Human activities impact on rivers and wetlands in many different ways. Many of the impacts interact with each other and what happens on land affects what happens in the aquatic environment and vice versa.The severity of the threats and the balance of the impacts is different in different catchments.
We have grouped the threats and pressures due to human activity under 5 different headings:
Each category of threat can and does create changes in the river’s flow regime which is the key determinant of river health.
Until the Wonthaggi desalination plant comes into use, every single drop of water that is consumed in Victoria comes from a river, a lake, a wetland or from groundwater., These resources are not separate but are all interconnected with each other through the water cycle. Depending on weather conditions and water availability, Victorians use up to 5,000 GL (that is five thousand billion litres) of water per year.
We use water for many things – drinking, cooking, washing, bathing, watering our gardens, in our offices, schools and hospitals, in our industries and for power generation – but the biggest use (60-80% of the water) is for irrigation. Irrigation helps to grow our food (especially dairy products, fruit and vegetables) but also powers a major export market for dried milk, butter and cheese and increasingly for other products like almonds, meat and wine .
The big irrigation areas are in northern Victoria where large rivers like the Murray and the Goulburn have historically had plenty of water –. Big dams have been built that hold thousands of gigalitres of water to supply the industry – Lake Eildon on the Goulburn and Hume and Dartmouth dams on the Murray. In southern Victoria the Macalister and the Werribee Rivers feed much smaller irrigation areas which largely supply the domestic market with milk and vegetables.
There are an increasing number of irrigators who are not part of a formal irrigation scheme but who pump water straight from rivers or groundwater. If you take a summer trip to Colac in the west of the state or Sale in the east, you are almost certain to see irrigation in progress. To give an example: the development of centre pivot irrigation using groundwater in the western Wimmera over the last 30 years has had a big impact on the landscape. Clearing to accommodate the irrigation systems has caused the wholesale destruction of paddock trees, especially Buloke which is a crucial food source for the endangered south-eastern Red-tailed Black cockatoo . This irrigation development uses water from an aquifer which has a very low rate of recharge – the water being used to grow crops was deposited over 20,000 years ago when the climate was wetter .
Not all the water that is extracted from rivers is actually consumed – much of it returns as drainage or from water treatment plants. Unfortunately it does not return to the environment in the same condition that it left and is often polluted with nutrients, agricultural chemicals, salt or industrial contaminants.
Over-exploitation has devastating consequences for river systems. These threats can be classified as :
a) Over-extraction of water for consumptive use – demand for water means that some river systems give up most of their water, particularly in dry years. For example the Goulburn can have up to 80% of its water diverted for irrigation and the Moorabool can lose up to 90% for urban and agricultural use. Some rivers like the Avoca or Wimmera cease to flow altogether and others like the Merri or Powlett have such low flows that their estuaries close to the sea. As a rule of thumb, a river can cope with one third of its water being extracted without long-term consequences, but if more than that is taken the river’s health will decline. Many of Victoria’s rivers suffer from over-extraction.
b) Altered flow regime – which is a consequence of consumptive demand for water and the construction of dams and weirs to meet that demand.
Under natural conditions Victoria’s rivers have highly variable patterns of flow, both from year to year and within years. In general flows are high in winter when it rains and low during the hot, dry summer. Consumptive demand for water shows the opposite pattern and is generally highest in summer when river flows are naturally at their lowest.
The construction of dams and water storages means that water can be captured in winter and stored for use in summer. This can create an inversion of the river’s flow regime with unnaturally low flows in winter (when water is being captured by the dams) and high flows in summer (when water is being delivered to irrigators). The dams also smooth out natural variations in the flow regime by capturing small floods.
All these changes have a serious impact on native fish populations. Native fish may receive unnaturally high or low flows, and/or water at the wrong temperature for the time of year, upsetting their breeding cycles and migration patterns. Altered flow regimes also advantage introduced species like carp and trout.
In addition the capture of water that would normally flow down the river in dams means that floodplains are starved of water. Periods of drought can be greatly extended, with consequent stress on flood-dependent vegetation such as River Red Gums.
Wetlands can also suffer altered flow regimes – some are drained and have their water removed, while other are used as repositories for drainage water and have the opposite problem of too much water. Wetlands on floodplains can be disconnected from the river channel by infrastructure such as levee banks or irrigation channels, and can be subject to greatly extended periods of drought because water is intercepted by dams.
c) Pollution – agriculture is a big user of chemicals such as fertilisers, herbicides, fungicides and more. While toxicity is an important issue, the most common problems for river systems and wetlands are caused by the extra nutrients (nitrogen and phosphorus) applied as fertiliser that get washed into them.
Raised levels of nutrients encourage algal growth and if flows are low and the weather is warm, algal blooms can develop rapidly. These can block light and smother other organisms, and in the case of blue-green algae can release toxins that are harmful to stock and humans.
Their rapid decomposition can reduce the oxygen content of the water which means that fish find it difficult to meet their oxygen needs and can die. Recycled water from urban treatment plants is also high in nutrients. Salt is another common pollutant of agricultural and industrial water that ends up in rivers and wetlands.
Bushfires can cause major pollution and have a big impact on water quality, especially if it rains following the fire. Fire retardants and ash can be washed into rivers, and significant erosion often follows.
When habitat is broken up and fragmented into smaller pieces, it makes it difficult for animals to get around, to find food, to find mates, to find nesting sites or burrows. It also makes it more difficult for plants to disperse and for pollination to occur if plants are too far apart. In the aquatic environment, habitat fragmentation results from the construction of barriers:
a) Instream barriers – dams and weirs that have been constructed across the course of the river. These are very common. Most Victorian rivers are dammed and have many barriers of varying sizes which present a major obstacle to fish mobility and migration. The only way for fish to get round them is by the construction of a suitable fish ladder.
The only major rivers in Victoria free of barriers are in East Gippsland and the Otways.
b) Farm dams – these are small, private dams, generally offstream, that capture water before it ever gets into the river system. Some are licenced for irrigation, others are for stock and domestic use. Individually they have little impact but in some catchments there are so many of them that their cumulative impact is very significant and in dry years most of the available water can end up in the dams and not in the river. They can be just as much a problem as the large public dams.
Victoria is the most cleared state in Australia, with about 70% of our native vegetation removed. The most significant damage is in native grassland (less than 1% remains), temperate woodlands and box-ironbark forests, and (not too surprisingly) these are the areas with greatest disturbance to related fauna and the highest numbers of threatened species of both plants and animals . The problems caused by clearing are very widespread– for example migration opportunities are limited for mammals and birds if less than 30% of native vegetation cover remains . When land is cleared habitat of all types is lost and even common species can suffer a decline in numbers.
We have identified three types of damage that have particular impact on rivers:
a) Land clearing – can have very significant impacts on the aquatic environment. Removal of vegetation can include riparian vegetation on river banks and many wetlands have been bulldozed to make way for agriculture or urban development. Some river channels have been cleared as well with the removal of logs, snags and other woody habitat. Land clearing increases erosion and mobilises salinity in the landscape as saline water tables rise.
b) Erosion – the impacts can be both direct where denuded river banks erode and indirect where extra material washes into rivers. For example, past gold mining has had a serious impact on the Ovens and Loddon Rivers with sediment being deposited all along the river bed, clogging up holes and riffles. A series of massive sand slugs are gradually travelling down the Glenelg River, filling up the deep holes that act as drought refuges and are favoured by native fish, and reducing habitat diversity.
c) Salinity – much of the groundwater in Victoria is naturally saline but when native vegetation is intact it holds the water table well below the surface. Clearing of the vegetation has caused a rise in the water table and with it has come dryland salinity. The threat receded during the drought as the water table declined, but recent rainfall has brought it back into focus, with approximately 240,000ha affected across Victoria . Much of the saline groundwater drains into river systems, raising salinity, sometimes with extreme results. Groundwater intrusion into the Wimmera River during the drought raised its salinity levels to more than double that of seawater.
Excess irrigation water also contributes to salinity. When more water is provided than can be used by crops or evaporated, the excess drains down to groundwater. This in turn raises the water table with the same end result as clearing native vegetation
Pest plants and animals are just as much as a problem in water as they are on land. Stock animals have a major impact on waterways when they are allowed unrestricted access so we have included them in this category.
a) Stock access – in Victoria over 30,000 km of land bordering a river belongs to the state. This land, known as a Crown frontage, varies in width from a few metres up to kilometres, and was reserved in the 19th century as a public resource. Some of this land is in public reserves, but about 17,000km is managed by the adjacent land holder under a licence . Most of the licences are used for stock grazing which includes access to the bed and banks of the stream. Animals can and do wander into the water to drink, wallow or cool off with devastating consequences, particularly for water quality and for bank stability and erosion. Every single CMA identifies stock access as a key manageable impact on river health, and spends millions of dollars fencing off streams, yet the grazing licences are regularly renewed without amendment.
b) Weeds – are a key problem both in and out of the water. Willows were planted on riverbanks throughout Victoria during the 20thcentury as an aid to bank stabilisation. However they had the opposite effect and have been responsible for increased flooding, degraded water quality and damage of aquatic habitat . They are now classified as a Weed of National Significance and millions of dollars are spent every year on their removal. Aquatic weeds include the nationally significant cabomba and parrot feathers and many others such as Azolla which can affect water quality. Spartina is a recurrent problem in estuaries.
c) Noxious fish – the sad fact is that introduced fish are far more numerous in Victorian waterways than native fish. Carp were introduced in the 1880s but only became widespread across south eastern Australia in the 1970s. They can now account for up to 90% of fish biomass across the Murray-Darling Basin  and have been described as ‘the freshwater rabbit’. They can out-compete native fish and their bottom feeding habits stir up sediment, affecting water quality and bank stability.
Trout have been common for a lot longer in cooler waters and are still frequently stocked for recreational fishing. Their predatory habits pose a massive threat to small native fish such as endangered mountain galaxids, and trout exclusion zones have had to be established in some mountain streams to protect these small fish. Mosquito fish and redfin are also highly invasive and locally very common. Alterations to flow regimes can advantage introduced species over native fish – for example, cold water releases from large dams favour trout over large-bodied native fish like Murray cod.
Climate projections indicate that Victoria’s climate is likely to be warmer and, for most of the state, drier than during the second half of the 20th century. By 2030, stream flow may vary from no change or slight increases in East Gippsland to 25-40% decreases in river systems in northern and western Victoria. By 2070, streamflow may decrease by up to 50% across much of the state. A rise in temperature of just 1°C in the Murray-Darling Basin would reduce annual inflow by 15% even if rainfall does not change .
The situation is made worse by the way water is shared between users and the environment. In most river systems there is a volumetric cap on the amount of water that can be diverted for use, and any water that is left over after that (‘above cap’ water) is for environmental purposes. When inflows are reduced by drought or climate change, above cap water – the environment’s share – is impacted the most . Users may have to cope with less water through restrictions or reduced allocations, but the environment may have to go without its share altogether. That’s why it is so important that the environment has access to secure, reliable water entitlements and the risk of drought and climate change is shared equitably between different users.