The LA River is a minor location in Fear the Walking Dead appearing twice in Season 1 of the show.
In the "Pilot", after Nick made contact with Calvin, the LA river is where Calvin takes Nick to kill him and where Calvin himself dies instead. It's also where Nick meets up with Madison and Travis the following day in order to explain to them what happened. It is the scene of Nick's unsuccessful attempted to put down the reanimated Calvin.
It is seen again late in the season when Victor Strand directs Madison Clark to use it as a shortcut to reach his house. Since it was last seen, the river and its surrounding embankment has now become home to several wrecked and burned out vehicles and helicopters, as well as at least one Infected. There was no sign of Calvin or his car; it may be that he was taken care of by the military.