The value of waterways

Waterways in their natural state are of great scenic and aesthetic value. In Australia, as in other parts of the world, rivers, estuaries and their associated catchments have traditionally been a focus for urban development, industry, transport and recreation. They are a significant tourist asset and, as available leisure time and population increases, it is expected their value as a recreational asset will also increase. Because major cities are often located close to waterways this trend will be accentuated as city dwellers seek convenient areas for their recreation.

Waterways are the breeding and nursery grounds for many marine organisms. Large quantities of organic matter (detritus) are produced from the breakdown of shoreline vegetation. This detritus forms the basis of all aquatic food chains.

Estuarine waterways (where rivers meet the sea) are considered to be among the most productive natural ecosystems in the world. A very large percentage of the fish that are caught along the Australian coast by commercial and amateur fishers use estuaries as feeding grounds and nurseries. The continuing profitability of these industries is strongly tied to the water quality of the estuaries. If the water quality declines, it's very likely that the fish stocks will decline too.

Waterbirds make use of the waterways as breeding, resting and feeding grounds. Waterways also provide a drought refuge for many species of inland waterbirds.

Waterways are valuable educational tools. They provide a range of natural ecosystems which can be easily and conveniently demonstrated to students. They also permit the study of interactions within an ecological framework in relatively simple and accessible habitats. The majority of the principles of marine ecology can be demonstrated within an estuarine system.

Waterway problems

The basic human desire to live and recreate near water results in large demands being placed on waterway environments. Unchecked, these demands can often conflict and result in the degradation of water quality and foreshore condition. The care and management of waterways must be coordinated to balance all the conflicting demands and protect the waterways.

Waterway problems are different in different areas. The type of problem depends a lot on how much human interference or development there is around the waterways margins and within their catchments. The list following provides an outline of the major problems facing the waterways of south-western Western Australia. These problems are not listed in order of severity or management priority.

Salination

More information about salinity is available in section 4.4

Before European settlement streams and rivers in low rainfall areas tended to be fresh water. The salination (increased saltiness) of river systems has occurred since native vegetation was cleared and shallow-rooted, introduced crop and pasture species were planted. The introduced species do not use up as much water as the native vegetation. Consequently, the watertable has risen and brought to the surface salts usually stored deeper underground. Clearing has increased the water flow throughout catchments and caused saline ground and surface waters to reach river systems.

Nutrient enrichment and other pollution

More information about nutrients in section 2.4. More about polution in section 4

Nutrient enrichment, or eutrophication, of waterways is a major problem in south-western Western Australia. High levels of nutrients (chemical compounds that promote growth) enter waterways through surface and groundwater

flow and result in increased growth of algae (microscopic or macroscopic algae, depending on the environmental conditions). Increased algae growth can severely disrupt the ecology of the waterways. Other pollutants also enter waterways from surface run off, groundwater and man-made drainage systems. These pollutants originate from different land uses across the catchment. Toxins, pathogens and physical pollutants such as litter can all result in water quality changes. The impact of pollution on a waterway depends on the type of pollution, the degree of contamination and the characteristics of the waterway.

River training and diversion

For more about this see section 7.2 and section 4.2

Many of the rivers of south-western Western Australia have been modified to reduce flooding of populated areas and farmland. These modifications include: desnagging (removing logs and vegetation from the river bed to increase river flow); changing the course of the river; ripping the river bed to encourage a deeper channel; construction of levee banks to constrain flood waters; dredging to make the river deeper; landfill of the river's mud banks; and changes to natural drainage lines. Physical disturbances such as these often upset the natural balance of the waterway's environment.

Alteration to estuary bars

Estuaries in south-western Western Australia naturally form sand bars at their openings. These openings often break open in winter, when water flow is greater. Sometimes these bars are opened by people at other times during the year to stop flooding, to provide access through to the open ocean for boats, and sometimes to flush the estuary when nutrient enrichment is a problem. A channel is dredged to let the water flow freely between the sea and the estuary. This can often result in sand being shifted into the estuary and the consequent growth of flood tide deltas at the estuary's mouth. This artificial opening may also affect the estuary's fishery because it causes changes to the salinity of the water.

Damming, inundation and flow regulation

For more about this see section 7.2 and section 4.2

Many rivers in south-western Western Australia have been dammed to supply water for drinking and for agricultural purposes. Damming can cause inundation of the foreshore land and alter its habitats. Dams also create barriers to the migration of fish and other species and can change the flow, temperature and oxygen content of the water.

Removal of fringing vegetation

For more about this see section 2.2 - fringing vegetation

Along many south-west rivers and around estuaries, fringing (also called 'foreshore') vegetation has been removed by clearing for agricultural purposes, use of the foreshores by stock and through uncontrolled public access. Fringing vegetation plays an important role in the natural workings of a waterway. It stabilises banks - reducing the chances of erosion - and traps nutrients and pollutants draining from surrounding land. Important animal habitats - for waterbirds and other creatures - have been lost along foreshores when the vegetation has been removed.

Erosion owing to clearing of vegetation (Ellen Brook)

Development near waterways

Introduced animals and plants

Many introduced species of animals and plants now inhabit foreshores and waterways. These species often dominate and exclude native species. The major introduced fish in South- West rivers are the mosquito fish (also called `gambusia'), redfin perch, brown and rainbow trout and the goldfish. Introduced stock also do considerable damage to foreshore areas. Feral pigs are thought to spread dieback, rabbits cause soil erosion and foxes, rats and feral cats prey on native fauna. Introduced flora species - such as the blackberry, arum lily and watsonia - have also infested foreshore areas and displaced native vegetation.

Bank erosion

Bank erosion occurs along many waterways. The severest erosion occurs on banks which have been cleared of vegetation and where stock or public access to the water's edge is uncontrolled. Power boats on narrow stretches of waterways have also caused bank erosion.

Siltation

Loss of biodiversity

'Biodiversity' means the number of different plant and animal species in a particular area. The loss of biodiversity is a major issue for Australia. Development in and around waterways causes disturbance to many important habitats which support a wide variety of species. Species are disappearing from their normal areas. This reduces biodiversity. The loss of bushland from foreshores, and the loss of seagrass meadows within waterways, are two examples of habitat disturbance which results in an overall reduction in biodiversity in the waterways environment.

Climate change

The warming of the Earth's atmosphere, as a result of the build-up of carbon dioxide and other gases, is likely to lead to an increase in global atmospheric temperatures. Temperature increases (estimates range from 1.5°C through to 4.5°C over the next 30 to 50 years) may have a significant effect on extreme weather events. For example, droughts may be longer or there might be more big storms. There may also be a rise in sea level of up to 0.7 m over the next 100 years. The point where salt water from the ocean and fresh water from inland meet in a waterway (the `saltwater-freshwater interface') 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 alterations to the environment will result in changes in the distribution of water and land plants and creatures along the estuary. Existing habitats and ecosystems will tend to move inland. Insect populations are also expected to increase. This could increase the level and distribution of certain human diseases such as malaria and Ross River fever.