This was supposed to be a black walnut live edge waterfall bookshelf but the slab wasn’t long enough to make even one true waterfall side. So I used a piece of cedar on one side and ash on the other to finish the project. The top is black walnut. The cedar and ash slabs were wider than the black walnut slab which created some shaping opportunities after mitre cut at 45 degrees for the joint. It’s a lot more work to try to match unrelated grain patterns and deal with different widths on a waterfall corner but it is possible! Enjoy the process!

Supplies

Three pieces of wood any will do but preferably same thickness at least. If they aren’t the same thickness more shaping will be required in the joint section that’s all.
Tools: All of them!
Circular saw, hand saw, belt sander/palm sander, hand tools and some screws maybe some clamps.

Step 1: Getting Started

The pictures show the slabs I started with.
First step is deciding the length of the top of the shelf. Mine was 46”. Now mitre at a 45 degree angle both ends. Your mitre cut should be oriented like the one in the second picture. This is true for both ends. My 7 1/4 circular saw didn’t cut all the way through the slab so I had to finish both cuts with the handsaw.

Step 2: Cutting the Sides

Now you need to decide how tall the shelf will be. Allow enough height to accommodate books or whatever that will be under the shelf. This is the step that rendered my black walnut slab too short to use for the whole “waterfall” process. I didn’t think the overall height would be as high as it was! Make sure the side pieces are wider than the top piece where the mitre cut will go. It’s easier to blend the sides into the top as opposed to the top into the sides. The mitre cut is same angle as top 45 degrees. Cut only one side for now. I find it’s safer to do the sides individually when using live edge because you can’t be sure exactly how long(tall) each side will need to be after cleaning up and straightening the live edge piece. It’s better to miss the height long by a bit on the side pieces because you can always trim off the bottom of the side piece. The bottom is flat. The first picture is the top piece. The second is the first side piece. Notice the side is wider.

Step 3: Shaping the Mitre Joint of the Top and Side Piece

The second picture shows the side piece (ash) mitre cut held against the top piece mitre. The top piece is centered on the wider ash piece. Draw a line down the side piece where it needs to be trimmed to be flush with the top. Do this for both sides of the ash. When done it should look something like picture one.
The fourth picture shows the 1/4 inch gap between the top and side pieces at the underside of the joint. There’s a gap because the thicknesses of the two pieces is different. In this case the top piece will be shaped to “meet” the side piece. Eyeball where the two pieces should meet up and draw a line on the top piece where you will sand (or cut) away the wood to achieve this.
The third picture shows the same joint after the top piece was shaped to fit the mitre on the side piece. Do this for both sides.

Step 4: The Second Side Piece

To get an accurate idea of how tall to cut the second side piece, I mocked it up after trimming its bottom flat. This is what the picture shows. Cut the mitre 45 degrees again at the desired length (height). The same process will be required to blend this side to the top if this side is also wider than the top piece. See previous step for that.

Step 5: Finishing and Assembling

Picture one you can see where I’ve used three screws to attach the sides to the top along with lots of glue as well. Countersink the screw holes so they can be filled and sanded smooth to hide them (picture two).
Now sand the entire shelf until you are satisfied with the feel of it.
I used mineral oil to finish the entire shelf

Step 6: Live Edge Bookshelf Video

I made this video to document the whole process. Some of the sanding/shaping is shown here. Also the application of the mineral oil to the finished bookshelf.