Minecraft wiki entry for Building_water_features, associated text: Build a 1 deep, at least 3 wide all, however high you would like, to start. Then, build a barrier 1 block above the top of the wall, to prevent the water from flowing out the wrong way. Finally, add water on the top of the wall. Also, if you would like, you can build a 1 block deep trench at the bottom of the waterfall, so that the water goes straight down instead of all over the place.