Rocky, acidic upland streams are potentially the most ‘natural’ of the watercourses in Wales. The banks have rarely been modified and carbon dioxide is freely available in the water, allowing mosses and liverworts, for example, to thrive. They are also less prone to pollution from industry and sewage plants, though they will still be impacted by atmospheric depositions, e.g. acid rain and nitrogen deposition, and by farm run-off and sheep dips.