An 18-year-old man has admitted karate kicking a 74-year-old man into a river.
Benjamin Bridgeman attacked the man while he was fishing on the banks of the River Mersey in Warrington on 25 April.
A 15-year-old boy filmed the attack and then posted it on the social media platform Snapchat.
Bridgeman, of Warrington, pleaded guilty to assault by beating and the 15-year-old pleaded guilty to common assault.
The pair will be sentenced at Warrington Magistrates' Court on 10 June.
The Crown Prosecution Service (CPS) said Bridgeman, of Quebec Road, was seen to run at the man and inflict a karate-style flying kick at him which pushed him into the river.
The pair ran off afterwards but could be seen later looking at the footage together and laughing.
The man, who was rescued from the water by some young girls nearby, suffered a cut to his nose and was shaken up by the shock of the assault and the effect of the cold water, the CPS said.
Bridgeman and the boy, who cannot be named for legal reasons, handed themselves in a few days later.
In a victim personal statement, the man's wife said her husband believed he could have drowned and since the attack has become aggressive, depressed and is reluctant to go out.
Lydia Durkin, from CPS Mersey Cheshire, said: "This was an entirely motiveless attack on an elderly man who was simply enjoying a day's fishing by the banks of the River Mersey.
"The early guilty pleas by the boys are welcome but why they chose to do this in the first place is a mystery."
The man's daughter Jacinta Bradbury previously said her father, who does not want to be named, had been on an "emotional rollercoaster" since the attack.
She also said the two 14-year-old girls who helped her father out of the water were "amazing".