Mill Point, South Perth before reclamation, 1932. Courtesy Battye Library Series A4051
How humans affect waterways
Mill Point, South Perth before reclamation, 1932. Courtesy Battye Library Series A4051
Human activity has been harmful to the habitats and ecosystems of waterways in south-western Western Australia. Some activities can directly disturb, reverse or accelerate the basic processes that have shaped these waterways. For example, dam building has reduced how much fresh water flows into rivers, and clearing catchments and `training' rivers has increased the amount of soil and silt in rivers.
Historically, most Western Australian waterways have suffered from indiscriminate or poorly-planned land use. To reverse this trend and to try and repair past damage, better management and planning is needed for estuaries, rivers and their catchments.
| For more about the effects of dredging see section 4,4 |
There are short-term and long-term effects of dredging. Short-term effects are limited to the period of dredging and for some time afterwards. There might be a temporary disappearance of an organism from the community, mainly because water is too turbid, or because their habitat has been altered or destroyed. Points (g) to (k) inclusive above are short term.
Long-term effects are difficult to judge. An organism might die or disappear permanently from the area (become locally extinct) because where it lived was destroyed and it wasn't able to find somewhere else nearby. There might also be less growth, some animals and plants mightn't be able to reproduce properly, and others might behave strangely. Points (a) to (j) inclusive above are long term.
Reclaiming (filling in shallows or wetlands with soil) areas around waterways is often done to build houses, roads, parks and industrial estates. More often than not, saltmarsh and seagrass areas, which are amongst the most productive and most important habitats of a waterway, are destroyed or severely degraded. The existing foreshore is buried, intertidal areas may be smothered by filling operations, or dug up and destroyed if used as a source of fill.
Mill Point, South Perth after reclamation, 1997
Reclamation can reduce the range of an intertidal area, or reduce how much a freshwater area is `flushed' with sea water at high tide (and vice versa). These changes can affect the animals and plants that are used to particular mixes of fresh and salt water.
Wetlands and low-lying areas are important `filter' areas, catching nutrients and cleaning pollutants before they get into the main waterway. They are also places where animals live. Wetlands and shallows are breeding grounds for insects. Insects are an important food for many birds. These areas also play an important role in water balance and drainage, helping to keep nearby areas free of floodwater and salinity. Reclaiming these areas results in the loss of an already reduced and valuable foreshore environment.
Dams and weirs have varying effects on the ecosystem of the waterway downstream from their location, depending on how big the dam is and where it's positioned. What use is made of the water they store can affect the waterway, too. The main changes which have an impact on estuaries are:
Sediment and nutrient inputs
| For more about this see section 5.3 - Sediment transport |
If there is less sediment reaching the coast than before a dam was built, there can be serious erosion problems along the coast because there is less soil to protect the coast from the action of the sea and wind. This has happened elsewhere in the world. However, reducing sediment input to an estuary could be an advantage in some circumstances, particularly where catchment clearing has increased erosion in the catchment and filled up the waterway and estuary with sediment.
| For more on this see section 2.4 |
Nutrients are affected by dams and weirs in different ways, depending on whether the nutrients are in the form of organic detritus, bound to sediment particles or dissolved in the water.
Freshwater flow and flooding pattern
How much the freshwater flow is reduced by a dam or weir depends on how much of the catchment run off flows into it, how much water it holds, and what proportion of the water is diverted away from the stream for other use. If there is less freshwater flow, it's easier for the waterway to become more salty downstream. Fresh water flowing into an estuary also stimulates a lot of animal and plant productivity. When the fresh water mixes with the salty water of the estuary, it can help fish larvae and other plankton grow much more than they do in sea water.
Dams and weirs tend to restrict both the size and frequency of floods. Flooding is important to many freshwater wetlands, and stimulates the breeding and migration of many fish species inhabiting coastal rivers and estuaries. Larger floods also flush accumulated sediments out of estuaries, along with toxins and nutrients in the sediments.
Water quality
Dams and weirs can help change the chemical balance of the water, too. Problems can happen downstream from the dam because cold, deoxygenated water is released from the deeper parts of the dam into the stream. Toxic substances such as hydrogen sulphide may also be released. As well as affecting how well aquatic plants and animals are able to live day-to-day, temperature and chemical changes can also affect breeding and migration behaviour. If phytoplankton blooms in a dam, the turbidity of the waterway downstream can be increased a lot when water filled with the blooms is released from the dam.
Barriers to migration
Many fish species use both estuarine and freshwater habitats. Weirs may be barriers to migration, although species differ in their abilities to swim, leap or even climb over barriers when the river is in flood.
Dams and water rediversion projects have had massive impacts on marine life in the northern hemisphere, but little information exists about their effect on Australian estuaries. In California, the striped bass, Morone saxatilis, has almost disappeared from the San Francisco area owing to the diversion of the Sacramento and San Joaquin rivers for agricultural and urban purposes. On the Nile River in Africa, the construction of the Aswan High Dam destroyed the sardine fishery in the river's delta.
Roads and bridges across estuaries can cause problems for estuarine habitats and ecosystems. Causeways across an estuary can severely restrict tidal flows and reduce the range of the tides. This in turn can lead to more silting happening on the bed of the estuary near the causeway, which can lead to the decline and death of seagrass beds and other habitats.
| For more about tides see section 5.2 |
Also, limited or unsuitable waterway openings may restrict fish movement past the structure.
Road construction along and across estuaries often involves wetland reclamation. Dredge spoil is dumped elsewhere in the wetland or estuary, and fringing vegetation is destroyed. These activities result in the loss of foreshore, wetland, tidal and possibly intertidal habitat.
The long-term harmful effects of road and bridge construction can be quite dramatic. The tides might not go as high or as far upstream. The tides can also move slower because they're held up or slowed by the road. This alters what sorts of plants and animals live upstream of the road because the water becomes completely different (less saline and not regularly flushed, for example) if it's not as affected by the tides. So, mangroves and upstream seagrass beds will decline and die.
When designing roads, causeways and bridges, it's important to think about their effects and build them to minimise harm to the waterways.
Two common and often very harmful results of canal-side housing estates on an estuary are the initial destruction of estuarine habitat - often wetlands or saltmarsh - and the subsequent continuing pollution and disturbance of the waters by urban run off, boating etc. Canal estates in particular, like most waterfront developments, may have harmful effects on wader birds (such as loss of habitat or disturbance of nesting birds).
| For more about tides see section 5.2 |
Canal estates are usually built on low-lying land next to estuaries. Sediments from the new canals are excavated and used as fill to raise the nearby land. In the past, canals through an estate were planned to make as much money as possible. For example, they were designed so that there were as many waterfront housing blocks as possible, and so that plenty of fill could be taken from them to raise the blocks above flood level. These days, canals are more likely to be designed with these things in mind: retaining as much of the wetlands as possible; lessening harmful effects on the nearby estuary (including reducing sedimentation and pollution); making sure that the tides flush the canals adequately; and providing public access to the foreshores.
Waterfront land is a valuable commodity. In the past, demand for waterfront land has led to the foreshores being reclaimed haphazardly. In turn, natural foreshore and intertidal habitats have been destroyed and there has been less public access to the foreshores. Also, there has been significant pollution of estuaries from sewage disposal, other domestic wastes and urban run off.
Waterfront developments can also have a harmful effect on animals and plants that rely on the water: important habitat areas may be lost or reduced; nesting birds may be disturbed.
Well-designed and sympathetic waterfront developments can increase public access to the foreshores and make recreation - both passive and active - more pleasant.
Flood mitigation works are any constructions or alterations to the environment that try and reduce damage caused by floods. In the past, a variety of flood mitigation works were constructed along estuaries to protect agricultural and urban areas from freshwater and coastal flooding. Such works included levees, drains and floodgates, together with the clearing and lining of stream channels to help flood waters move away. These works can have a number of harmful effects on estuarine habitats.
Creek, river and channel `improvements' aimed at helping the flow of floodwaters, as well as destroying habitats, can cause erosion problems and the waterway downstream silting up.
Perhaps the most critical effect of flood mitigation works is the loss of wetland areas. In the past, many agricultural areas on floodplains were created by draining wetlands, and restricting saltwater inflows to tidal creeks (by building weirs and floodgates). Also, levee banks were constructed to keep floodwaters off the land. All of these activities led to the loss of wetlands or changed how wetlands worked because they were no longer filled up regularly by floodwaters. As well, the tidal water no longer had anywhere to go if it couldn't get past a weir or floodgate. This meant that animals and plants that relied on that influx of salty water either moved or died. Silting also increased near the weir, and water quality became worse.
The construction of tidal barriers, such as weirs and floodgates, eventually converts upstream reaches from a brackish to a freshwater environment. Also, these barriers slow or stop fish and prawns moving along the waterway. They can also lead to prolific weed growth upstream of the barriers. This weed growth is caused by nutrients building up in the largely stagnant waters upstream of the barrier. Herbicides may be used to control the weeds, with possibly even more harmful effects on living things in the waterway.
| For more about dispolved oxygen see section 4.4 - Parameter 4 |
When this water is released through the floodgate, it takes a while before it mixes with the better quality water below the floodgate. In a number of cases, such releases have caused disease and death to downstream fish and other aquatic organisms.
These days, the potentially harmful effects of flood mitigation works are understood. It's unlikely that biologically significant wetlands will be drained for agricultural or other purposes. Good design and management of new flood mitigation works can preserve existing wetlands and still provide flood protection.
Mosquitoes, midges and other insects irritate nearby humans. At times they can be a health hazard. For example, Ross River fever is caused by a virus carried by mosquitoes whose numbers can increase to epidemic proportions in stagnant brackish water.
Insects breed in wetlands, saltmarshes and the tidal fringes of an estuary and are an important part of a healthy and balanced estuarine ecosystem. Often there is pressure on the local council to kill or reduce insects that are a nuisance by filling or draining breeding areas or by using pesticides. All of these activities have harmful effects on the estuarine environment. Some pesticides can destroy other estuarine plants and animals as well as the insects they're intended for. For example, a pesticide used to control biting midges is toxic to crustacea, has killed large numbers of wading birds in Western Australia, and has affected waterfowl in New South Wales.
Managing nuisance insects in important estuarine areas such as wetlands and saltmarshes has to juggle the needs of the ecosystem with those of the nearby humans. Managers have to consider the possible harmful effects of control methods.
Expanding urban areas can have a lot of effects on wetlands and also the groundwater which helps keep wetlands wet. Subdivision of land for urban development has resulted in wetlands either being filled and reclaimed for development, drained, or modified and set aside as stormwater basins.
Filling and draining wetlands for urban development is a very direct harmful effect. However, more subtle harm is done to those wetlands and waterways which are left. Towns and cities alter the water balance, affect the water quality and alter water consumption dramatically.
Estuaries are popular areas for active recreation such as swimming, boating, waterskiing and fishing. These activities need shore-based facilities such as marinas, boat ramps and car parks. Often, these structures are built along the foreshore, reducing or destroying completely mangroves and the natural fringing vegetation. Also, dredging is sometimes needed so that boats can more easily get to marinas.
Harmful effects of boating include pollution by unburnt exhaust gases from powerboats; fuel spills and litter; disturbance to fish; bank erosion; bank undercutting and increased turbidity levels caused by powerboat wash; and damage to seagrass beds caused by propellers, anchors and digging for bait.
Marinas and recreational facilities can have a number of harmful effects on wader birds, including disturbing their roosts and beach nesting areas, damaging their feeding areas, and reducing how much food there is for them by collecting bait (by fishers) and delicacies (such as oysters).
Some estuaries have berthing and cargo facilities for large fishing fleets; some also have extensive facilities for coastal and international trading vessels. These maritime operations can have a number of harmful effects on estuaries. These include loss of shoreline and habitat; disturbance of animals; oil spills; and the introduction, via ballast water from ships, of foreign organisms that may be harmful to local animals or even to humans eating animals and plants from the surrounding area.
Aquaculture is relatively new in Western Australian waterways. However, it is an industry which is expected to increase rapidly in the years to come. Intensive aquaculture can have a number of harmful effects on waterways. Exotic animals can escape into the waterway and alter the ecosystem, and the set-up of the aquaculture operation can affect water flow and quality. The main problems come from locating aquaculture near sensitive areas and the wastes that the farm may create.
For example, if yabbies spread into rivers of the South-West they could compete with native crustacea such as marron and may damage river banks and irrigation channels because they like to burrow into them. For another example, a simple aquaculture project takes water from a perennial stream, passes it through one or more holding ponds containing the farmed animals, then returns the water to the river at a lower point. The river's flow is reduced between the intake and discharge points because its water is diverted, and the water which is returned to the stream has higher nutrient levels owing to animal faeces and uneaten food, and less oxygen content owing to the farm animals' respiration.
Effluent resulting from aquaculture activities can contain many pollutants which are harmful to the waterways environment. These can include nutrients and organic matter. Organic waste from excess food and fish faeces, or shellfish pseudo-faeces, which settles onto the waterway's floor can pollute the area.
Oyster farming, with the oysters raised on racks in shallow estuarine waters, can also have a number of harmful effects on the estuarine environment. First, oyster leases are `out of bounds' to other waterway users (sailors, boat fishers etc.). Second, if oyster racks are poorly aligned to the prevailing currents, they don't flush very well, which means that wastes can build up in the area. Realigning oyster racks to suit ebb and flood tide currents helps both the flushing of the lease areas and oyster growth.
One potential advantage of aquaculture is that the harmful effects of this form of fishing should be much more controllable - because it is focussed in one place and usually on land - than `open water' fishing. Shifting fishing operations from the water to the land may allow more stringent fishing controls to be imposed in the estuary itself, and thereby hasten its recovery. And commercial fishing can continue, but on land. A second potential advantage is that aquaculture is usually dependent on good water quality. The water would be monitored to make sure of this.
Over-fishing and degrading habitats harms fish. There are also less fish available to catch. Habitat degradation can be physical - for example the smothering of seagrasses with sediments - or come from poor water quality. Apart from these direct effects, fishing may also disturb the roosting and feeding patterns of birds.
To conserve fish and allow fishing to continue, natural habitats need to be protected. Fishers (and other people who use the waterway environment) must be responsibly managed to make sure that everyone benefits.
Clearing catchments for agriculture in south-western Western Australia has had a lot of harmful effects on waterways. Clearing has increased soil erosion. The added sediment has built up in waterways. The increased sediment load entering waterways has also carried adsorbed nutrients, pesticides and other pollutants downstream and deposited them in other waterways such as estuaries.
Clearing in some areas has also increased salinity in waterways. The loss of trees and other plants has allowed more rainwater to enter the groundwater (because it hasn't been used by the plants), causing the watertable to rise and bringing salts with it to the surface. Freshwater invertebrates and fish are found in fewer and fewer places because the water is becoming too salty for them.
Clearing also affects the hydrologic balance of catchments: there are more and bigger floods; run off patterns change; and changes to the groundwater table may occur.
Clearing riverbanks commonly causes bank erosion and collapse. Cropping also increases sediment run off because ploughing and tilling can destabilise the upper soil and expose it to the wind, rain and sun. Run off from agricultural land, apart from carrying a large sediment load, can also contain high levels of toxic materials (pesticides and herbicides) and nutrients.
Grazing causes a progressive loss of tree cover because seedlings are eaten by stock. Other harmful effects of grazing include trampling plants, compacting soils, increasing nutrient levels in run off (from faeces) and the destabilisation of river banks (unless they are fenced off).
Intensive horticulture includes growing plants such as vegetables and flowers. Intensive animal industries include agriculture-related industries such as piggeries, dairies, poultry farms, abattoirs, saleyards and feedlots.
Horticulture usually involves large amounts of nutrients being applied to the soil to keep soil fertility high and, therefore, production. Animal industries don't need large amounts of nutrients but instead generate large amounts of nutrients in the form of solid and liquid waste.
Depending on the soil types these nutrients can enter watercourses in surface and groundwater flow eventually reaching the main waterways and adding to the problems of nutrient enrichment.
Forestry operations next to estuaries or in catchment areas upstream can cause significant soil erosion. This can increase turbidity and sedimentation within an estuary. Forestry can also destroy the beauty of the environment around an estuary. Roads through forests can act as wildlife barriers, stopping or restricting the movement of certain native animals. They can also help the spread of exotic animals and plants.
Planting trees (called `tree farming' or `tree plantations') can reverse salination trends in cleared areas because the trees take up more water and hold stable or reduce the watertable. However, tree farming can involve spraying the ground with pesticides before and after planting, use of fertilisers, and encourage increased run off when the trees are cut.
Waste materials discharged from industrial processes can contain high levels of nutrients and other pollutants. These wastes are often directly discharged into waterways through outfall drains or enter the waterways via run off and groundwater from industrial areas.
Materials such as soil, plant matter, nutrients and pollutants travel through estuaries from the land to the sea. Many activities in upstream catchments can pollute estuarine waters. This pollution can make fishing and recreation in the estuary less pleasant or stop it altogether, reduce or destroy habitats, and restrict the use of the estuary for other purposes.
Pollutants can enter an estuary as `point source' discharges such as sewage effluent, or as `diffuse source' pollutants such as urban stormwater run off. Common pollutants include toxic chemicals, suspended solids, nutrients, microbial contamination and oils.
Some pollution comes from sources which can be pin-pointed (such as a factory or a piggery discharging its wastes into a drain which ends up in the river). These are called `point sources'. Point source pollution can come from sewage treatment works, abattoirs, sugar mills, dairy factories, dredging activities, sand and gravel extraction, gravel washing plants, land reclamation, canal estates, marinas, waterfront developments and effluent discharges from industrial operations. Oil spills are another hazardous point source of pollution.
The harmful effects of point source pollution on water quality depend on the nature of the pollutant, its volume, concentration and frequency of discharge (called the `pollutant load'), and how well the estuary can take in pollutants and store them or get rid of them.
Diffuse source pollution comes from many diverse land uses, urban and rural. Urban development, by taking away vegetation, exposes soil to erosion. Urban development also creates impermeable surfaces - such as pavements, roads and roofs - which increase the speed and volume of stormwater run off because the water can't seep through them into the ground. Run off from roads includes oil, heavy metals, fuels and chemicals. If a truck carrying chemicals is in a road accident those chemicals can spill into nearby waterways. Stormwater run off from urban areas contains various pollutants such as litter (plastic, paper, cans), animal wastes, vegetation (grass, clippings, leaves), oil and grease, car oils, waxes, metals (filings, rust, petrol additives), bacteria and viruses, nutrients (nitrogen and phosphorus), pesticides, noxious weeds and seeds, and sediments.
Rural run off is increased by clearing vegetation from catchments and the banks of waterways, road making and tilling soils. All of this together contributes substantially to the silt loads of waterways. Burning off agricultural and forest lands frequently causes massive losses of topsoil after rain. The topsoil is carried into waterways which then silt up. Rural run off can contain agricultural chemicals (nutrients, herbicides and pesticides).
| For more about this see section 4 - Water quality |
The warming of the earth's atmosphere because of the build-up of carbon dioxide and other gases in the atmosphere, is likely to lead to a warmer climate.
Global temperatures may rise 1.5°C to 4.5°C over the next 30 to 50 years causing changes in climate, weather and sea levels. The changes are not expected to be uniform over the earth's surface or throughout the year. Warming is expected to be greater at higher latitudes and in winter time. In Australia, it's possible that surface temperatures may increase by 2°C in the tropics and by 3°C to 4°C in southern latitudes.
Temperature increases of these amounts may have a significant effect on extreme weather events. For example, tropical cyclones are likely to come further south along the Western Australian coast.
There may also be a rise in sea level up to 0.7 metres over the next 100 years.
The saltwater-freshwater interface (the point where salt water meets with fresh water) will move further inland; existing wetlands will be flooded more regularly and possibly become wetland lagoons, while new wetlands will develop in low-lying areas along estuaries.
These changes to water and salinity levels will also cause changes to where aquatic and land plants and animals live along an estuary. For example, plants that grow along the foreshore will move inland as the sea levels rise and create new foreshores further inland. Animals that live amongst those plants will move with them.
Development (such as roads, houses, factories) close to waterways could be more likely to be flooded, and agricultural land could be lost. It's important to set development back from waterways for this reason alone.
There will also be other changes: insect populations are expected to increase, which in turn could increase the level and distribution of certain human diseases such as malaria and Ross River fever.