River of No Return (1954)

   Matthew (Robert Mitchum) is a man with a past but he wants to just move on and start a farm.  As soon as he's able, he sends East for his son, Mark (Tommy Rettig), but the man he entrusted to bring the kid out disappears as soon as he hits town.  Mark gets in with saloon singer, Kay (Marilyn Monroe), in order to stay out of trouble until his dad can pick him up.  Matthew has very dim views on Kay and the lifestyle she promotes.  These are reinforced when she and her card sharp boyfriend (Rory Calhoun) have to be rescued from the river outside of Matt's property.  The boyfriend needs to get to the nearest real city in order to file a gold claim he won in a poker hand and doesn't have time for niceties.  He steals Matt's gun and horse, leaving them defenseless against the dangers of the wild West, and takes off.  Kay stays behind in protest against this callousness.  Father, son, and showgirl are forced to take to the river when Indians attack, hoping to survive long enough to reach the town themselves.

In this intrepid adventure, we get to see our heroine battling the great outdoors.  So far, I think this the most activity I've seen from Marilyn and the only role that hasn't required her to be a dumb blonde.  She's naive but not stupid, and that's a big step up.