A river nigger is a Mexican as they use the river to cross over to the USA and jump fences or cross at openings in fences to gain access to the USA. A fence jumper, if you will.
An expression used in the card game "Texas Hold 'em Poker" for someone who was losing the hand to one or more players, and was saved by the 5th card, the river, due only to luck, and won the hand.
Bob: "All in"
Stew: "... I call"
Bob: "haha, I'm beating you with the odds of 4:1 in favour of me, you only have 3 cards you can hit out of the deck, with one card left, what a crappy call and a horrible read"