A 35-year-old man died at Casuarina Sands on Thursday afternoon. His body was found in the river by police divers who had responded to reports of a missing swimmer.
ACT police said they would prepare a report for the coroner, although they did not consider the death suspicious.
A witness told The Canberra Times it was a "very sad day". The man was swimming with his wife and two children about 5pm when he drowned.
"He began to struggle and was out of his depth. He attempted to keep his children afloat and then disappeared into the water," the witness said.
"Many people attempted to dive and help try and find him, while others helped his wife and children ashore.
"Unfortunately no one was able to locate the man as the water was too deep with no visibility. Members of the public searched for over half an hour until they were told to move out of the area by police."
Police reminded Canberrans to take care when swimming and to watch out for each other in and around the water.
Preliminary data from Royal Life Saving Australia shows 50 people died in waterways and at beaches between December 1 and January 2 across the country.
More than 40 per cent of the people who drowned had died at rivers, lakes and dams, the data showed.