LogFAQs > #965251161

He talks about it in the Giant Bomb interview. The river is meant to be a nudge for players that didn't notice the mechanic earlier.

The environmental puzzles' origin is pretty interesting. The genesis of The Witness was as a fantasy game where you cast magic spells by drawing runes. The most powerful spells would be subtly hidden in the environment. Obviously The Witness became a very different game, but the idea of things hidden in the environment remained.