 Even after the construction of sewage treatment works at Miskin and Rhiwsaeson, the quality of the effluent continued to cause pollution until the late 1980s.These problems were exacerbated by the highly polluting discharge from Coedely coke ovens.At the mouth of the river, the effluent a large paper mill rendered large parts of the estuary anoxic for most of the tidal cycle preventing the passage of migratory fish.