Generally the water quality of rivers is best in the headwaters, where rainfall is
often abundant. Water quality often declines as rivers flow through regions where land use
and water use are intense and pollution from intensive agriculture, large towns, industry
and recreation areas increases. There are of course exceptions to this rule and water
quality may improve downstream, behind dams and weirs, at points where tributaries or
better quality groundwater enter the mainstream, and in wetlands.
The condition of NSW rivers and estuaries has deteriorated
since European settlement. This is a result of major
changes in their catchments, instream conditions and
estuary entrance characteristics. Catchment changes
may vary from deforestation and soil erosion in the
inland rivers to extensive urbanisation in coastal
regions. In the NSW South East Region, the rivers,
lakes and estuaries are some of the healithiest in
the country. Only 10% are regarded as significantly
changed. Of this 10%, the most degraded system is
the Bega River, but even this is only regarded as
"stressed" for 10% of the time.
Rivers frequently act as conduits for pollutants
by collecting and carrying wastewater from catchments
and, ultimately, discharging it into the ocean. Stormwater,
which can also be rich in nutrients, organic matter
and pollutants, finds its way into rivers and oceans
mostly via the stormwater drain network. Beach water
quality in NSW may also be affected by bacteria from
sewer overflows (mainly after heavy rainfall) or other
runoff into stormwater drains. Both Councils on the
NSW South East Coast regularly monitor beach waters
over summer and sporadically throughout the year.
The data collected, coupled with Council pollution
control measures and effluent treatment, suggests
that the NSW beaches are some of the best in the world;
in terms of water quality and intact native vegetation.
For some rivers frequent heavy demand for for water is causing major changes in flow
patterns and flooding characteristics with subsequent impacts on aquatic ecosystems. At
the same time the development of riverine plains (which often involves the clearing of
riverside and bankside vegetation) has reduced channel stability, biological diversity and
the capacity of the streams to cycle nutrients.
Water quality is closely linked to the surrounding environment and
land use. Other than in its vapour form, water is never pure and is affected by community
uses such as agriculture, urban and industrial use, and recreation. 
