Hollywood power couple Rooney Mara and Joaquin Phoenix is celebrating the arrival of their first child together. And they have chosen an adorable name for their firstborn.

According to Us Weekly, the happy news was not confirmed by the couple, but Russian filmmaker Victor Kossakovsky. The latter is the creator of the documentary "Gunda" which has been executive produced by Joaquin Phoenix.

The director of the American-Norwegian film was attending a Q&A session at the 2020 Zurich Film Festival following the screening of the movie when he made the big announcement. The documentary filmmaker revealed why Phoenix didn't attend the big event. "He just got a baby, by the way, his name was ... a beautiful son called River, so he cannot promote it now," Kossakovsky said.

Rooney Mara and Phoenix has reportedly decided to name their baby boy River after the actor's late brother who passed away in the year 1993. Phoenix's older brother was only 23 when he died due to drug overdose. He was an actor, a musician, and an activist. And among his several memorable performances, he is well known for his role as Danny Pope in the movie "Running on Empty" (1988) for which he was nominated for Academy Award for Best Supporting Actor. The name of their baby is believed to be a tribute to the actor's late brother who was last seen in 2012's "Dark Blood," which was released unfinished 19 years after his death.

The news about Mara and her fiancé Phoenix's first baby was confirmed by a source of the abovementioned publication in May. At the time the "Mary Magdalene" actress was six months pregnant. "Rooney is about six months along," a source said at the time. "Joaquin is great and has been accompanying her to appointments."

Rooney Mara
American Actress Rooney Mara Mike Windle / Getty Images

The "Joker" actor and his fiancée are apparently a very private pair who is believed to be dating since the year 2016. Things became official in 2019 when Mara was spotted wearing a sparkling rock on her ring finger. In October 2019, Phoenix also spoke about her openly at the Golden Globes and People's Choice Award functions.