Sergeant Aaron Nicholson, of Wanaka, said an immediate search was initiated involving LandSAR teams, the Hawea Marine Group, Wanaka Coastguard and a night vision equipped helicopter from Queenstown.
The 16ft aluminium jet boat was located early in the search, beached in a shallow section of river braid near the mouth of the Makarora River but there was no sign of Mr Rivers.
Police deployed teams again this morning and at 9am a LandSAR team discovered the missing man's body in the river, snagged below the surface by a sunken willow tree about 150 metres from the jet boat.
Police have referred the matter to the coroner for a full investigation.