Drainage Lines Research Articles

Drainage evolution in south-central Africa since the breakup of Gondwana

The drainage system in south-central Africa has undergone major reorganisations since the disruption of Gondwana. Isopachs of the Kalahari sequence and a variety of geomorphological features can be used to pinpoint abandoned drainage lines. Continental fluvial sediments of Mesozoic-Cenozoic age reflect river systems which existed prior to and immediately following continental break-up. The east coast sedimentary sequence documents changes in the location of major supplies of terrigenous sediments, and provides a framework for establishing the timing of changes in drainage configuration. This evidence indicates that during the upper Jurassic to Cretaceous, the Okavango, Cuando and Zambezi-Luangwa rivers formed the headwaters of the proto-Limpopo. The lower-Zambezi-Shire formed a separate graben-bound river system with a discharge point into the Indian Ocean in the vicinity of mouth of the present-day Zambezi. A third major drainage entered the Indian in the vicinity of the modern Save mouth. End Cretaceous uplift along the Okavango-Kalahari-Zimbabwe Axis severed the links between the Limpopo and the Okavango, Cuando and Zambezi-Luangwa. This resulted in a senile endoreic drainage system which supplied sediment to the Kalahari basin. However, the uplift rejuvenated the lower Zambezi, initiating headward erosion and progressive capture of the Luangwa, upper Zambezi and Kafue. Predatory headward extension of the Zambezi is still active, and this river will eventually capture the Okavango. The model developed for drainage reorganisation provides a framework for interpreting kimberlitic heavy mineral dispersion patterns. It also forms the basis for explaining fish and plant dispersion patterns, and understanding recent water level fluctuations in the Makgadigadi pans system in Botswana.

Flora and fauna assemblages vary with local topography in a subtropical eucalypt forest

AbstractLocal‐scale spatial patterning (over hundreds of metres) in terrestrial assemblages was investigated by sampling a variety of organisms within a 400 ha eucalypt forest area in the lowlands of south‐east Queensland. Organisms were trees, shrubs, birds, insects extracted from the litter layer, and insects caught in pitfall traps. Each group was sampled using a standardized methodology, and the component taxa were counted and identified to a level commonly used in ecological studies of that organism – varying from species to order levels. Sites adjacent to drainage lines or ephemeral streams were biotically more similar to one another than they were to paired upslope sites 100–200 m distant and 15–35 m higher in altitude. This phenomenon occurred irrespective of the level of taxonomic resolution or type of organism. Within each taxonomic group, some components were mainly riparian, while others were more characteristic of upslope sites. Characteristically riparian taxa included trees in the genus Glochidion, the shrub genus Leptospermum, birds in the Pachycephalidae and Meliphagidae families, and litter invertebrates in the order Acarina. Upslope taxa included shrubs in the Rutaceae, birds in the Artamidae, and ants. Within the groups of trees, birds and litter invertebrates, more taxa were characteristic of riparian than upslope sites. Local scale biotic patterns were more strongly correlated with altitude than with measured soil characteristics; however, microtopographical differences would also be highly correlated with a large suite of covarying environmental features. The patterns of diversity and the implications for survey design and conservation are discussed.

Assessment of landslide susceptibility on the natural terrain of Lantau Island, Hong Kong

Steep terrain and the high frequency of tropical rainstorms make landslide occurrence on natural terrain a common phenomenon in Hong Kong. For example, more than 800 slope failures were triggered by a rainstorm in November 1993 on Lantau Island, Hong Kong. Maps of recent landslides interpreted from aerial photographs, in combination with a geographical information system, were used to evaluate the frequency and distribution of landslides, with particular reference to such physical parameters as lithology, slope gradient, slope aspect, elevation, vegetation cover, and proximity to drainage line, all of which are considered to be influential in the occurrence of landslides. A stepwise logistic regression model was obtained between landslide susceptibility and the above mentioned physical parameters. The study area has been classified into five classes of relative landslide susceptibility, namely, very low, low, moderate, high, and very high, based on this methodology.

Toward capturing hydrologically significant connectivity in spatial patterns

Many spatial fields exhibit connectivity features that have an important influence on hydrologic behavior. Examples include high‐conductivity preferred flow paths in aquifers and saturated source areas in drainage lines. Connected features can be considered as arbitrarily shaped bands or pathways of connected pixels having similar (e.g., high) values. Connectivity is a property that is not captured by standard geostatistical approaches, which assume that spatial variation occurs in the most random possible way that is consistent with the spatial correlation, nor is it captured by indicator geostatistics. An alternative approach is to use connectivity functions. In this paper we apply connectivity functions to 13 observed soil moisture patterns from the Tarrawarra catchment and two synthetic aquifer conductivity patterns. It is shown that the connectivity functions are able to distinguish between connected and disconnected patterns. The importance of the connectivity in determining hydrologic behavior is explored using rainfall‐runoff simulations and groundwater transport simulations. We propose the integral connectivity scale as a measure of the presence of hydrologic connectivity. Links between the connectivity functions and integral connectivity scale and simulated hydrologic behavior are demonstrated and explained from a hydrologic process perspective. Connectivity functions and the integral connectivity scale provide promising means for characterizing features that exist in observed spatial fields and that have an important influence on hydrologic behavior. Previously, this has not been possible within a statistical framework.

Nested clade analysis of the freshwater shrimp, Caridina zebra (Decapoda: Atyidae), from north-eastern Australia.

The freshwater shrimp, Caridina zebra, is endemic to montane rainforest streams of the Atherton Tableland, north-eastern Australia. As the confluences of many of the headwater streams are in unsuitable habitat, dispersal is expected to be highly restricted. Results from a previous allozyme survey for this species suggested that historical dispersal between separate river drainages had occurred due to rearrangements of the drainage lines at some stage in the recent past. The aim of this study was to use temporal information from the mitochondrial cytochrome oxidase subunit I (CO-I) gene to determine whether the observed genetic structure was a result of historical processes, or alternatively, due to low levels of terrestrial dispersal. The mitochondrial DNA (mtDNA) data were analysed using nested clade analysis, which can differentiate between historical fragmentation and range expansion vs. contemporary restricted gene flow. The results displayed three divergent clades that were likely to have arisen in allopatry. One widespread clade, with individuals in more than one river drainage, reflected a pattern of restricted gene flow. This suggests that this species is capable of terrestrial dispersal.

Terrain-based mapping of landslide susceptibility using a geographical information system: a case study

This paper deals with the development of a technique for mapping landslide susceptibility using a geographical information system (GIS), with particular reference to landslides on natural terrain. The method has been applied to Lantau Island, the largest outlying island within the territory of Hong Kong. Landslide susceptibility in the study area is related to a number of terrain variables, viz., lithology, slope gradient, slope aspect, elevation, land cover, and distance to drainage line. Multiple correspondence analysis (MCA) was carried out to generate the principal axes that are linear combinations of these terrain variables using occurrence data of landslides and terrain variables. A GIS is used to project the values of the principal axes, and subsequently to relate these principal axes to landslide susceptibility by logistic regression modeling. The spatial landslide susceptibility response in the study area can then be obtained by applying this logistic regression model to the study area. The results from this study indicate that such a GIS-based model is useful and suitable for the scale adopted in this study.Key words: landslides, geographical information systems, multiple correspondence analysis, logistic regression, terrain analysis.

The ecology of a stressful site: Mount To wrong, Central Victoria 1967–1997

Summary Species on Mt Towrong are distributed along soil moisture gradients which are dictated chiefly by aspect, altitude, soil depth and proximity to drainage lines. On the warm and dry western aspect large areas of shallow, rocky soils alternate with bands of deeper soils. On the cooler eastern aspect, soils are relatively deep throughout. Drought damage has occurred on the rocky soils of the western aspect six times in the last 25 years, whenever the annual rainfall was less than about 50% of the long-term mean. Fires are also recurrent perturbations that are most severe in drought years, especially when accompanied by hot NW winds. The fire of 1983 caused severe damage over the whole western aspect of Mt Towrong. Damage on the eastern lee slope was much less severe. An assessment of relative recovery of tree species from these perturbations was based on various combinations of species pairs growing in close proximity to one another. Hie rank order of drought resistance was Allocasuarina verticillata > Eucalyptus goniocalyx > E. radiata > E. obliqua (= E. viminalis) which was the reverse of that in relation to fire+ drought, that is, E. obliqua > E. radiata = E. goniocalyx > Allocasuarina verticillata. Seedling regeneration of tree species in the inter-drought periods was meagre but nevertheless the most mesic species, E. obliqua, consistently dispersed onto adjacent dry rocky sites, only to be killed in each of the ensuing drought periods. After fire, seedling regeneration of eucalypts (especially E. obliqua) was generally prolific, although the vigour and density of seedlings was patchy and related to the pattern of redistributed nutrient-rich hillwash. After 10–12 years, survival of the best seedlings was evident only in gaps in the reconstituted canopy. On this stressful site the indigenous species have been resilient to serious perturbations of the environment. It seems likely that, for the retention of species on a site, frequencies of fires and droughts may be more critical than the occasional exceptionally severe event.

How livestock grazing affects vegetation structures and small mammal distribution in the semi-arid Karoo

In this study we investigated vegetation changes superimposed by grazing and their effect on small mammals in the Karoo (South Africa) on grazed farmland and an adjacent, 10-year livestock enclosure. Plains and drainage line habitats were compared by monitoring vegetation height and cover, and small mammal species composition and abundance along transects. Animals were captured by live trapping. Vegetation cover was low on the grazed compared to the ungrazed study site, but vegetation height did not differ. The number of small mammal individuals and the number of species captured was higher at the ungrazed study site. Two species of climbing rodents captured in the ungrazed drainage line were absent from the grazed drainage line. Numbers of small mammals captured on the plains were similar for grazed and ungrazed land, but grazed plains were dominated by a single species of gerbil.

Wetland vegetation in the North-eastern Sandy Highveld, Mpumalanga, South Africa

The wetland vegetation of the high mountain grasslands of Mpumalanga w as sampled by using stratification based on geology and land types. Floristic data were classified by TWINSPAN procedures and refined by using the Braun-Blanquet method. This resulted in the recognition of four major w etland plant communities w hich are subdiv ided into eleven minor plant communities. The major communities include the Phragmites australis Wetland occurring in relatively deep water, the Miscanthus junceus Wetland from moist river banks and wet drainage lines, the Eragrostis biflora-Stihurus allopecuroides Moist Grassland restricted to moist, poorly drained soils w ith a high water table, and Arundinella nepalensis Moist Grasslands on black vertic soils.

Characterization of alluvial bajada facies distribution using TM imagery

Automatic mapping techniques using multiband satellite image information have been used to study sediment grain‐size variations on an alluvial bajada. A previous study of sparsely vegetated alluvial surfaces in central‐western Argentina showed that the reflectivity recorded in seven‐band Landsat TM images is controlled by sediment composition, age and grain size. At diameters >4 mm, clast composition influences image information, while at grain sizes smaller than sand, clay mineralogy begins to influence spectral characteristics. The progressive increase in desert varnish and the loss of fine grain‐size fractions as a result of deflation cause the age influence. The bajada sedimentary environment is well suited for testing the influence of grain‐size variability on image recovery, as most drainage basins are small and sourced by a single geological unit, which produces compositionally homogeneous alluvial surfaces. Additionally, most drainage lines are active within intervals of 10–50 years, reducing the effect of surface ageing. Larger than average drainage basins produce oversized drainage lines that disrupt the bajada, generate individual alluvial fans and have slightly different compositions. Two typical bajadas were selected to map grain‐size characteristics using automatic classification techniques. The obtained classes were checked in the field, and grain‐size and compositional counts were completed. Although both bajadas are very different in composition, most grain‐size curves showed similar shapes, suggesting that deposition took place by the same process (hyperconcentrated flash floods). The values of the median and mean were consistent across the same class between both bajadas. Thus, unsupervised classification techniques are useful for mapping sediment grain size, although minor field control is needed. Image classes represent areas of similar grain size, which are elongated parallel to the mountain front in an active alluvial bajada, indicating a homogeneous distribution of sedimentary processes along strike. Changes in the width of image classes indicate different downstream fining rates closely related to the topographic gap and the slope change rate. In contrast to bajadas, alluvial fans have semi‐circular belts and a pie‐piece‐shaped area in which most active streams are located. Thus, unlike fans, bajadas lack autocyclic mechanisms for producing heterogeneous sedimentary sequences. The sedimentary log of an ancient bajada was measured in order to show the influence of allocyclic factors in the absence of autocyclicity.

Comparison of plant diversity in protected and communal lands in the Bushbuckridge lowveld savanna, South Africa

Patterns of higher plant species richness and beta diversity were assessed using standard Modified-Whittaker plots in relation to landuse, slope position and mean annual rainfall across a rainfall gradient in the savanna areas of the Bushbuckridge lowveld, South Africa. In particular, comparison of communal areas with adjacent protected areas was important in showing the impacts of potential changes in landuse within an overarching catchment management plan. Although most of the protected areas considered preservation of biodiversity as their primary goal, they were characterised by significantly fewer plant species than the adjacent, highly utilised, communal lands, at both the plot and point scale. Slope position also had a significant effect on plant species richness, with eutrophic bottomlands having c. 30% more species than the dystrophic toplands. This adds weight to the need for greater public awareness for the judicious use and management of the sensitive bottomlands, which fringe the primary drainage lines that are vital for sustained supplies of good quality surface water in this semi-arid environment. The total number of species increased with increasing mean annual rainfall across the rainfall gradient. This suggests that, if the catchment management plan aims to identify additional areas for conservation, the higher rainfall areas should be the first to be assessed. Species turnover was greater along the rainfall gradient than the catenal gradient between toplands and bottomlands.

The conservation value of mesic gullies in dry forest landscapes: avian assemblages in the box–ironbark ecosystem of southern Australia

Drainage lines and shallow gullies generally have different microclimates and hence flora to surrounding upland ridges and slopes (abbreviated throughout as ‘ridges'). Gullies often have mesic, or at least less xeric, conditions compared with surrounding ridges. Gullies are frequently subjected to greater human impacts, such as clearance for agriculture, logging, grazing-damage and mining for alluvial deposits. We tested the hypothesis that mesic gullies in the generally dry (400–700 mm precipitation p. a.) box–ironbark forests of central Victoria, Australia, harbour a richer and different avifauna to surrounding ridges. Ten pairs of adjacent gullies and ridge sites were surveyed eight times over 1 year. Species richness was one-third greater in gullies than in ridges, while mean total densities of birds were almost twice as great. Assemblage composition also differed significantly, which reflected: (1) significantly different densities of those species common to both gullies and ridges (several more abundant in gullies, some more abundant in ridges); and (2) differences in composition wherein some species occurred only in one or the other habitat, even though pairs of gullies/ridges were only 400–1900 m apart. These results indicate that avian assemblages within gullies are distinct from those in ridges and slopes, and that integrated management is required to conserve the entire avifauna of these dry forests and woodlands. The depletion of gully habitats through human disturbance makes prescriptive management of gullies the highest priority at present.

Managing Nitrogen for Water Quality—Lessons from Management Systems Evaluation Area

AbstractThe Management Systems Evaluation Area (MSEA) project was initiated in 1990 to evaluate existing and develop new N management technologies to reduce the potential adverse impacts of agricultural practices on surface and ground water quality. Field research sites were established in nine Midwestern states. Results from MSEA research showed that nitrate leaching was greatly reduced by changing from furrow to sprinkler irrigation. At least 95% of the nitrate N percolating through tiled soils was intercepted and discharged into surface waters. Computer models indicated that routing tile discharge through wetlands would greatly reduce the nitrate load. Nitrate losses also were reduced by establishing controlled water tables using drainage lines for subirrigation. Preplant and presidedress soil nitrate tests were effective in determining proper N fertilizer rates and reducing nitrate losses. Banding ammoniated fertilizers slowed nitrification rates and nitrate leaching, especially if soil over the bands was packed. A major new technology was proof that crop greenness can be used to monitor crop N sufficiency, and that N deficiencies after the V8 stage can be corrected by sidedressing or fertigation (reactive N management). Inexpensive sensors or aerial photographs can be used to assess crop greenness. Using Global Positioning Systems (GPS), N‐deficient areas of the field can be managed differently from the remainder of the field. These results point to the need to develop site‐specific or precision farming systems to control nitrate losses to water resources and reduce the impact of natural variability in both soils and weather.

Assessing the quality for hydrological applications of digital elevation models derived from contours

Digital elevation models (DEMs) are becoming increasingly important tools in hydrological research and in water resources management. The quality of DEMs, however, normally is reported simply as the root mean square error of elevation, a statistic that fails to capture the numerous sources of error in DEMs or to predict their effect on the result of using the DEM. This paper presents a review of other approaches to assessing DEM quality, and argues that a full assessment of DEM quality must focus on the accuracy and reliability of the final product of the DEM analysis. A number of DEMs for the Slapton Ley catchments in Devon derived from digitized contour data are compared in an initial assessment of their sustainability for use in hydrological work. Two are available for purchase from data suppliers, and five more were created using a variety of interpolation techniques in widely available geographical information system software. The different interpretation methods produce DEMs with different artefacts, although analyses of the distribution of elevation values, and visual techniques, suggested that none of these were of a particularly pronounced nature. The results of using the DEMs to derive drainage networks and catchment areas showed that at the broad scale there was a high level of agreement between the DEMs. There were, however, important differences of detail. For example, some DEMs predicted drainage lines that occasionally crossed the original contours. The results of calculating the TOPMODEL topographic index showed far more variation, because the index is calculated for each pixel in the area, rather than being an aggregate result derived from numerous pixels. The main conclusion was that care should always be taken to assess the quality of a DEM before attempting to use it, and that results should always be checked to ensure that they appear to be reasonable. Copyright © 2000 John Wiley & Sons, Ltd.

Evolution of the Molopo drainage and its possible disruption by the Morokweng impact event at the Jurassic-Cretaceous boundary

A large, deeply eroded impact structure, known as the Morokweng impact structure, has recently been positively identified in the North West Province of South Africa, centred on the Ganyesa Dome about 80 km northwest of Vryburg. The marked geomorphic effect of the Morokweng impact event at 145 ± 2 Ma is identified through large-scale analysis of the long-term evolution of the Molopo River drainage system. A comparison of the pre- and post-impact drainage patterns shows that a dramatic change in both drainage direction and pattern occurred after the impact event. Prior to the impact, the regional drainage direction of the proto-Molopo had been towards the northwest, where it connected with the major southwest trending drainage line of the Permo-Carboniferous (∼300 Ma) Kalahari Basin. Reconstruction of the post-impact drainage pattern, as it existed in the Late Cretaceous (∼ 75 Ma), just before the formation of the rejuvenated Cenozoic Kalahari Basin, shows a generally southward flowing system with annular drainage pattern components. With the filling of the Kalahari Basin since Tertiary times, the affinity between the Molopo drainage and the Morokweng impact structure became less obvious.

Water Table Management, Nitrogen Dynamics, and Yields of Corn and Soybean

Concern about NO−3 contamination of surface waters has prompted the development of agricultural water table management systems to reduce NO−3 loss in subsurface drainage outflow by subirrigating through the existing subsurface drainage lines during the growing season and controlling off‐season outflows. We hypothesized that soil N pools, crop yields, and N uptake in a corn (Zea mays L.)–soybean (Glycine max.) rotation differ between subirrigation (water table at 40 cm) with controlled drainage (SI/CD) vs. subsurface drainage (SD) alone on Omulga silt loam (Aeric Fragiaqualfs). Mean microbial biomass N, potentially mineralizable N, dissolved organic N, and ammonia N were not affected by the water table management system. Mean NO−3–N was not affected by the water table management system at 0‐ to 15‐cm and 15‐ to 30‐cm depths, but the 2‐yr mean soil NO−3 concentration at the 30‐ to 75‐cm depth was 46% lower in SI/CD compared with SD. The average corn yield was 19% greater, and the average soybean yield was 64% greater, in SI/CD plots, compared with SD. Corn N uptake was 13% greater and soybean N uptake was 62% greater with SI/CD, compared with SD. The SI/CD water table management system increased plant N uptake and reduced deep‐profile NO−3 concentrations, thereby reducing the amount of NO−3 potentially available to move via drains to surface waters.

Ordination and classification of vegetation of Songimvelo Game Reserve in the Barberton Mountainland, South Africa for the assess­ ment of wildlife habitat distribution and quality

A vegetation survey was undertaken of the 49 000 ha Songimvelo Game Reserve in the Barberton Mountainland of Mpumalanga. South Africa with the aim to identify constituent plant communities and to assess their relative value to wild herbivores. The vegetation is highly diverse with representation of three biomes: Savanna. Grassland and Forest A total of 428 plots were sampled by means of a semi-quantitative technique. Data were subjected to ordination (CANOCO) and clas­sification (PATN). The composition of the 19 distinct communities is determined through an intricate combination of environmental factors as evident from the ordination results. Firstly drainage line' position is critical, followed by land use history and further by the interplay between elevation and geology. These findings are in line with results obtained from other studies along the eastern Escarpment. Alluvium, mafic and ultramafic lavas support mixed veld, whereas felsic lavas, sandstones and quartzites support sour veld which has a very low forage value in the dry’ season. Each community, through its specific species assemblage, structure and location, forms a distinctly different habitat in terms of its value to the various species of herbivores in the SGR

Impact of an introduced ant on native rain forest invertebrates: Pheidole megacephala in monsoonal Australia.

Pheidole megacephala is an exotic ant species that has severely affected native invertebrate biodiversity throughout the tropics. Its impacts have been documented extensively in relatively depauperate invertebrate communities, but not in species-rich habitats such as tropical rain forests. Here we describe the local distribution of P. megacephala and its impacts on native invertebrate assemblages in and around a rain forest patch at Howard Springs, in Australia's monsoonal tropics. P. megacephala was found to be confined to a single area of approximately 25 ha, with its distribution centered on drainage lines and the rain forest. Significant but weak correlations were found between its abundance and vegetative canopy cover (positive) and distance from the rain forest (negative). In the most heavily infested area within the rain forest, the abundance of P. megacephala was 37-110 times that of total native ant abundance found within uninfested plots, as measured by pitfall traps. The abundance and richness of native ants and other invertebrates were significantly reduced in litter samples, pitfall catches and foliage beats where P. megacephala was present, inversely relative to the abundance of P. megacephala. Only two individuals of a single native ant species were found within the most infested plot, with native ant richness being reduced to about half in the least infested plot. The most persistent functional groups of native ants in infested plots were Cryptic species, which forage primarily within soil and leaf litter, and Opportunists, which exhibit highly generalised foraging behaviour. The highest abundance of P. megacephala corresponded with a 42-85% decrease in the abundance of other native invertebrates. Insect larvae were totally absent from foliage beats collected at the most heavily infested plot. P. megacephala was found overall to be expanding its range, averaging 12 m range expansion in the dry season and contracting 7 m in the wet season. It is able to spread into surrounding savanna habitats by occupying relatively sheltered microsites, such as beneath logs and at the bases of trees. However, it is unlikely to attain high population densities in open savanna habitats because of its relative intolerance of desiccation, and the prevalence of behaviourally dominant native ant species. Howard Springs is currently the only rain forest patch in monsoonal Australia known to be infested by P. megacephala, but clearly this ant is a serious potential threat to the region's rain forest invertebrate fauna.

Development of a simplified unsaturated module for providing recharge estimates to saturated groundwater models

This paper describes the development of an integrated saturated/unsaturated catchment model of salinity processes and its application to an irrigated and dryland district in northern Victoria, Australia. The model simulates a coupled surface water and groundwater system in which a lumped conceptual model is used to model one-dimensional movement through the unsaturated zone, and a distributed model is used to simulate three-dimensional saturated groundwater flow. Movement of water through the unsaturated zone was found to be dominated by macropore flow, and accordingly the classical matrix flow approximations described by the Richards equation were not used. Instead, a conceptual model of the unsaturated zone was developed to simulate soil infiltration, recharge to the underlying groundwater system, capillary rise and crop evaporation on a daily time-step for each active cell within the model domain. Subsurface lateral flow between each vertical soil column is not permitted, though once in the saturated zone lateral flows are handled by the groundwater model. Salt movement through the profile is simulated using a mass balance approach, and separate conceptual formulations are used to generate salt export along the drainage lines. Copyright © 1999 John Wiley & Sons, Ltd.

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

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 m 3, 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.