Alteration of natural water flows
Water is pumped from rivers and underground water supplies for use by rural towns, farms, industries and cities. Many rivers also feed dams and reservoirs for public water supplies and hydro-power, and are used as transport routes for boats.
While these activities provide economic and social benefits, there are many adverse environmental effects caused by altering the natural flow of rivers (river regulation). These include the decline and loss of native species of plants and animals, encouragement of habitats favourable to pest species (carp, gambusia and redfin), declining water quality and loss of amenity.
It is now widely recognised that changes to the flow regime have severely degraded most, if not all, regulated rivers in some way. The River Murray highlights this. So much water is removed from the river that less than 20% of the normal annual volume reaches the river mouth at Goolwa.
River regulation in the Murray-Darling Basin is so severe that giant river redgums which rely on frequent flooding are dying and the Murray cod is threatened.
Major efforts are now underway to understand the impact of river regulation, and to develop strategies to restore and/or protect the natural flow regime of rivers and creeks to improve the environmental condition of our waterways.
Native vegetation clearance has wide-ranging effects on water quality, habitats and biodiversity. Clearing the landscape of trees and shrubs changes the direction and rate of runoff and increases erosion. This means more sediment, nutrients, salt, pesticides and other toxicants are transported into rivers and streams.
Towns and cities increase the volume of stormwater due to their large area of impervious surfaces (roads, roofs, footpaths, carparks) compared with well-vegetated catchments.
The National Land and Water Resources Audit found that, of the river length assessed in South Australia, 95% had water with elevated loads of suspended solids, total phosphorus and total nitrogen.
Habitats are where organisms live. Loss of habitat can range from the removal of whole wetland ecosystems, to the loss of a small stand of reeds in a swamp or creek.
The effect of habitat loss is invariably a reduction in biological diversity. This can limit the ability of the environment to tolerate climatic variation and the effects of human activities. It can also affect the ability of the environment to recover from a major event such as a drought, or a significant pollutant discharge.
Pests or invasive species are usually introduced by humans. They threaten the survival of native plants and animals, and can also damage valuable agricultural and personal resources.
Terrestrial and aquatic pests affect the health of our waterways as well as native animals and plants. For example, the mosquito fish (Gambusia holbrooki) was introduced from the USA to control mosquitoes. However, it now outnumbers native fish in many parts of south eastern Australia, as they out compete indigenous species for food.
Exotic trees such as willows (Salix sp) are another problem. Willows produce dense shade, suppressing understorey growth, resulting in bare banks that are susceptible to erosion. The trees are a poor habitat for land animals, and the population and diversity of aquatic invertebrates and native fish is greatly reduced under their canopy.