YO1 Beach Club in Tower Gardens was not able to open until 12.30pm while forensic enquiries took place at the riverside park.
North Yorkshire Police have released brief details about a 'disturbance' in Tower Gardens overnight which included sexual activity and resulted in a man being arrested.
The force said: "Police in York have launched an investigation after members of the public reported a disturbance involving a group of people, including people involved in sexual activity, in the Tower Gardens area of York in the early hours of Sunday morning.
"Officers received the report at around 3.20am and are carrying out extensive enquiries into the incident and into allegations of a serious sexual assault.
"A man in his 20s has been arrested in connection with the incident and has been released on conditional police bail to allow for further enquiries to be carried out.
"Anyone who was in the area at the time and witnessed the incident but has not yet spoken to the police is asked to get in touch.
"Please call North Yorkshire Police on 101 if you have any information that could assist the investigation."
Please quote incident number 12210162345 when passing on information. If you wish to remain anonymous, you can pass information to Crimestoppers on 0800 555 111 or online at crimestoppers-uk.org