As pollution control becomes more rigorous it is expected that ambient or "average" environmental quality will rise steadily. But more stringent legislation, improved wastewater collection and treatment, more sophisticated operational control and other developments that enable environmental improvement also serve to heighten the expectations of a more environmentally aware public. In this context even short-term pollution events will be regarded as unacceptable. Yet by their nature such transient and "unpredictable" events constitute a severe challenge to existing concepts of water quality management. In particular they set problems that cut accross traditional disciplinary boundaries. New planning and operations strategies are needed to provide adequate answers to these problems. The twenty papers collected in these proceedings were presented at a pioneering symposium at which professionals from a wide range of disciplines and viewpoints sought to achieve the necessary synthesis of ideas, approaches and experience to resolve these issues. Papers deal with the nature and extent of the problems, the concepts of uncertainty and risk in water management, the monitoring of impacts, and modelling and management strategies. They provide a comprehensive update of the state of the art in this area.