"Things I Don't Understand" #3

COLLOQUIAL SAYINGS

Where does the saying "We'll cross that bridge when we come to it" come from? It means you'll overcome some future obstacle, but that would imply that the bridge WAS an obstacle, wouldn't it? The line should be "we'll ford that river when we come to it." Or maybe, "we'll cross that rickety bridge with the 500-foot fall to a raging river below when we come to it."

It's unclear. All I know is I have a sudden hankering to play Oregon Trail.