A dog walker discovered the male dog in the middle of the canal at around 12.20pm on October 5.
The RSPCA said what appeared to be a pinky-red lead and matching collar were wrapped around his head and front leg.
The charity hopes the description will help identify the dog, which did not have a registered microchip.
Animal rescue officer Beth Fazackerley said: “This was an incredibly upsetting incident for the member of the public who made this distressing discovery, and it is heartbreaking that this poor dog was found in such circumstances.
“We are keen to establish what happened, and would urge anyone who was in the area around Middleton Road at midday, on October 5, and may have information about what happened, to get in touch.”
Anyone with information should call the RSPCA appeals line on 0300 123 8018, quoting incident number 01919098.
