Acid rain's legacy for Britain's waterways is far more serious than previously thought and will take decades to repair, research suggests.

A study of rivers and streams in Scotland and Wales has shown that environmental damage caused by the acidic downpours of the 1970s and 80s is ongoing, despite a massive reduction in industrial emissions.

It may take 30 years until Ph levels in upland waterways and surrounding soils return to normal, researchers from Cardiff University have said.

This has implications for fish and freshwater insects, such as the mayfly, whose survival, in turn, affects birds such as dippers. In some streams 90% of salmon stocks have disappeared.

Monitoring by the Department for Environment, Food and Rural Affairs, based on 22 sampled sites, has given an optimistic impression of Britain's waterways with the return of native algae and insects to affected areas.

But the latest findings show damage to wildlife is more serious than estimated and recovery rates slower.

The researchers took samples from 90 rivers and streams in Galloway, the Highlands, the Scottish Borders and in Wales, at normal water levels and after heavy rainfall. They found that acid levels were at their highest after heavy rainfall and that this was having a "disproportionate" effect on wildlife.

Professor Steve Ormerod, one of the report's authors, said: "The surprise is that, given we seem to have tackled air pollution - there's an 80% decline in sulphur dioxide emissions and 40% reduction in nitrogen oxide emissions - that we're still seeing this extremely slow recovery."