How did the poker terms "flop," "turn" and "river" originate?

Flop:
"Unknown, but one might guess that the dealer's action of "flopping" the cards on the table may have something to do with the evolution of the term."

http://pokerterms.com/flop.html

Turn:
"One reasonable possibility is that the term may be derived from horse racing (or any other race on a curved track). In a race, when the horses are coming around the final turn, they are nearing the end of the competition. Similarly, when the fourth community card is dealt in a poker hand, the hand is nearing an end."

http://pokerterms.com/turn.html

River:
"One credible theory is that the term came from the Mississippi riverboats on which poker was played in the 1800's. Sometimes, cheaters would purposely deal a final community card that would improve their own hand. If caught, they would often be thrown overboard, into the river, thus giving the "river" card its name."

http://pokerterms.com/river.html