Water temperature, dissolved oxygen, and conductivity measured alongside zooplankton sampling in the Chicago River before and after a major CSO event in 2023. Three locations were sampled, with the disturbed location receiving 9.1 billion litres over two weeks. Shared letters indicate statistical similarity among locations within each period (‘a/b’ before the CSO; ‘x/y’ after the CSO), while asterisks denote significant changes within locations before and after the event.