Riverdale has done OK for The CW, posting a 0.8 rating and 2 million viewers in Live+7 day numbers. While those aren't terribly exciting stats, they're better than The CW's other new shows this season, No Tomorrow and Frequency, both of which are expected to be canceled.
The series has also generated a passionate fan base and positive reviews from critics, and is streaming well (a figure that doesn't show up in standard ratings). That all adds up to a big thumbs up from The CW, which welcomes programming that isn't just about superheroes (even though Riverdale shares the commonality of being inspired by comics).
With the renewal, Riverdale joins Arrow, Supernatural, Supergirl, DC's Legends of Tomorrow, Jane the Virgin, Crazy Ex-Girlfriend and The Flash as guaranteed returns for the 2017-2018 season. Riverdale is also super producer Greg Berlanti's fifth show on that list.
Riverdale airs Thursday nights at 9/8c on The CW, after Supernatural.