river is a tiling window manager for Wayland. Most of the big tiling wms are for X, and to my knowledge not many are planning Wayland versions. It makes sense, as for a tiling wm you'd just be rewriting most of the code. So instead you get these roughly parallel projects that start at attempts to basically port wms to Wayland: Sway for i3, river for bspwm, and dwl for dwm.

Sway is by far the most popular, but river and dwl are both functional alternatives, even if the feature sets are a little barebones. Personally I'm waiting for a Wayland version of herbstluftwm, it's my favorite.