There is a lot of public land downstream from the Lorenzo boat ramp. Surely the boat ramp is not the only access point for the public to get to their land?
IDFG does not maintain any access points between Lorenzo and Menan. There are areas with public land, however. Most of it belongs to the Bureau of Land Management. As such, obtaining one of their land ownership maps would be the best way to determine public vs private lands along the South Fork (link below). Alternatively, there appears to be an app designed to provide this information (link below).