Horowitz, vice president of product management since 2008, announced the move in a Google+ post late on Sunday.
"It's important to me that these changes are properly understood to be positive improvements to both our products and how they reach users," Horowitz wrote.
It was not immediately known what the company called "streams" product.
"I think increasingly you'll see us focus on communications, photos and the Google+ Stream as three important areas, rather than being thought of as one area," Forbes quoted Pichai as saying.
Google+ marked the company's most concerted effort to catch up with Facebook Inc in the fast-growing social networking market, but the service has struggled to match the rival's popularity.
It was not immediately clear what role David Besbris, who last April replaced Google's head of social networking, Vic Gundotra, will be taking on.
Google representatives did not immediately respond to a request for comment.
The changes were first reported by technology blog Techcrunch, which also said that the Google+ team has halved in size since Gundotra's departure.
© Thomson Reuters 2015