The kawa (河, "river") is each player's own discard area, laid face up in front of their seat. Tiles are placed there in the exact order they were discarded, traditionally in rows of six. Together the four kawa form the public record of the hand.
On every turn, the tile a player chooses not to keep joins their kawa, placed face up in the exact order of play. The tile that declares riichi is turned sideways to mark the declaration. If it is called for chi, pon or daiminkan, riichi remains valid and the next discard is turned sideways. A called discard leaves the river.
A tile visible in an opponent's kawa is genbutsu and safe against their ron. A called tile leaves the river visually but remains one of that player's previous discards for furiten. The order of other discards gives suji and shape clues, never one certain answer.