PART of Huddersfield Narrow Canal has been left almost empty after vandals damaged two locks at Aspley.
British Waterways is reviewing security at lock gates following the vandalism.
Lock One on the Narrow Canal and Lock Nine on Huddersfield Broad Canal were forced open and the mechanisms jammed. Several items were also thrown in the water.
Damage has been put at about £1,000 and engineers are busy repairing it.
A British Waterways spokesman said the environmental damage was devastating.
He said fish would have been killed, as well as other wildlife and plants.
The spokesman also said it would take some time to "re-water" the section which had been drained.
Vandalism was a continuing problem and he urged witnesses to tell the police.
A 24-hour phone number - 0800 47 999 47 - is available to report pollution or litter on waterways.