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, along with most smaller streams, suffered varying degrees of pollution as a consequence of acid rain, nitrogen deposition, sheep dips, sewage treatment plants, eutrophication from land run-off etc. For the purposes of nature conservation, watercourses in the UK have been assessed for changes in their expected or known trophic status, with a view to detecting signs of acidification or eutrophication, while invertebrate sampling has been used as an indicator of water quality.