Although water quality improvement has continued or been maintained in the north- west, Wales, the Midlands and the south-west, it has declined in the Southern, Thames and Anglian regions - the areas hit hardest by low rainfall.
An Environment Agency spokesman said yesterday: "In low flowing, warmer rivers like those in the south-east there is less dissolved oxygen than in cooler, fast flowing rivers of the north and west. River wildlife needs oxygen to survive and the lack of dissolved oxygen could have a serious impact."
Algae, which is more prevalent in low-flowing warmer rivers, also affects the amount of dissolved oxygen in a river and many rivers in the south- east are fed by groundwater springs, which contain less dissolved oxygen than surface waters.
The latest results show that last year the chemical quality of river water was 21 per cent better than in 1990, the baseline year against which such changes are now measured. This is not as good as for the previous two years - 1996 (26 per cent) and 1995 (28 per cent).
Yesterday's report by the Environment Agency called for investigations into the effect of the changing weather patterns on rivers and canals and warned that, if nothing was done, there could be a "severe impact" on the amount of water available to the public, industry and the enviroment.
Dr Jan Pentreath, the Environment Agency's chief scientist, said that climate change will affect not only the supply of water, but demand for it, such as increased crop irrigation in warmer, drier areas.
He said the situation could be aggravated by other factors, such as the 4.4 million new homes that are planned for the south-east.Reuse content