You can build a waterfall on an above-ground pond, although it will be more challenging than working with an in-ground pond. The easiest way is to use a prefabricated waterfall form or a container, though building one with natural rocks can work. The most important part of building a waterfall for an above-ground pond is creating enough sturdy structure to support the water.
One of the biggest concerns when building a waterfall for an above-ground pond is the stability and support of the structure. Since both prefabricated waterfall structures and hand-built rock waterfalls are wider than the edge of an above-ground pond, addition support must be given.
Use the same type of material as the above-ground pond sides to create this stable support structure: rock, brick, cement blocks or wooden timbers. As long as these are fastened together securely, the waterfall will not collapse.
Fasteners should be as unobtrusive as possible. Use silicone sealant to glue rocks together. Black expanding pond foam can not only fill spaces between rocks, but also can be shaped to create water courses. Connect wood timbers with metal brackets and screws.
To deal with concerns about water flowing up from your pond to your waterfall, use a waterfall pump. Purchase a pump larger than you need for the volume of water in your pond. Since above-ground pond waterfalls are built vertically rather than horizontally, more power is needed to suck the water out of the pond and transfer it up to the top of the falls. A large-diameter hose can help create water flow that is natural and attractive.
Position the pump at the top of the waterfall and insert a rigid tube, such as PVC pipe, into the pond water. Cover the opening with a filter to prevent debris from entering and slowing the flow of water. With an above-ground pond waterfall, the base supporting the falls can also easily house a filter and pond pump.
Make your above-ground pond waterfall look natural with additional rocks and potted plants around the edges of the structure. Above-ground ponds tend to look less natural than in-ground ones, and a waterfall stuck on the side of one can make it look even less natural. Create a wide base, hide hoses and hardware, and add plants to help alleviate the concern that the structure will look stuck on and not authentic.