sentences:  To further improve fish stocks (especially salmonid) 'V' weirs were constructed to trap gravel and create suitable spawning habitats.At the river's source in Blaenavon, Conservation Volunteers and the Forgotten Landscapes Partnership are re-greening the river where it runs through the Blaenavon World Heritage Site.The river has been invaded by the American signal crayfish, which is affecting the native protected white-clawed crayfish.