A peat landslide which occurred in Upper Teesdale in 1983 is described. The slide followed thunderstorm rainfall of a very rare intensity (104.8 mm in 2.5 h) and a long predicted return period (2500 years). A large mass of peat (30,000 tonnes wet weight) entered the headwaters of the River Tees, resulting in chemical changes such as increases in suspended solids, iron, aluminium and lead in the river, and a consequent fish kill. The probable cause of the fish kill was the high suspended solid content (particularly peat and iron hydroxide) of the flood water. Effects on the invertebrate fauna of the River Tees are also considered.