Officers today released an e-fit image of the man, who they believed could be linked to a number of incidents.
A spokeswoman said: "Officers are investigating an incident along the River Wharfe near the viaduct in Tadcaster and Church Hill, Selby.
"In both incidents, the description of the man has been the same.
"Both incidents happened on June 19, however there have been other reports of this nature recently along the river in Tadcaster."
Anyone with any information is asked to call North Yorkshire Police 101, press 2 and ask to speak to PC Steve Shaw.
Alternatively, contact Crimestopppers anonymously on 0800 555 111.
Please quote reference number 12170107029 when passing on any information.