eroded sediment is stored on the channel bed along this reach Žaround 150 = 10 3 m3 . and is yet to be transferred to lower Frogs Hollow Creek and hence Wolumla Creek. 3.5. Post-disturbance sediment budget for Wolumla catchment Of the 5500 = 10 3 m3 of material eroded from alluvial sediment stores in Wolumla catchment, only 1300 = 10 3 m3 remains stored on the channel bed and at channel margins. Around 4200 = 10 3 m3 has been supplied to the Bega River. This gives an alluvial sediment delivery ratio from the catchment of over 75%. This contrasts with overseas studies, in which sediment delivery ratios of around 10% have been recorded for catchments which drain more than a few hundred square kilometres Žsee Meade, 1982; Trimble, 1983; Phillips, 1991, 1992, 1993, 1995.. In these cases, this has been attributed to poor within-stream coupling Žsensu Phillips, 1995. and storage of sediment along river courses for residence times of up to a few thousand years. While Wolumla catchment is smaller than these documented cases, the system has experienced similar disturbance and sediment release. However, the relatively narrow valleys present limited opportunity for sediment re-storage, and efficiently convey materials through the catchment. 3.6. Summary contributions from differing sediment sources Incision and expansion of channels into valley fill deposits has been the dominant sediment source in the period following European settlement of Wolumla catchment, accounting for around 75% of the total volume of sediments released Ži.e., 4150 = 10 3 m3 of material out of a total of 5500 = 10 3 m3 ; see Table 1.. Removal of valley fill in Wolumla subcatchment alone totals 3480 = 10 3 m3. This is around 85% of the total material released from valley fill sources. The volume of sediment released from channel expansion sites totals 990 = 10 3 m3 , almost 40% of which has been contributed by the Greendale channel expansion site. Around 380 = 10 3 m3 of material has been released from gully systems in Wolumla catchment. This represents - 10% of the total sediment removed from the catchment. 4. Slope-channel decoupling in Wolumla catchment Sediment generated by hillslope and soil erosion processes are a vital constituent of any drainage system and have traditionally been seen as one of the most important sources of sediment. In order to fully appreciate the spatial and temporal aspects of sediment yield and delivery, catchment-based assessments of colluvial and alluvial

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systems are required. While many studies have dealt with the colluvial component of the sediment system Že.g., Loughran, 1990; Loughran et al., 1992; Loughran and Elliott, 1996. and the alluvial component ŽPhillips, 1991; Erskine, 1992, 1993; Brizga and Finlayson, 1994., very few have assessed both ŽTrimble, 1983; Phillips, 1995; Wasson et al., 1996. or the link between the two ŽHarvey, 1991, 1992.. In tectonically active settings, landscapes are highly dissected. This produces high drainage densities, presenting significant opportunity for slope-stream coupling. In these settings, channels and slopes are connected and sediment yields are high Že.g., Harvey, 1992, 1994, 1996.. In contrast, large parts of the tectonically stable Australian landscape are characterised by low relief. In many instances in southeastern Australia, lower order drainage lines remain unincised Ži.e., intact. or are disconnected from the primary drainage lines by valley marginal floodplain. Hence, while sediment reworking and soil erosion on hillslopes have been relatively high in many disturbed landscapes of southeastern Australia ŽOlive and Walker, 1982; Olive and Rieger, 1986., sediment has not been efficiently transported to the drainage network Žcf., Walling, 1983; Phillips, 1995.. Slopes and channels are effectively decoupled, and sediments released from slopes are contributing little to the sediment budgets of these systems Žcf. Olive and Rieger, 1986; Olley et al., 1993; Loughran et al., 1992; Brizga and Finlayson, 1994; Wasson, 1994; Wasson et al., 1996, 1998.. Processes operating along the valley floors of small upland basins in Wolumla catchment have determined the volume and yield of sediment transferred to Bega River. Stream bank erosion in lowland areas constitutes a relatively minor sediment source. These findings parallel the observations of Wasson Ž1994. and Wasson et al. Ž1998.. In Wolumla catchment, it is estimated that over 95% of the sediment released from the catchment in the period following European settlement has been derived from alluvial rather than colluvial sediment stores. Transfer of materials from hillslopes has been negligible, reflecting ongoing decoupling of the slope–channel system Žcf., Walling, 1983; Phillips, 1989; Harvey, 1994, 1996.. Various sources of evidence suggest that the volume of material removed from hillslopes in Wolumla catchment has been relatively small compared to that released from alluvial sediment stores in the period following European settlement. Ø The majority of Bega catchment consists of ‘transferral soils’ ŽTulau, 1997., defined as deposits of eroded material washed from upslope. Sand has accumulated behind fences on slopes, reflecting re-storage of materials on-slope. Analysis of soil depth along a slope profile in Pulpit Park subcatchment showed that the thickness of soils increased down-slope from 10 cm at the ridge crest to 25 cm thick at the toe of the slope ŽFig. 8.. Ø Although significant material movement on slopes is acknowledged, this material did not extend atop the valley fill surface, as indicated by the distinct composition of the topsoil and valley fill material ŽFig. 8.. Ø There is little evidence for extensive sand accumulation atop swampy valley floor surfaces in other subcatchments. For example, the valley floor surfaces of Ivonlea and upper Frogs Hollow subcatchments have less than 20 cm of sand accumulation atop fine grained swamp deposits, while no sand is evident on many other tributary valley floors ŽFryirs, 1995..

58 K. Fryirs, G.J. Brierleyr Catena 35 (1999) 41–63 Fig. 8. Valley scale cross section through Pulpit Park tributary fill. Note the exaggerated thickness of the topsoil on this cross-section. Superimposed on the slope profile is the depth of soil over bedrock. Note that the soil thickens in a down-slope direction, but the toe of the slope has a marked break in slope to the valley fill surface. The valley floor is comprised of deep valley fill which has a distinct sedimentology to that of the hillslope material.

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Fig. 9. Valley cross sections depicting the contrast between upstream trapped tributary fills and downstream tributary fills. ŽA. Upstream trunk stream valley fills are up to 12 m deep and are perched above tributary valley fills, which have become ‘trapped’. ŽB. Downstream trunk stream valley fills are up to 4 m deep and have flat surfaces. Note the distinct break in slope at the base of the hillslope suggesting that slope derived materials are not stored in this section of the profile.

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Ø Valley cross-sections throughout Wolumla catchment show little indication of material accumulation at the toe of slopes. Abrupt breaks in slope at valley margins indicate that there has been minimal transfer of materials to the valley floor ŽFig. 9.. Ø In upstream sections of the catchment, where valley fills are 4 to 12 m deep, the surface of the valley fill declines with distance away from the central axis of the valley ŽFig. 9a.. This suggests that rates of valley floor aggradation along the trunk stream exceeded the rate of material accumulation along tributary courses, effectively perching valley fill deposits above tributary fills. To date, sediments derived from slopes have barely filled these ‘hollows’ and sand accumulations are thin or non-existent atop these intact tributary valley fills. Any material that is supplied to these tributaries is effectively trapped ŽTimms, 1986; Brierley and Fryirs, in press.. Elevated trunk stream valley fills buffer Žor decouple. sediment delivery from hillslopes to the channel. It is only when these valley fill buffers are incised, and gully networks develop, that the slopes and the channel become coupled. In mid-catchment and downstream locations, where valley fills range from 1 to 4 m deep, tributary fills are generally not trapped behind a perched trunk stream valley floor ŽFig. 9b.. Rather, the valley floor is essentially flat at these sites, but there is an abrupt break in slope at valley margins. This suggests that delivery of sediments from slopes to the valley floor is limited throughout the catchment. Ø As the valley floors were unincised at the time of European settlement, any sand materials that lined the valley floor would remain in the upper catchment until incision took place. The only proportion that would subsequently be released would be that fraction that lay atop the valley fill in the section of the valley floor that was incised. From this, it is inferred that hillslope-derived materials have played a relatively minor role in terms of sediment release from Wolumla catchment in the period following European settlement. While the rate of erosion of materials on slopes in the period immediately following disturbance was initially high, actual contributions to the valley floor were small Žcf., Olive and Rieger, 1986; Prosser et al., 1994; Rinaldo et al., 1995.. Slopes were decoupled from the stream network and, consequently, the primary mechanisms of downstream sediment transfer, at the time of European settlement. This relationship did not change following disturbance of slopes in the post-settlement period. Removal of materials from valley fill deposits has dominated sediment release from Wolumla catchment. 5. Implications and conclusions In Wolumla catchment, alluvial sediment supply has greatly outweighed contributions from colluvial sources. In terms of future sediment transfer, it is likely that the following will occur. Ø Hillslopes will retain contemporary low rates of sediment supply. Ø Sediment release from valley fills will decrease further, most materials remain trapped between bedrock spurs at valley margins and behind bedrock steps ŽTable 1.. Ø Valley fills may refill, but further reworking of sediments stored on the channel bed may occur. Ø Nickpoints could retreat into the intact swamp and floodout in Frogs Hollow subcatchment where large volumes of material are stored.

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Ø Sediment release from established gully networks will diminish given limited opportunity for further headcutting. However, throughout most of the catchment, tributary fills remain unincised and are effectively protected from erosion by elevated valley marginal valley fill ŽBrierley and Fryirs, in press.. If lower order drainage lines become connected to the trunk stream, gully networks may establish along lower order tributary valley fills. Ø Channel expansion will continue whenever possible, especially along Greendale Creek. Although the clearance and cultivation of slopes in Wolumla catchment induced significant erosion within a few decades of European settlement, most of the material derived has been stored on-slope and in trapped tributary fills. This sediment has not been directly transferred to the fluvial system. Despite the dramatic transformation to the landscape over the last 150 years, the channels and slopes in Wolumla catchment remain decoupled. Materials derived from alluvial sediment sources have dominated the sediment budget. In total, around 5500 = 10 3 m3 of material has been eroded from valley fill, channel expansion and gully sources since 1865, most of which has been derived from valley fills at the base of the escarpment. Sediment transfer is the key underpinning of landscape change. Unless sediment sources, storage units, and potential sites of erosion are mapped accurately, and process controlling sediment delivery are understood, the effectiveness of catchment management practices may be compromised. Catchment-based management of colluvial and alluvial sediment stores, and their connectedness, should be a critical component of catchment planning. This planning needs to be flexible such that the changing nature of sediment release and delivery within a system, over time, can be managed effectively. There are no obvious reasons why the findings from this study should not have direct parallels in other drainage basins in southeastern Australia or elsewhere. Indeed, one of the intriguing research questions that emerges from this study is the need to develop further our understanding of the relative contributions from colluvial and alluvial sediment stores to sediment budgets in differing landscape settings, and their changing role under differing land management practices. Acknowledgements Much of this work has emerged from LWRRDC Project MQU1 coordinated by Dr Gary Brierley at Macquarie University. Special thanks are extended to the landowners of Wolumla catchment who filled out extensive questionnaires and allowed access to farm dams, as well as students from the 1996 Applied Geomorphology class at Macquarie University who helped survey cross sections in numerous gully networks. References Bird, J.F., 1982. Channel incision at Eaglehawk Creek, Gippsland, Victoria, Australia. Proc. R. Soc. Victoria 94, 11–22. Bird, J.F., 1985. Review of channel changes along creeks in the northern part of the Latrobe River basin, Gippsland, Victoria, Australia. Z.F. Geomorph. 55, 97–111.

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