Experts say tests will reveal the full extent of a pollution incident in watercourses near Richmond.
Hundreds of fish and other wildlife are thought to have died after slurry got into the stream which runs through Gilling West and Skeeby.
The waste got into the watercourse from tanks at a site near Hutton Magna, with farm bosses saying vandals were to blame for the release.
Experts from the Environment Agency attended the scene last week.
A spokesperson said: “Our officers attended a pollution incident last week spanning a number of watercourses near Richmond, North Yorkshire to assess ecological impacts and mitigate any harm to the environment.
“The discharge has now been stopped and the water is running clear.
“Ecologists have taken samples to evaluate the impact to wildlife and we are awaiting lab results to understand the full extent and impact of the incident.
“Members of the public can report environmental incidents to our 24/7 incident hotline on 0800 807060.”
More than £4,700 has been raised in an appeal to help restore the watercourse.
To donate to the appeal click here.