The river Wye, 250 km long and draining a predominantly rural catchment of 4183 km2, rises in mid-Wales (677m O.D.) and flows to the Severn Estuary (Fig 1).
Samples for chemical analysis were obtained from sites 1 to 14 at two-week intervals over the period April 1975 to July 1976 and from site 15 at weekly intervals from 1973
