Firstly, I would not initially drain the pond. If you have a waterfall, turn it off. Watch for additional water loss. If the water loss stops, the leak is either in the liner at the level of the pond water, in the waterfall or in the piping above the water level of the pond. If the leak does not stop, the leak is likely not in the waterfall area and is in the pond proper or in the pipes beneath the pond or below the water level of the pond. If it is still leaking, keep watching until it stops leaking. That will be the level at which the leak is occurring. Of course, if the leak is in the bottom or in the plumbing that may be under the pond, the pond will empty. Secondly, you will have to make provision for your fish as you continue to watch the pond. Get a liner, make a wooden frame and setup a temporary pond to hold them. Koi show tanks can also be purchased to hold them. Depending on the pond setup and you are able to inspect pipes and tubing, check for frozen and burst pipes. If you have flexible tubing, especially the thin-walled corrugated tubing that is underground, check for pinholes caused by insects, particularly termites that eat through them. Due to the seemingly sudden nature of this major leak, and your residence in Colorado, I'd suspect a frozen pipe. Again, depending on pond design, check the bottom drain integrity and the connections to skimmers and waterfall filters. By working logically, you should be able to identify the area to investigate. If, in the end, the whole pond drains, you may have to tear it all out to find out what's going on underneath. repair and/or replace. Let's hope when you turn off the waterfall that the leak stops. Then it's likely a liner that has slipped and water is flowing over the edge of the liner.
