SummaryChanges in discharge and chemical composition are related to biological and physical conditions at eight sites on the South Winterbourne from the source to the confluence with the River Frome. The results show a complex pattern of discharge with nitrate peaks at high discharge values. Mean phosphate values varied from 34·2 μg l−1(at the source) to 164·6 μg l−1.The Winterbourne has a rich flora of algae and higher plants of which the annual sequence is influenced by the flow regime and the effects of human interference by cutting and removal of plants and addition of nutrients. The fauna has much in common with permanent chalk streams but temporary flow conditions favour insects having prolonged resting stages or those which are able to colonize quickly from other areas when flow commences. Many non‐insect groups particularly snails, are able to withstand relatively long dr