Steinbeck grew up in Salinas, CA, and many of his works are set in the Salinas River Valley area, most likely because he knew the area so well. Steinbeck stressed realism in his writing. He lived among migrant workers to study their dialect and to be able to sympathize with their plight. He wanted to expose the struggles of the outcast or downtrodden, and visiting areas around his hometown allowed him plenty of opportunities to get to know those whom he championed.
The Salinas River does play a literal and figurative role in Of Mice and Men. During the Great Depression, the Salinas River Valley was one of the few places in the country where work could be found. The land there was not barren from drought, and people from across the country migrated there. Thus, it is realistic for George and Lennie to wander from job to job and even for them to be from somewhere that is most likely far away from the Tyler Ranch. Figuratively, a river is where the novella begins and ends, and it represents a peaceful, soothing place for both George and Lennie. By the river, they are able to be "their own boss," and no one harasses Lennie or forces George to defend him. In this aspect, the river plays a similar role as the Mississippi River in Huck Finn--it's a place free of social and cultural disdain or rules.
My question is, is it fiction or not?