Changing Natural River Flows

The harnessing of rivers to provide secure water supplies for towns and irrigation has had profound effects on the ecology of rivers, floodplains and estuaries. The introduction of dams and other regulating structures, of diversions from streams, of groundwater bores and of small catchment dams have affected the natural flows of our rivers. Large releases of water for irrigation often occur during summer, when flows should naturally be low. The quality of water released from storages is also altered, commonly being far colder than normal. These changes to volume, seasonality and water quality reduce the suitability of rivers and streams for many plants and animals. For example, storage during high flow periods has reduced the natural flooding of the floodplain, affecting some fish, bird and plant species.

Dams, Weirs and Other Structures

Barriers to the migration of fish affecting their feeding and breeding cycles (Read a story about this). Even small structures such as weirs or poorly constructed road crossings can create barriers to fish movement.

Changing Land Use

Land use change has altered the quality of water in rivers and streams. Clearing of native vegetation, particularly in association with irrigation, has led to increased river salinity, while degradation of river bank condition through grazing and other activities has increased turbidity levels in some streams. Agricultural practices and domestic and industrial discharges have increased the levels of various toxic chemicals and nutrients in streams.

Removing Snags

Many bird, invertebrate and fish species rely on woody debris (snags) for resting or breeding. Removing snags for navigation, and during flood and erosion mitigation works, has reduced the habitat quality of many rivers for native animals. Increased runoff of sediment has filled in natural depressions in the streambed or deep pools, further reducing the available habitat.

Introduced Species

Several introduced species of fish and invertebrates have caused declines in populations of a range of native species. Exotic plants such as willows (Read a story about this) and blackberries dominate many river banks. Introduced species can change the ecology of rivers and streams, making them less suitable for native plants and animals. Carp, for example, have become the dominant freshwater fish in many fish communities in south-eastern Australia. In addition to displacing native fish, they increase water turbidity and damage aquatic plants.

Aquatic weeds, including cord-grass, can be serious problems where they become abundant.

Poor Water Quality

Deterioration of water quality is a major threat to Victorian rivers. It is important to maintain good quality water to protect agriculture, industry and environmental systems, and for human consumption.

When the health of waterways deteriorates, so does the quality of services they provide for our communities and the economy. For example:

• The Gippsland Lakes now experience algal blooms each summer and major blooms roughly every four years, causing major losses to both tourist revenue and the commercial fishing industry.
• Lake Boga, a recreational and tourist attraction, now experiences regular algal blooms at a cost to the region of $600,000 per bloom.

Degrading Riverbanks

There are a number of major processes that degrade the health of riverbanks and their surrounding vegetation (the riparian zone). These include vegetation clearing, erosion, uncontrolled stock access, recreational use (e.g. trampling of understorey layers), weed invasion, road crossings (e.g. roads, powerlines), removal of timber for firewood, salinity and water management.

Because of their importance to agriculture, and because their other values were poorly understood, a large proportion of riparian areas were cleared or heavily grazed in the past. Recent assesments of the condition of riparian land shows that less than 14% was in good or excellent condition, around 54% in reasonable to good condition, and over 32% in poor to very poor condition.

Fire

Fire-induced changes are capable of exerting major effects on the hydrology of forested areas, with consequent short to long-term impacts on water yield. Runoff from fire-affected areas is expected to increase immediately after the fire, and remain at elevated levels until the tree canopy is re-established. If trees survive, this is likely to take around three to five years. If they are killed, the forest must regenerate from seed. Where an old growth forest has been burnt the regrowth forest can develop total leaf areas much larger than in the original mature forest. With denser canopies intercepting more rainfall and transpiring more than the unburnt old growth forest. Therefore, leaving significantly less “left-over rainfall” to appear as streamflow, ie water yield from regrowth forest catchments is less than from mature forests. In the case of mountain ash forests, runoff typically drops below pre-fire levels after about five to ten years after a fire, reaching a minimum about 25-30 years after the fire. It then slowly recovers to pre-fire levels as the forest ages.

Immediate changes in water temperature and chemistry can kill fish and other animals. Enduring changes in the physical and chemical attributes of catchments and watercourses have long term effects on populations of aquatic biota