AVALANCHES caused by heavy rain in the Yorkshire Dales have turned rivers the colour of chocolate prompting concerns that the unusual phenomenon could harm fish.
Experts say a localised cloudburst on top of Great Whernside and Little Whernside, between Coverdale and Wharfedale, caused a series of landslides at the weekend.
Tonnes of peat and clay from the moors was washed into the River Cover, which flows into the River Ure to the north of the hills, and the River Wharfe to the south.
The discolouration was spotted by regular river watchers including Dave Bamford, river manager at the Ure Salmon Trust, who is concerned it could harm fish.
He said: "We first noticed it on Monday morning. The River Ure at Masham looked like liquid chocolate and we knew something had happened.
"The Environment Agency found the source of the problem at Cover Head. Some people in Coverdale said the rain on Sunday night had been so heavy that it was difficult to stand up - it was monsoon rain apparently."
Mr Bamford, who is working to increase salmon numbers in the Ure, said he hoped the discolouration would not affect the large number of migratory fish which came up the river in the spring.
"At the moment the water temperature is 20 degrees and the salmon are just about coping.
"However, following the landslide we've had reports that some fish have been found dead but they've not been substantiated."
The Environment Agency said peat could irritate fish gills, although it was hoped the incident would not cause a serious problem.
A spokeswoman said: "The discolouration has been caused by an intense and localised cloudburst which fell in the Great and Little Whernside area over the weekend .
"This extremely intense rain caused an number of peat landslides and avalanches which ran off to either side of the hills - into the River Cover in Wensleydale, a tributary of the River Ure and into a tributary of the River Wharf above Kettlewell in Wharfedale.
"The force of the flow in one location knocked out a dry stone wall and disturbed a number of small trees."
Water levels in the rivers are low because of the time of year, meaning sediment will take more time to flow through the river system, experts said.
Anyone who notices fish in a distressed state in the Cover, Ure or Wharfe should call the Environment Agency incident number on 0800-807060.