The Underground Rivers patch adds streams of water or lava to random rooms-and-corridors levels in the Dungeons of Doom. It is implemented in Slash'EM Extended by default but can be made to work for other NetHack variants as well. Taken from the readme:
Dungeons below dlvl 13 occasionally have rivers, either of water or lava. The code doesn't check for blocking the way, so there's a chance you might need either cross the river or dig around it to get to the stairs. Special rooms are not affected by the river. Does not invalidate bones or saves.