Two men pushed a fisherman into the river and then stole his fishing bag which had his Rolex watch inside, police have said.
The 48-year-old was fishing on the River Leven in Alexandria, West Dunbartonshire, at 22:30 on Friday when he was approached by two men on bikes.
The bag they stole also contained fishing equipment and other personal items.
Officers said the man was uninjured and managed to get home to call the police.
The suspects are described as white, in their late 20s and wearing dark clothing. Both had baseball caps on, one dark, the other light a light colour which was possibly a Burberry style.
Det Con David McKee said the victim regularly night fished in the Bonhill area of Alexandria.
"The two suspects came up to him (from the Alexandria side of the path) and started chatting but as he stood up to carry on fishing, one of the men pushed him into the water before the two of them made off with his belongings," he said.
"The Rolex watch is an old style, one he has had for over 20 years and wears all the time. It is of great sentimental value to him and he is devastated by its loss."
The officer appealed to anyone with information to contact the police.