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Ilkley residents are being urged to not panic if they see the River Wharfe or one of its becks turn green on Friday or over the weekend.

A fluorescent green tracing dye is being put into a watercourse that comes off Ilkley Tarn on Friday morning to try to better understand the route it takes to the River Wharfe.  It’s unlikely to be visible in the river until Friday lunchtime.

The purpose is to discover whether the watercourse flows into Backstone Beck and contributes to it flooding, particularly onto Leeds Road, in times of heavy rainfall.

Simon Stokes, of the Environment Agency, said: “Our maps suggest that the culvert below the tarn carries the flow north-west to Backstone Beck – joining the watercourse between Ben Rhydding Road and Clifton Road - but on the ground investigations have shown that this might not the case. Water can then be heard passing under manhole covers on Craiglands Park but from there the course is unknown.

“Old maps indicate that the tarn previously drained west before disappearing close to the Trout Beck Hotel, now a care home.”

If the mysterious watercourse does flow into Backstone Beck it will need to be modelled as part of an ongoing natural flood management (NFM) project.

The Backstone Beck NFM project was granted £167,000 out of a pot of £14 million allocated by the government to similar projects around the country in 2017.

The Agency is currently developing the project with Bradford Council, which owns the moor, in order to begin work on the natural flood management scheme once the ground nesting bird breeding season is complete.

Mr Stokes said: “The small size of the Backstone Beck catchment and the fact that the A65 Leeds Road floods relatively frequently after periods of heavy rainfall suggests that it is an area in which natural flood management could potentially have a positive impact.

“After putting the dye in the watercourse we will follow it as best we can - obviously we won’t walk through culverts or watercourses - to identify which catchment the water from the tarn joins.

“If the water joins Backstone Beck then the catchment for the tarn is within scope of our project.”

Last year, the Environment Agency appealed for information about Ghyll Head Reservoir and Mr Stokes thinks he has got to the bottom of it. 


He said: “Further up the Backstone Beck catchment is Ghyll Head Reservoir, it’s a covered reservoir which had me baffled for months.  Myself and staff from Bradford Council spent a couple of days looking for the inlet and outlet for it wondering if it could be repurposed for storage.

“It turns out, to the best of our understanding, that it’s a filtration reservoir built in the first half of the 1800s, likely to have served one of the hydropathic establishments in the town, and water is fed to it through hundreds of metres of iron pipe from springs on the top of the moor; filtered water was taken from the reservoir via the same means.

“One assumes that its located there for no other than the course of the beck created a nice flat spot on which they could site it – the beck itself is passed around the reservoir in a stone channel without, it seems, any hydrological interaction with the internal parts of the structure.”


The water tracing dye is a safe, nontoxic, and biodegradable way to determine the direction and rate of water flow in various systems. The water tracing dye dissolves rapidly in water, producing a vivid, easy-to-see colour used to trace the diffusion. Water tracing dyes are excellent for identifying diffusion and dispersion patterns, rates of flow, discharge locations, seepage areas, sea marking, and more.

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