In mahjong, the discard area where a player's discarded tiles are placed (also read 'hō'); used to track discards and infer opponents' hands.
相手の河を見て、あの牌はもう使えないと判断した。
Looking at the opponent's discards, I decided that tile was no longer usable.
河に牌が重なっていて、前の捨て牌が見づらい。
The discards are piled up in the river, making it hard to see previous discarded tiles.