The deal is expected to be finalised at the end of the week and will see River join the recently merged above-the-line agencies Nitro and Soul.
River's chairman, Mark Thornton, will work alongside Nitro's Chris Clarke and Paul Shearer and Soul's senior management team. This line-up includes Andy Bird, Duncan Bird, Kevin Brown and Bruce Crouch.
River's client list includes Samsung, the Royal Bank of Scotland and Thames Water. Its services incorporate sales promotion, sponsorship and film production.
Last month, Nitro, the agency founded by Clarke, bought a significant stake in Soul, which has £12 million of billings from clients including Fanta and Tresemme. Nitro, meanwhile, handles global assignments for Mars, Unilever and Swatch.
The merged agency picked up its first piece of business last week, taking a slice of Nike for Europe, the Middle East and Africa from Wieden & Kennedy Amsterdam. It has been briefed to create a brand campaign to run this summer. Shearer worked with Nike for two years while he was a creative director at W&K in Amsterdam.
Nitro was founded in China in 2001. The agreements with Soul and River add a UK presence to the company, which has offices in Melbourne, Sydney, Shanghai, Hong Kong, Taipei and New York.