The River is a large, flowing body of water, which generates in the Forest in all gamemodes. This was added in the 'Water Is Better With Sugar' update, being, alongside the previously-added lake, a natural water source inside the forest. It can be anywhere in the Forest depending on which server you play on, and usually travels from the top of the forest to the bottom.
The river has a current, which pushes the player downwards [verify]. However, if you just continue moving left or right, you will not be affected by it. Also, the current will not affect you if you are on a bridge.
At the lake or ocean that represents the end of the river, the current stops.
With the high quality setting, you can see lily pads and rocks; however, with the low quality setting, even the current is not visible.