Ilkley Tarn Renewal Plan
A plan to restore Ilkley Tarn as a thriving natural habitat
50 years ago, there were sticklebacks in Ilkley Tarn. 30 years ago, the population of toads breeding in the Tarn was so large that, on an April evening, walkers had to be careful not to tread on them when walking around it. Now Ilkley Tarn is in poor condition. The water is polluted. The principle reason for this sorry state has been the huge increase in the duck population.
The number of ducks on the Tarn is simply determined by the amount of food. The Tarn, in its natural state could support one or two ducks. The fact that there are more is because people feed them. We all know that taking small children to feed ducks is one of the great pleasures of life, and when it was a matter of grandparents taking a few crusts with grandchildren on a weekend, then the effect on the duck population was modest. But recently we have had a few (very well meaning) people taking large containers of duck food every day.
The result is that the duck population swelled to more than 100. This number of ducks defaecating and dabbling in the water, stirring up sediments means that the water has become so polluted and turbid as to restrict light penetration into the water, making it incapable of supporting other aquatic life, especially aquatic plants. Aquatic plants, both submerged and emergent, are essential as habitat for invertebrates and the submerged plants act as oxygenators.
Thus, the first step in our plan to bring life back to the Tarn has been to discourage duck feeding at the Tarn (we suggest feeding at the riverside instead) by putting up educational notices. This policy has had some success. At the last count there were about 20 ducks on the Tarn. It is a message that needs repeating and reinforcing as we would like to see a further reduction in duck numbers.
The second step in our plan is much more ambitious. We should like to increase the flow of fresh, clean and well-oxygenated water into the Tarn and we think this can be achieved by replicating, on a smaller scale, a version of the original Victorian water engineering.
When the Tarn was constructed, contemporary maps show water being brought, in a ditch, from Backstone Beck into the eastern end of the Tarn. The outflow was then from the western end and ran down into Mill Gill. The current outflow, into the sewage network via a culvert by Craiglands is a later addition.
We have carried out a preliminary assessment of flows into and out of the tarn, and flows within Mill Gill, using a simplistic but conservative approach looking at catchment areas and likely run-off rates. We calculate that (at peak flow) this outflow takes about 80 litres per sec (80 l/s). Yorkshire Water has in the past expressed a wish to take this water out of the sewage system to reduce the load on Ilkley Sewage Treatment works.
We think that our calculation of 80l/s is of the right order and this value corresponds well with the capacity of the culvert adjacent to Craiglands that has an assessed maximum capacity of 110 l/s. Under our plan this outflow will only operate in the most severe rainfall events.
Our proposal is to bury a small diameter flexible plastic pipe (probably 63mm external diameter), following the approximate route of the Victorian ditch that will bring 4 l/s fresh, oxygenated water into the Tarn. The pipe would be installed in short sections using volunteer labour and using only a mini-excavator, to cause minimal disruption to the moorland habitat. The work would be carried on outside of bird nesting seasons. Extracting this amount of water from Backstone Beck will require a permit from the Environment Agency. Bearing in mind the EA concern about flooding in Ilkley caused by Backstone Beck, we do not think that gaining this permission will be difficult.
4 l/s is, we calculate a sufficient flow to replace the water in the Tarn over two to three weeks, thus it should be sufficient to have a measurable effect on water quality.
The water would then be taken by a ditch or a pipe alongside the path up to the Tarn to Mill Gill. Our calculation is that the current peak flow in Mill Gill is about 1250 l/s, so that the addition of 80 l/s will make only a small percentage effect. Mill Gill has given rise to overland flooding onto Brook Street in the past, but this was caused by a build-up of debris on the trash screen at the entrance to the Brook Street culvert, and not by lack of flow capacity in the culvert itself. The Tarn would act as a large attenuation tank and thus there would be no increase in the amount of debris into Mill Gill. We are looking at options to use the Tarn to attenuate outflows into Mill Gill, to reduce the maximum outflow to significantly less than the stated 80l/s. Note also that the Tarn will not generate any significant amount of debris so will not exacerbate the build-up on the trash screen.
The culvert running down Brook Street is about 950mm x 950mm, and has a gradient typically greater than 1 in 100. It is about 400m from the inlet adjacent to the new fountain to the outfall in the Wharfe, and falls some 21m over that distance. The culvert walls are rough, but the flow capacity appears to be between 3000 and 4000 l/s. The additional flow from the Tarn would have no discernible effect on a culvert with a capacity of this magnitude.
Thus, our plan, to reduce the duck population and to increase the flow of fresh, oxygenated water into the Tarn should have a measurable effect on biodiversity. Our intention is to allow aquatic plants to re-colonise the site through natural dispersal allowing two to three years for that process to occur. Within the same period of time a greater range of invertebrates should appear and amphibians should restart breeding in the Tarn. And who knows, but a few years thereafter, sticklebacks might return.
Ilkley Moor is a Site of Special Scientific Interest. It is now part of a National Nature Reserve. Under our plans, the Tarn could play its part as a valuable thriving pond, rather than the polluted pond that it has become and contribute significantly to the overall biodiversity of the Moor.
Having taken advice, the first stage of our plan will be to employ hydrological experts to carry out a thorough analysis of the water quality, the flow rates at different times and the existing water flows into the Tarn. We believe that the Tarn is of sufficient importance that we should proceed cautiously and with expert advice. At the moment, we believe that the commissioning of an expert report is within our financial resources. If the report suggests that we should proceed, then we think that other sources of finance would become available.
Friends of Ilkley Moor
October 2025