Australian Heritage Commission, 1998
ISBN 0 6425 4590 1
Part A - Wild River Values and Impacts (continued)
3. Impacts on Wild River Values
This section is intended to provide a general overview of major impacts from widely occurring activities relevant to rivers, rather than a detailed review of relevant literature. Specific mention is also given to likely impacts in the riparian zone. Further detail is available in reports by O'Brien, McGregor & Crawshaw (1983), Land Conservation Council (1989), and Robertson, Vang & Brown (1992).
Disturbances associated with a range of human activities lead directly or indirectly to physical, chemical, or biological impacts, or other impacts causing a less tangible reduction in wild river values. Some activities have an immediate and obvious impact, for instance the construction of a dam. Others bring about incremental changes, for example grazing in the riparian zone. Impacts can occur on water quality, flow regime, morphology, biological communities, or aesthetic quality.
The impacts of greatest concern are those which result from activities in areas of the catchment where the biological, hydrological and geomorphological processes are linked with the river. The extent and location of these areas will vary according to the river type. However it can generally be assumed that with increasing distance from the river and its associated flows, impacts are reduced. Certain types of disturbance in the catchment may have substantial impacts on natural values, but negligible effect on wild river values.
Cattle and sheep grazing is the most extensive land use across Australia, occurring from alpine areas to arid rangelands. Stock tend to concentrate along river frontages, waterholes and other waterbodies for water supply and grazing. Impacts on the riparian zone include direct contamination of the water, trampling, soil compaction, spread of exotic species, changes in species composition of plant communities, and inhibition of regeneration. In the broader catchment, reduced vegetation cover due to grazing leads to greater erosion potential. Accelerated catchment and stream-bank erosion may result in channel widening and alterations to the substrate. Temperature, nutrients, suspended sediment and bacterial levels increase in the water (Land Conservation Council 1989). In the arid rangelands, Chewings, Bastin & Pickup (1992) found that on the more preferred landscapes, vegetation has a reduced capacity to recover when in the vicinity of water and when it has been damaged by grazing (Landsberg et al. 1997).
Soil which has been cultivated is vulnerable to water and wind erosion, with concomitant increases in turbidity and sedimentation in streams. Aerial spraying and runoff from croplands and horticultural areas can carry nutrients and pesticides into watercourses. These impacts are much greater if the riparian zone is cultivated, because this removes the protective buffer of riparian vegetation which can prevent sediment, nutrients and other pollutants reaching streams (Woodfull et al. 1993).
Clearing of native riparian vegetation results in a decrease in supply of nutrients to the stream from litter and insect fall, leading to changes in the food chains dependent on such nutrients. The supply of woody debris by limb fall also stops. This debris is important for the creation of a range of in-stream habitats (Land Conservation Council 1991). The loss of shade over the stream leads to increased water temperature, with consequent effects on sensitive instream biota.
Catchment clearing has substantial impacts on flow regimes, erosion and siltation, soil conditions, fire regimes, and direct ecological impacts. The effects and level of impacts depend on the percentage of catchment cleared and the combination of soil type, slope, rainfall and method of clearing. In some areas, land clearance has contributed to rising saline water tables, resulting in extensive salinisation of soils, wetlands and streams (Barmuta, Marchant & Lake 1992). Commonly, streams flowing in cleared land change their morphology through bed and bank erosion. Flow regimes in small cleared catchments can have higher, shorter peaks and can be less perennial than under natural conditions.
The supply of water for irrigated agriculture has been the main reason for river regulation and water harvesting in parts of Australia (see below for discussion of impacts). In many irrigated areas, water which has percolated into sub-strata causes a saline mound to develop, eventually leading to saline discharges into river systems. Irrigation has been the main cause of change to the riverine environments of western New South Wales (Helman & Estella 1983).
3.5 Flow regulation and water diversion
River flows are regulated by the construction of dams, weirs, and barrages for a variety of purposes including irrigation, hydro-electric power generation, domestic and industrial water supply, flood mitigation and navigation. Most of these purposes require not only storage of water but also diversion of part of the river flow. Effects of reduced downstream flow can include accelerated channel erosion through the increased erosive power of water with a reduced sediment load, altered oxygen levels and temperatures, and a decrease in population size and diversity of fish and invertebrates. Many native river fish are migratory, and instream barriers have greatly depleted their stocks by preventing them from completing the migratory stages of their life cycle, particularly the spawning migration (O'Brien, McGregor & Crawshaw 1983 and Walker 1985).
Dam Construction on Gordon River, Tas. Dams established for water storage generally also involve diversion of flow and an altered pattern of downstream flows. Physical works involved in dam construction can have significant effects on the catchment.
Photo: Max Bourke © 1998
Dams alter the natural pattern of flooding, thereby affecting aquatic, riparian and floodplain ecosystems dependent on flood cycles. Major floods trigger the reproductive cycles of many Australian plants and animals, including the River Red Gum, and many species of waterbirds (Helman & Estella 1983). For inland river systems with very flat gradients, a levee of just 10 cm in height can substantially alter water movement on the floodplain (J Puckridge pers. comm. 8/1995).
Kingsford, Bedward & Porter (1994) in reference to the Paroo River in north-western New South Wales, cite the importance of maintaining natural flooding and drying regimes for wetland and floodplain habitat.They consider that the catchment protection is the most critical conservation issue, as during periods of high rainfall, flood flows in the upstream reaches of the Paroo catchment are often diverted to impoundments for irrigation in the neighbouring State of Queensland. See also Walker et al. (1997).
Activities giving rise to the above impacts include engineering works affecting the river, catchment activities, and river-related recreation.
3.6 River management works
River management works include snag removal, widening, deepening and straightening of stream channels, diversion of flows, and bank stabilisation. These activities which are undertaken to reduce loss of adjacent land and structures through bed and bank erosion, and to facilitate water flow to reduce flooding. Such works may alter channel shape, bed structure and flow characteristics and decrease habitat diversity (Land Conservation Council 1989).
3.7 Mining and sand, gravel and soil extraction
Activities associated with mineral exploration are generally transient and non-intrusive. For example, common activities such as remote sensing, geological mapping and most geochemical and geophysical surveys have little or no impact on river environments. Some more intensive activities such as trenching and costeaning, closely spaced drilling, track construction and line-clearing, if carried out in the riparian zone, have the potential to significantly disturb river environments. However, exploration activities are subject to legislation and conditions designed to minimise such disturbance.
Some past mining activities have caused significant ecological damage, which persists in some rivers and streams, particularly where the river or river valley has been dredged or tailings have been discharged. The potential still exists for operating mines to result in water pollution, which even at low concentrations, can exert chronic effects. Such effects include failure of organisms to reproduce (Lake 1992).
'Deep River', Dee River near Bouldercombe, Qld. Abandoned and operating mines can produce water pollution which, even at low concentrations, can exert chronic effects. Sections of the Dee River are biologically dead due to copper sulphate leachate from the Mount Morgan mine.
Photo: John Casey © 1998
However, in all States and Territories there are stringent mining approval processes which ensure that, if approved, mining is subject to strict environmental management and rehabilitation conditions designed to minimise, amongst other things, pollution of rivers and streams. Special constraints generally apply to any activities, including exploration and mining, in rivers and streams and in the riparian zone. Licensing of water discharges from mine sites is generally a legislative requirement.
Sand and gravel are in many places extracted from river beds and floodplains, and in places soil is extracted from floodplains. Unless effectively controlled, these activities can have undesirable impacts on local geomorphological, hydrological and biological processes.
3.8 Roads and tracks
Impacts of roads and tracks are greatest in riparian zones and can include landform disturbance, disturbance of drainage patterns, erosion, siltation, turbidity, vegetation clearance, fragmentation of habitat, spread of disease and non-indigenous plants and animals, increased visitation, and visual intrusion (reviewed by Robertson, Vang and Brown 1992). The significance of the impact depends on factors such as design of watercourse crossings, route location, road surface, discharge points from drains, level of use, extent and nature of maintenance, and time since construction.
Pipelines, power transmission lines, telecommunications installations, garbage depots and sewage treatment works are an integral part of modern society. Where they are located within a riparian zone or cross a watercourse, impacts can arise from vegetation clearing, associated access tracks, erosion and sedimentation, polluted discharge, and degradation of aesthetic quality.
3.10 Urban settlement
Downstream of urban areas, runoff patterns within the catchment are modified by increasing the areas of hard surfaces and decreasing areas of infiltration. Water quality also deteriorates from the input of industrial and domestic wastes from point and non-point sources (Land Conservation Council 1989). These impacts are magnified where settlement is within the riparian zone. Urban areas also demand increasing volumes of potable water for domestic, industrial and institutional use, resulting in pressures for water harvesting from more remote catchments.
Isolated dwellings and fishing or holiday shacks may degrade wild river values.
3.11 Water based recreation and tourism
River cruises, rafting, kayaking and canoeing attract considerable numbers of tourists and private recreation seekers in regions such as Tasmania's West Coast, Queensland's Wet Tropics and the Top End of the Northern Territory. Manidis Roberts (1993) have documented the current annual use by commercial operators of wild and scenic rivers in the Wet Tropics, and suggest that some sections of rivers are already at or near capacity. Rafting brings demands for road access, disturbance to banks at launching sites, and in some places, hut construction and disposal of human wastes in the riparian zone. For recreationists seeking solitude and a 'remote' recreation experience, encounters with other visitors or evidence of past users reduces the value of the setting.
Franklin River, south-west Tas. River cruises, rafting, kayaking and canoeing attract considerable numbers of tourists and private recreation seekers in regions such as Tasmania's west coast, Queensland's wet tropics and the top end of the Northern Territory.
Photo: Peter Canty © 1998
Impacts of motorboats may include: oil and fuel spills, littering of waterbodies, noise disturbance to wildlife and recreational users, and sewage disposal leading to increased nutrient load in the watercourse (Robertson, Vang & Brown 1992). Wave wash from vessels on confined water ways can be very damaging. For example on the Lower Gordon River in south-west Tasmania, tourist vessels have speed restrictions placed on their activities to reduce serious erosion effects. Despite these limits, bank erosion is still occurring.
Angling can have direct impacts on native fish stocks, particularly for the introduction of exotic species as bait. Angling and demands for vehicular access to wild rivers can also result in trampling and other damage to riparian vegetation. There is increasing concern in some areas, over the addition of lead to aquatic ecosystems, from shot used in hunting and anglers' sinkers (Land Conservation Council 1989).
3.12 Water associated recreation
Rivers provide pleasant settings that attract people for a variety of recreational activities. Inappropriate or over-use of river areas for camping can lead to soil compaction and trampling of vegetation, or its removal for campfires and disturbance at camp sites. Littering, escape of fires, and faecal contamination of water are other potential impacts. The use of off-road vehicles in riparian zones can cause severe erosion and turbidity, as well as disturbance to wildlife and other visitors. Fossicking in riparian zones can result in soil disturbance and vegetation clearance and increased water turbidity from disturbed stream sediments. Horses used in group trekking parties may cause degradation such as trampling of vegetation, erosion and the spread of weeds.
3.13 Introduced plant and animal species
The impacts of introduced plant and animal species on river systems are extensive and diverse. Willows (Salix spp.) and blackberries (Rubus spp.) have modified channel morphology and riparian habitat in many temperate upland streams (Barmuta, Marchant & Lake 1992). The Athel Pine (Tamarix aphylla) is colonising areas of active stream erosion in arid parts of the Northern Territory (P Latz pers. comm. 10/4/95). Some aquatic plants can spread very rapidly and impede water flow, discharge toxic substances reduce oxygen levels, and impair recreation values. There are more than 100 water weeds in Australia, the most significant of which are the Water Hyacinth (Eichhornia crassipes) and Salvinia (Salvinia molesta) (O'Brien, McGregor & Crawshaw 1983).
A number of feral animal species cause damage to river systems, including Water Buffalo (Bubalus bubalis) in the Northern Territory, the ubiquitous rabbit, which can exert much heavier grazing pressure than native herbivores, horses, donkeys, goats and pigs. Buffalo populations have been dramatically reduced in the last decade due to the successful brucellosis and tuberculosis eradication campaign in the Northern Territory.
In Cape York Peninsula species of particular concern are wild pigs and wild horses. Pigs dig up the rhizomatous plants after receding water levels and are believed responsible for the localised extinction of the lotus lily on the lower Mitchell floodplain (Herbert et al. 1994). Horses also tear up aquatic vegetation, particularly eelgrass, which provides important fish habitat. Both species increase water turbidity (Herbert et al. 1994). The potential for exotic aquatic plant invasion in Cape York Peninsula is high, as is the possible introduction of exotic or non-native fish, such as the Tilapia which is used as live bait for Barramundi (Herbert et al. 1994).
Eighteen exotic fish species have become established in Australian rivers. A number of these are stocked for angling. It appears that the introduced salmonoids (eg. trout species) affect native fish by predation or competition. The European Carp (Cyprinus carpio) increases turbidity and destroys aquatic vegetation, thereby affecting native fish and waterbirds (O'Brien, McGregor & Crawshaw 1983).
The Cane Toad (Bufo marinus) is now by far the most abundant amphibian in Australia, and in large numbers can give rise to fetid conditions in natural waterbodies (O'Brien, McGregor & Crawshaw 1983).
3.14 Fisheries and aquaculture
Numerous fish species, both exotic and indigenous are being cultured in freshwater catchments across Australia. In the Northern Territory for example, aquaculture is seen to have considerable potential as a future industry. The key species that may impact on fish habitat value are freshwater crayfish (red claw), and prawn and barramundi in estuarine areas (Power and Water Authority 1992). There is a potential risk associated with aquaculture that non-indigenous plant or animal species or pathogens will escape from the aquaculture site and establish in natural river systems.
3.15 Timber production and harvesting
The nature and severity of impacts from timber production vary with the stage of the production cycle, topography and climatic conditions, logging techniques used, and protective measures applied, such as the retention of vegetated buffer strips along drainage lines. The replacement of mature forest with regrowth can reduce runoff (Alexandra and Eyre 1993); soil surfaces exposed during logging are likely to result in sediment movement and turbidity unless properly managed; and use of residual herbicides during plantation establishment can affect aquatic ecosystems (Kunert and McGregor 1995). In many parts of Australia, timber such as red gum is harvested from the floodplain which may result in the creation of access tracks, log dump sites (Land Conservation Council 1989) and disruption to the riverine floodplain flora and fauna communities. Effective codes of practice which are in place for logging and forest management can prevent, or minimise these impacts.
Fire is a natural occurrence in most Australian ecosystems and an essential component of their dynamics. It is widely recognised that in most areas Indigenous people traditionally managed the landscape with the aid of fire. Small and numerous patch burns were lit, which resulted in a mosaic of different aged vegetation at various stages of structural development.
However, when the frequency, intensity or time of occurrence of fire and area burned are changed significantly, catchment vegetation may be affected. Wildfire can lead to extensive areas becoming temporarily denuded which may impact on river values. Methods used to suppress wildfire may also affect naturalness (Land Conservation Council 1989).
Many areas are now subject to prescribed management burns to protect people and property. However, such burns do not necessarily consider the ecological requirements for burning regimes, or the ecological need for some vegetation communities to have infrequent, large scale and intense wildfires (Preece 1990). Some documents, such as the Victorian Code of Practice for Fire Management on Public Land recognise ecological burning requirements to preserve particular plant communities.
A change in fire regime can alter vegetation communities, such as the Pandanus and vine thickets in northern Western Australia (J Dell pers. comm. 4/4/95), by altering the influx of nutrients into the waterholes. In northern Australia, large scale fires in the dry season can lead to massive erosion when the rains come, with increased turbidity and associated impacts. Recent research by Douglas (unpub) in Northern Australia suggests that catchment burning may be necessary for the maintenance of the biodiversity of aquatic plant and animal communities (See also Department of Environment, Sport and Territories Biodiversity Papers numbers Three and Eight, which relate to fire and biodiversity).