Abstract. An overview of hydrochemical studies of the upper River Severn catchment in the Plynlimon region of mid-Wales, as linked to twenty years of research at CEH Wallingford, is presented. The work provides a bibliography of publications to date and illustrates that the water quality changes associated with felling are often small when compared with the effects of climate variability. The felling effects are manifested most clearly at the local scale and with clear as opposed to phased felling. Phased felling over several years, which is standard forestry practice, provides a much reduced response. The highly heterogeneous nature of the catchment is emphasised. This complexity makes identification and modelling of the changes very difficult to describe in terms of space and time. Long-term records (forty years or more) are needed to allow assessment of the changing water quality patterns associated with climate variability, climate change, changing pollutant deposition patterns and forestry rotation cycles. More detailed but still long-term (daily to sub-daily) records are required to examine ``fractal processes": within the fine structure is a high information content that can help unravel the dynamics of the internal workings of the catchment in a way that experiments at the plot scale cannot resolve.

Keywords: nutrients, acidity, trace metals, trace elements, Plynlimon, River Severn, forestry, fractals