The Environment Agency has confirmed it had been called upon to investigate the river’s unusual appearance.
The cause has yet to be confirmed but a green dye pumped into the drains in an attempt to locate a blockage is believed to be responsible.
Although it looks alarming, such dye is not harmful to aquatic life and poses no risk to the environment.
The Environment Agency said it had contacted Yorkshire Water and was waiting for confirmation but a harmless green tracing dye was believed to be responsible.
One walker said the Porter Brook had turned ‘vibrant green’ between Forge Dam Cafe and Common Lane on Tuesday afternoon (Apr 5).
It’s not the first time this has happened, with the river adopting a similar shade in July last year due to a tracing dye, as this photo taken by Tonia Lucas showed.