A Welsh Water spokesperson said:
“We are aware that some silt has been released into Roath Lake, Cardiff.
“We are working closely with Natural Resources Wales to help identify how this has happened.
“Our initial investigations have attributed the cause to the redevelopment of Llanishen and Lisvane reservoirs where we believe our contractors have accidentally disturbed some silt.
“This silt has made its way into Roath Lake via the Nant Fawr River.
“We will continue to work around the clock to confirm the cause of the incident and to fix this as soon as possible.”