There are very few, if any, truly natural water courses in Wales: most rivers, for example, have undergone one or more of channel straightening, bank modifications, water extractions, the insertion of weirs and dams or the construction of marinas and barrages where they enter the sea.. They have also suffered varying degrees of pollution as a consequence of acid rain, nitrogen deposition, sheep dips, sewage treatment plants, eutrophication from land run-off etc.