He knew what was in back of him and before him. In 10 cases of 10, I will take what I know over the unknown. Turning around was a major option, he knew the terrain he had already traversed. Continuing on into "unknown" territory, I say unknown as no terrain is truly known, it always varies, was not a wise option.
One small point JB.
This was a river trip. Apparently a big, fast river. Paddle upstream....?....I don't think so.
His choices were to either complete the trip, or get rescued.