Action is being taken to tackle pollution in the River Calder after fish were found dead between Copley and Cromwell Bottom, near Brighouse.
Staff at the Environment Agency will be working with Yorkshire Water to improve water quality close to Halifax Waste Water Treatment Works at Copley.
The Environment Agency investigated the pollution on April 25 and found 200 dead trout and grayling.
This stretch of the river will now be surveyed to assess whether the pollution has caused any long-term impact.
The results will be used to determine whether fish restocking is needed.
Environment Agency team leader for Calderdale Tracy Langdale said: "Our investigation found that the problem was caused when the power supply was cut to the treatment works. This meant that the sewage being discharged into the river was only partially treated."
Halifax treatment works is allowed to discharge under these conditions as part of its consent permit which is issued by the Environment Agency.
Staff are now reviewing this permit to reduce the amount of ammonia that is allowed to discharge into the river. Ammonia is present in sewage and is toxic to fish.
Yorkshire Water is also carrying out a programme of improvements at the treatment works which will further reduce the risk of pollution.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article