River Kennet warning over storm drain waste disposal
Storm drains linked directly to rivers are being labelled with yellow fish plaques to deter people from using them for waste disposal.
Volunteers from Action for the River Kennet (ARK) started installing the visual reminders on the streets of Newbury on Thursday.
ARK said what goes down the town's drains will eventually feed into nearby waterways untreated.
The signs on the drains say "rainwater only".
The initiative has been launched after a 3.5 mile (5.5km) stretch of water further downstream was contaminated with diesel earlier this month.
This led to 60 swans being evacuated from the area.
Project officer Anna Forbes said household products such as paint and insecticides had also had a "devastating" impact on the environment.
"Being careless about what goes into our rivers not only has a serious effect on the wildlife that depends on this special chalk stream, but is also dangerous to humans and domestic pets too," she added.
ARK is working on the scheme with the Renewal Project and West Berkshire Council.