The quick-thinking passerby waded into the water before pulling the struggling animal back to shore.
When officers from Nottinghamshire Police arrived, they discovered a large rock inside a carrier bag had been tied to her lead.
She was taken to a local vet for treatment and a check of her microchip revealed she was last known as Bella.
But officers said her name may have been changed by a previous owner.
Police Constable Pace urged anyone who was in the area to check their CCTV and dash-cam footage to help their investigation.
He said: ‘This was an evil and nasty thing to do to this poor dog and we are making every effort to find out who is responsible.
‘We are lucky that the kind member of the public found and rescued her.
‘Whilst the dog is still quite poorly she has been showing some encouraging signs by eating so hopefully she will be on the mend.’
Since the public appeal for information, a 31-year-old woman has been arrested on suspicion of animal cruelty.