There is no river more unspoilt in Yorkshire than the Esk. For much of its 25-mile-long flow eastwards to the sea at Whitby, it meanders almost secretively beneath overhanging trees and is visited mainly by otters and herons.

But crystal clear the Esk is not. It's a distinctive tea-brown colour owing to its origins high on the airy peat landscape of Westerdale Moor.

The murkiness isn't helped by the large number of livestock which depend on it for water. The river is an important source of water for farmers' stock, and grazing of the notoriously sandy riverbanks can strip away vegetation and leave them crumbling.

This doesn't simply create a further unpleasant discolouring of the water. Sediments being swept downstream can have harmful consequences for the gravel beds, where the Esk's famous salmon and sea trout come to spawn each year. More seriously, they are also threatening the survival of freshwater pearl mussels which depend on clean riverbed gravel for successful breeding.

The pearl mussel – it can grow as large as the human hand – is rare in England, and the Esk is the only Yorkshire river on which it is found. Once collected
in their thousands by pearl fishers in the last century, taking the mussels or even disturbing them is
now illegal.

Unlike the familiar seaside species, pearls grow inside their black shells, although not every mussel develops one. The pearl is a ball of calcium carbonate which the mussel secretes in response to the irritation it feels from a piece of grit or twig that managed to find its way into the shell.

Over many years, the pearl grows bigger and bigger as it envelops the source of irritation, and can sometimes appear translucent. More often than not, however, they are discoloured and misshapen, and lack the beauty of classic pearls.

The mussel population on the Esk is now estimated at not more than 200, spread around between 10 and 20 small colonies between Danby and Glaisdale, and a project to save the species is now under way. Led by the Environment Agency and North York Moors National Park, it will cost more than £140,000 over the next few years. The plan is also linked to work on improving the river gravels where salmon and sea trout spawn, safeguarding their populations for the future.

Central to the project is finding ways of stopping the siltation process. The fine sediments block up spaces between the river gravels, which stop oxygenated water from reaching fish eggs or the young pearl
mussels in their first years of development, effectively suffocating them.

To help persuade farmers in the Esk's catchment area to get involved in the project, a special demonstration farm is being created.

Peter Dowson's dairy herd at Furnace Farm, Fryupdale in the Esk Valley, was wiped out by the foot and mouth disaster of 2001. Since then he has restocked with suckler cows, and earlier this year he successfully applied to have his land used to show how siltation of the Esk could be successfully reduced.

The key objective, which will be illustrated by new work undertaken at Furnace Farm over the next few months, is to prevent livestock from gaining access to the water courses wherever possible.

In some places it's a simple job of putting up bank-side fences to keep stock away from the Esk. But according to Fraser Hugill, senior farm conservation officer with the North York Moors National Park – one of the partners in the Esk project – stock are very good at finding alternative routes to the river.

"Obviously, animals do need to drink," he says, "so we looked at the thin riparian corridor along each stretch of riverbank to see where it was practical to bring stock down to the water, and looked for other spots where the water needed to be brought up to the stock." A series of managed watering points will now be created at Furnace Farm. Innovatively, one will be a cattle water trough supplied directly from the Esk by a solar-powered pump, costing about £1,500.
This will give Peter Dowson's livestock all the water they require while the nearby riverbank can be safely fenced off.

At other locations, special cattle drinks on the Esk will be created using either crushed stone or re-cycled concrete railway sleepers embedded in the ground so that the livestock's hooves don't trample the riverbank and add to the siltation problem.

It is hoped that farmers along the Esk and its many small tributaries will visit Peter Howson's farm and see how they can reduce siltation from their own land.

Meanwhile, an attempt to breed the mussels artificially is under way. Almost 30 adult mussels have been taken to the Freshwater Biological Association at Windermere, where their complicated breeding cycle (see story opposite) is being reproduced in fish tanks. When the mussels are five or six years old they will be reintroduced to the Esk.

Roger Martin, Biodiversity team leader for the Environment Agency at York, says: "Pearl mussels are highly sensitive and a classic indicator species of ultra-pristine water. They are found only in the very best rivers.

"We could sit back and watch them become extinct on the Esk, or we can try and save them. The work we're doing will also benefit the salmon and sea trout, and the health of the river in general."

n Farmers and landowners along the River Esk wishing to learn more about the Esk pearl mussels and Salmon Recovery project can contact Fraser Hugill at the North York Moors National Park, The Old Vicarage, Bondgate, Helmsley, York YO62 5BP. Telephone 01439 770657 or email f.hugill@northyorkmoors-npa.gov.uk