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Rivers Summary

A key public resource

Australia's rivers have been significantly altered by land use - including agriculture, urban development and water resource development. Without informed and strategic management, the condition of Australia's rivers will continue to deteriorate. For the more intensively used Australian catchments:

• 33% of river length has impaired aquatic biota, with almost 25% having lost between 20% and 50% of aquatic macro-invertebrate groups
• Over 85% of river length is classified as significantly modified in terms of environmental features - New South Wales, South Australia and Western Australia have the greatest amounts of modified river length (97%, 96% and 93% respectively) and the Northern Territory has the smallest amount (34%)
• Over 80% of river length is affected by catchment disturbance with land uses affecting delivery of sediment, nutrients and water - reaches in Tasmania and the Northern Territory are the least affected by catchment disturbance
• Hydrologic change could be assessed in only 25% of river length because of limited data to support assessment of change of hydrology from natural flows - flow regimes are largely unmodified in approximately 20% of the regulated river length assessed (11% of river length is regulated)
• more than 50% of river length has modified habitat, mainly linked to changes in sediment loads that can alter channel morphology
• nutrients (mainly phosphorus) and suspended sediment loads are higher than natural loads in more than 90% of river length; 33% of river length is classified as substantially modified with respect to nutrients and sediments for mainland Australia

Management priorities

• Largely unmodified rivers - occurring especially in far north Queensland, in eastern Victoria and Tasmania - require protective management to ensure their condition is maintained
• Rivers with the most urgent need for rehabilitation and strategic management - located in parts of the Murray-Darling Basin, the Western Australian wheatbelt, western Victoria, and the South Australian wheat-growing areas - generally have highly modified catchments, are subject to high nutrient and suspended sediment loads, have lost much of their riparian vegetation, and have dams and levees that disrupt the movement of biota and material into, along and from the river
• Most river reaches in Queensland and northern coastal New South Wales, western Victoria and south-west Western Australia have largely unmodified habitat but very high nutrient and suspended sediment loads, and erosion from hill slopes and stream banks is high - control of nutrient and suspended sediment loads and revegetation of stream banks is essential for rehabilitation of these streams
• River reaches in central Tasmania, central Victoria, New South Wales and Northern Territory with habitat severely modified by from dams need protection and restoration of environmental flows and fish passage
• Clearer delineation of institutional and agency responsibilities for river management at State and Commonwealth levels is needed

Building better assessments

• Implementation of a standardised approach to data collection and data management, leading to cost-effective, Australia-wide collation, analysis and reporting, and including:
  • collection of finer-scale, management-relevant data on riparian vegetation
  • use of more representative and responsive indicators of biota condition, especially fish populations
  • use of an agreed set of river reaches as a basis for monitoring and reporting
  • development of methods for assessing riverine habitat
  • understanding the different river processes across Australia and refinement of monitoring measures for these different river types
  • information on changes in river hydrology, especially natural and current flow regimes

Introduction and Objectives

Australia's rivers have changed noticeably since European settlement, particularly in areas where human land use dominates the natural environment. The Audit river assessment aimed to:

• report on river condition for key river basins across Australia;
• assess aggregate impacts of land use; and
• identify priority management challenges.

It is recognised that the river assessment is one of many inputs for identifying priority management challenges. Other information (e.g. social and economic factors) should be used in conjunction with the biophysical condition assessment.

Australia's rivers

Australia is the driest inhabited continent, with only 12% of run-off collecting in rivers. Rainfall is distributed unevenly across the continent, so that river flows are nearly three times more variable than the world average (Boulton & Brock 1999, NLWRA 2001f). Characteristics of Australian rivers vary widely with differing climates (Figure 32):

• in northern Australia, monsoonal rains are common during the wet season; almost 50% of Australia's average annual run-off is from the Timor Sea and Gulf of Carpentaria drainage divisions (Boulton & Brock 1999, NLWRA 2001f);
• tropical cyclones may occasionally dump heavy rain in the arid interior causing spectacular flooding;
• in south-eastern Australia, rainfall is more evenly distributed and the climate is temperate.

Australia has perennial, intermittent and episodic rivers, fed by groundwater sources and surface rainfall (see measuring river flows box, p. 59). Intermittently flowing streams are common in semi-arid and arid Australia where water may be lost from the river to the watertable below the channel (Lake 1995).

Rivers are made of a wide range of environments, including various kinds of channel, riparian lands and floodplains, and their associated lakes and wetlands. These differing environments are sometimes found within a single basin. Connections between riverine components need to be maintained in healthy rivers, to maintain supply of energy, nutrients and habitat resources to the river channel.

Hynes (1975) suggested a way of bringing order into the complexity of interacting components that can influence river condition: in every respect the valley rules the stream. Hynes demonstrated that catchment geology, soil types and human activities in a catchment profoundly influence the physical and chemical characteristics of rivers flowing through that catchment. These characteristics in turn mainly determine the biological components and processes that occur in a river. The Audit assessment of river condition has been structured around three components:

• catchment interactions;
• riverine habitat; and
• biota.

Catchment interactions

Catchments influence a river through large-scale effects on the hydrological regime, sediment delivery and chemistry (Allan & Johnson 1997). The amount and timing of run-off from a catchment will be determined by climate, topography, soil type, geology, and vegetation. Land use changes affect the hydrological regime, resulting in an increase or decrease in flow and changes in the seasonal and daily timing of hydrological events. Land use activities can alter the hydrological regime by changing the rates and quantity of infiltration and overland flow as well as the extraction and release of water from storages. These changes affect the timing and volume of water that flows down a river (e.g. land surfaces that are dominated by impermeable surfaces, such as roads and roofs within urban areas, can markedly change the hydrological regimes of rivers).

Sediment and nutrient inputs to rivers from catchments are also determined by a complexity of natural factors. Extensive clearing for forestry and agriculture, grazing and cropping, the destruction of riparian zones, urbanisation, road construction and extractive industries can exacerbate the natural delivery of these materials to rivers (Waters 1995, Boulton & Brock 1999).

The natural chemical constituents in a river are mainly determined by catchment geology and soil types. Rivers range from naturally alkaline waters flowing from limestone catchments to the highly acidic streams in some of the upland, granitic catchments. The most significant changes to these natural conditions result from an augmented nutrient or sediment supply and the introduction of toxic chemicals and salt. Toxicants include hydrocarbons, trace metals, pesticides and herbicides. A wide and growing range of toxicants is entering rivers, and their long-term effects are still far from clear. Toxicant sources tend to be linked to particular land uses (e.g. hydrocarbons and trace metals from urban areas, pesticides and herbicides from agricultural areas).

Salinisation of catchments and rivers is a problem that has recently received national attention. While some Australian soils and rivers have naturally high salinity levels, the currently increasing levels of river salinity is attributed to extensive clearing of deep-rooted vegetation and increasing irrigation (NLWRA 2001e).

Riverine habitat

The riverine habitat includes the river and its floodplain, associated riparian land, channel features and river form, flow regime and water quantity and quality (Figure 33).

Floodplain

Floodplain water bodies include billabongs, lakes, wetlands, flood runner and distributary channels. These water bodies are naturally connected to rivers during high flows and are critical parts of the river ecosystem. Material and organisms are supplied and trapped by the floodplain as water levels rise and fall. Floodplain water bodies can be highly productive when filled, providing an extensive and complex variety of aquatic habitats (e.g. distinctive habitats such as reed beds that are important for frogs, invertebrates and water birds). As water levels recede, organisms and materials such as nutrients released from organic matter are fed back to the river, replenishing resources in the stream. This exchange of materials between river and floodplain is essential for maintaining biodiversity and supporting river function. Maintenance of natural wetting and drying regimes is essential to ensure the breeding of organisms whose life cycles are cued by flood events and release of nutrients.

Floodplain water bodies can be affected by changes to the flow regime (e.g. changes to flood volume, seasonality and frequency). The connections between floodplains and rivers can be influenced by constructed levee banks and flow regulation structures. During the assessment of river condition, floodplain-river connectivity was assessed on the basis of the existence of local levee banks and flow regulation structures.

Riparian land

Riparian land is the land adjoining, directly influencing or influenced by a river. A major component of riparian land influencing the river is riparian vegetation. Riparian vegetation provides shade; and supplies energy, nutrients and habitat to the stream and the floodplain (Figure 33). Specifically, riparian land supports river health by:

• regulating instream primary production;
• trapping sediment, nutrients and other contaminants;
• protecting river banks from erosion; and
• providing a food source and habitat (e.g. fine organic matter, leaves, sticks and snags) for aquatic animals.

Degradation of riparian land is mainly caused by the removal of vegetation, but also, in some cases, by the introduction of alien species (e.g. willows). Removal of native riparian vegetation can affect a stream by (Figure 34):

• increasing the warmth and light reaching the stream, favouring algae and macrophyte growth;
• reducing the loads of organic matter and woody debris entering the stream, thereby decreasing sources of habitat and food;
• allowing larger inputs of sediments and nutrients to the stream from the catchment;
• destabilising stream banks, leading to channel erosion; and
• allowing the local watertable to rise, with resulting potential increases in salt inputs to the stream.

Riparian vegetation is included in the river assessment (Table 9). It was calculated by measuring the extent of tree cover in the riparian zone. Composition and structure of the riparian zone are also important, but were not able to be assessed.

Flow regime

Flow regime is a key driver of river condition. The regime and variability of flow at various scales have been recognised as an important determinant of river habitat and biota (Poff & Allan 1995). Australian rivers, such as those of the Murray-Darling system, have some of the most variable natural flow regimes in the world (Puckridge et al. 1998). The biota inhabiting Australian rivers are well adapted to hydrological variability (e.g. Boulton & Brock 1999) and the ecological integrity of some rivers depends upon flooding over the floodplain as well as substantial drying-out periods.

Changes in river flow regimes are well recognised as a cause of changes to river geomorphology and habitat (e.g. Erskine et al. 1999). Geomorphology and habitat, in association with water flow and water chemistry, control the distribution, physiology, and abundance of organisms, as well as the dynamics of riverine communities. Flow affects river biota from habitat scale (e.g. Stazner & Higler 1986) to river basin scale (e.g. Power et al. 1996). Flow has been considered as the fundamental driver that orchestrates pattern and process in river systems (Walker et al. 1995).

The ecological significance of flow variability can be considered at four scales:

• flow regime;
• flow history;
• flood pulses; and
• flow hydraulics.

Australian rivers vary widely and therefore measurements and characterising functions will vary. This assessment has included flow regime where data were available. The lack of data on this aspect of river condition is recognised as one of the limitations of the assessment.

In the river assessment, the hydrological disturbance subindex is used to assess the change to flow regimes that typically result from river regulation and/or substantial flow diversion or extraction. An additional effect of river regulation is the impact of the river regulating structures themselves on biota. Many fish species and possibly other biota rely on upstream-downstream migration for breeding or other purposes. Physical impediments (e.g. dams and weirs) can affect these populations, with domino effects to the rest of the ecosystem.

Channel features and river form

Australian rivers are complex systems and vary in river form and channel features. Components of the physical habitat of a river include the river floodplain and channel form, in-channel habitat types, substratum, and organic matter (Figure 33).

• Upland part of a river: the valley is usually constrained and has minimal floodplain; the river is dominated by riffle and pool habitats.
• Mid-reaches: the valley floor widens, the river starts to meander, and in-channel benches and flood runners develop; sediment is highly mobile, eroded and deposited from point and lateral bars and pools.
• Lower river reaches: the river generally splits into several channels and there is extensive development of floodplain features; sediment is mostly deposited and distributed to the floodplain.

The characteristics of each section of river control the habitat available for biota:

• In upland sections, the substratum is dominated by boulders and cobbles; current velocities and riparian inputs are high.
• In the mid-reaches, substratum is mostly cobbles, pebbles and gravels; current velocities are lower; macrophytes appear; and organic inputs, including snags, are important.
• In the lowland reaches, pools, anabranch channels and floodplain features such as billabongs and wetlands form the main habitat types; sediment is dominated by smaller particles such as sand, silt and clay; current velocity is low; and woody debris, snags and macrophytes become the most important habitats.

Erosion of riverbeds and banks and transport of sediment along a river is an essential, natural and continuing process. However, excessive loads of sediment can swamp the natural physical features of rivers with sand or mud, causing a loss in variety of available habitats (Figure 34). Accumulation of sediment in a river channel will cause the channel to become narrow and shallow, facilitating colonisation by semi-aquatic plants and weedy species (e.g. willows), eventually choking the channel and reducing available habitat for aquatic species.

Organic matter can enter the stream in dissolved form or as small particles, leaves, sticks or even snags. Most organic matter transported in streams is dissolved, entering the stream through groundwater or run-off - mostly from storms. Dissolved organic matter is derived largely from the leaching of leaf litter and detritus and is consumed by microbes and bacteria. Coarse organic matter such as fallen trees, branches and leaf packs, is an important substrate, habitat and food source for microbes, algae, invertebrates, fish and other animals such as platypuses, tortoises and lizards. Snags provide fish with shade, shelter from predators and currents, spawning sites, and feeding areas. At least 34 native freshwater fish species use woody debris as a major habitat for spawning (Treadwell et al. 1999). Removal of riparian vegetation may severely reduce the supply of organic matter to streams. De-snagging has contributed to degradation of many Australian rivers. Without woody debris, current velocity can also increase, accelerating bank erosion.

In the assessment of river condition, no assessment could be made of channel form or in-channel habitat types. Increases in bedload and suspended sediment load were assessed by modelling hillslope, gully and bank erosion and hydrological and climatic features. Measurements of mean annual suspended sediment loads are scarce in Australia but some data are available to calibrate the model and evaluate the bounds of suspended sediment yield. Rivers used were LaTrobe, Brisbane, Tully, Johnstone, Avon, Lachlan, Murrumbidgee and Burdekin.

Water quality

Many aspects of water quality (e.g. nutrients, salt, turbidity, water temperature, dissolved oxygen, trace nutrients and toxicants) are important when assessing river health.

While nutrients are essential for river function, excess nutrients can disrupt normal ecosystem function, increasing the risk of algal blooms. The most significant nutrients for ecosystem function are nitrogen and phosphorus, since they are limiting to plant growth. They are released from instream processes, but may also enter the stream by run-off and inundation of the floodplain.

The main sources of salts to rivers are the atmosphere, catchment and groundwater, with the contributions from each source depending on location, climate and catchment features. The variable discharge of Australian rivers leads to temporal variability in dominant salt sources. Floods also play an important role:

• in low flows, more salts are generally contributed from groundwater and in arid areas salts may concentrate in surface water because of evaporation; and
• at high flows, surface water and atmospheric sources dominate.

Salinity generally increases downstream in Australian rivers because tributary inputs and impacts accumulate along the route. However, salinity is higher in headwater catchments in some rivers (e.g. in south-west Western Australia) because of catchment clearance and dryland salinisation (Boulton & Brock 1999). High concentrations of calcium salts can cause sediment to flocculate and settle. In naturally turbid water, calcium salt makes water clear, increasing light penetration and the risk of algal blooms.

The amount of suspended material (e.g. silt and clay) in the water alters the amount of light that can penetrate the water column.

• High turbidity can restrict instream primary production to the water's edge or the top few centimetres of the surface and smother instream habitat.
• Lowered turbidities in areas of naturally higher turbidity may lead to algal blooms.

The natural turbidity of a river can be affected by flow regulation, water abstraction, sediment entering the river from the catchment, changes in water chemistry and the removal of riparian vegetation, causing erosion. Many inland rivers are naturally turbid because of their loads of suspended sediment.

Water temperature is a major factor regulating instream production and provides cues for spawning and migration for organisms (e.g. fish). Water temperature is controlled primarily by shade, water colour and turbidity. It can be lowered unnaturally by the release of cold water from the bottoms of dams. Cold water affects fish and macro-invertebrate populations. Temperature can be increased by the removal of riparian vegetation and the shallowing of river channels (Figure 34).

Toxicants may arrive in a stream from run-off, the atmosphere or groundwater sources. The effects of toxicants on stream organisms are complex and the subject of considerable research.

A comprehensive data set was available for nutrients and suspended sediment loads. The subindex for this river assessment is called the 'nutrient and suspended sediment load subindex' to more accurately reflect its function. Nutrient loads were assessed using modelled data for total nitrogen and total phosphorus. Turbidity was estimated via suspended sediment loads.

Toxicants were assessed using National Pollutant Inventory data. Reach assessments did not include salinity (refer to the project report for the basin-scale assessment of salinity).

Biota

Water plants, algae, bacteria, macro-invertebrates (mostly insects), crustaceans, fish, amphibians, water birds, mammals (e.g. water rats, platypuses), reptiles (e.g. tortoises, crocodiles and lizards) are important riverine biota. Riparian vegetation is also considered part of the biota, but has been discussed as part of the riverine habitat.

Emergent and submerged plants provide habitat for water birds, insects, amphibians and fish, and are a substantial sink for nutrients (Boulton & Brock 1999). Algae are important for nutrient cycling and as a food source for other biota. Primary production is limited by light availability; riparian vegetation, turbidity and water depth all have an influence on instream production. The amounts of organic matter fixed by primary production are often smaller than those lost as carbon dioxide after breakdown, but without shade this relationship may be reversed. Shade also mediates daily and seasonal water temperatures. In upstream environments there is generally little instream photosynthesis, and most energy is derived from leaf litter and other organic matter. In lowland environments where there is more light, macrophytes are generally more abundant. However, the higher turbidity in lowland rivers can limit the zone where light is sufficient for plants to grow to a few centimetres, restricting plants to those that grow at the water's edge or float on the surface. Algal blooms generally occur when there are slow flows, clear water, high nutrient concentrations and high temperatures. Therefore, maintenance of native riparian vegetation and flushing flows is important in avoiding algal blooms.

Animals such as macro-invertebrates, crustaceans, fish and amphibians are affected by changes to light, temperature and nutrient levels and are dependent on food resources and habitat features at many scales. Increases in light, often combined with increased nutrients, may result in filamentous algae and macrophytes replacing riparian litter as a food source (Figure 34), resulting in a major change in the food chain, a loss of biodiversity and population structure changes. Woody debris and snags provide a critical habitat for macro-invertebrates and fish and changes to the physical habitat (e.g. loss of woody debris, removal of snags and deposition or erosion of sediment) also have major impacts on the plants and animals that live in rivers and streams. Sediment deposition may smother fish eggs and spawning sites, affecting fish reproduction and recruitment, and reduce macro-invertebrate habitat.

Despite the importance of each biotic component to ecosystem function, the only comprehensive national biological data set is the National River Health Program data set based on macro-invertebrates. Only macro-invertebrates could therefore be used as a biological measure of river condition for this assessment.

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