Step 2: Build the water containment system.
Next line the depression with a poly liner. I purchased mine from the local big-box hardware store in the paint department. I wrapped the plastic up and attached it to the lower edge of the wooden box with galvanized roofing nails. In the lowest corner, I did not fully attach the plastic at the top, so that water could run over when full.
Once your liner is installed, prepare a section of corrugated drainage pipe. This pipe is 4 inches in diameter and has small holes throughout the pipe to all water to escape. Place a cap on each end of the pipe. In one end, drill a hole and insert a PVC pipe which will be used to fill with water. No need to bother gluing everything together; the purpose is to let the water seep into the rocks anyway.
Next lay the prepared pipe into the depression. I recommend you size the pipe to run the full length of your bed, but it is not critical to loop the pipe as I have done here. The pipe simply speeds up distribution of the water throughout.
Next fill the depression with rocks. Ideally you will fill to the top of the corrugated pipe