A combined sewer overflow, where the sewer system releases untreated sewage mixed with rainwater into a river during heavy rain to stop it backing up into streets and homes. Water companies must now monitor every overflow with Event Duration Monitoring (EDM), which records how often and how long each one spills. Yorkshire Water publishes the results, and we show spills into the Don, Sheaf, Porter, Rivelin and Loxley.
Wikipedia: Storm overflow