Urban rivers cleanest for 20 years, Cardiff University says
Britain's urban rivers are the cleanest they have been for more than two decades, a study has found.
More than 2,300 river sites in England and Wales were analysed between 1991 and 2011 by Cardiff University.
They examined the presence and spread of 78 organisms - including insects, snails and others - and claim many are making a comeback following decline.
The scientists say the improvements are largely due to reductions in pollution levels.
The study measured the presence of "clean-river invertebrates" as a "yardstick for river health", using data from Natural Resources Wales and the Environment Agency.
Dr Ian Vaughan and Professor Steve Ormerod, from the university's School of Biosciences, analysed changes in the occurrence and spread of the organisms, and asked whether water quality, temperature or river flow best explained any biological changes.
They claim that - despite climate change warming Britain's rivers by up to 2C (35.6F) in recent decades - the "findings suggest that improved pollution control has managed to offset its damaging effects on river ecosystems".
The report authors claim it "indicates that society can prevent some undesirable climate change effects on the environment by improving habitat quality".
Of the 78 types of organisms studied, 40 have become more prevalent in rivers in England and Wales, due to reductions in gross pollution, the study claims. However, 19 have declined.
Improved water quality has resulted in some upland river species returning to previously-polluted lowland rivers, which the scientists claim could explain some northwards movement previously linked to climate-change.
Dr Vaughan said: "Our analysis showed clearly that many British river invertebrates are sensitive to climate, for example, because they require good supplies of oxygen that decline as rivers warm up.
"However, it seems that efforts over the last two to three decades to clean up pollution from sewage and other sources have allowed many of these sensitive organisms to expand their range despite 1-2°C warming trends and several periods of drought."
Prof Ormerod added: "These results reveal part of a larger pattern in which organisms dependent on cleaner waters, faster flows and high oxygen concentrations have been progressively re-colonising Britain's urban rivers.
"Atlantic salmon, mayflies, and dippers are prime examples.
"We need to protect these and other river organisms against climate change effects, and solving other problems such as pollution clearly helps.
"Away from Britain's urban areas, some pollution problems are increasing, and our analysis shows some negative trends among sensitive organisms such as stoneflies that are typical for rural hill-streams.
"It's important that our efforts to protect Britain's rivers against pollution or climate change are extended to the farmed, rural, upland landscape."
Last month, it was revealed that salmon are spawning in the upper reaches of the River Taff for the first time in 200 years.