Attach a pump (to calculate the correct size, see below) to the hose end at the bottom of the pond and set it on a brick. Turn on the pump and adjust the flow, moving the rocks around as necessary. Rather than opting for a single, concentrated flow, you may wish to lay the tubing horizontally across the top of the waterfall, cap the end, then puncture the tube with small holes and slits so the water flows in a wider face. Or, you can fit the end of the tubing into a PVC pipe (capped and drilled at the front with several holes) to distribute the flow.
Use rocks to hide the tubing at the top, and half-bury some in the earth mound. Use plants to further disguise it (see our planting guide).
When you achieve the desired water flow, remove the rocks and reset them in thick mortar or in a bed of spray-on polyurethane foam (such as Dap or Mono) available at most hardware stores. Use the mortar or foam to fill in the spaces between the rocks and prevent the water from running behind, instead of over, them. Poly foam expands to 40 times its volume, is frost-proof (unlike cement or mortar) and adheres well to the pond liner, but it can burn your skin, so wear gloves. To give the foam a more natural look, add pebbles or sand while it's still wet, or paint with non-toxic, waterproof latex once it's dry.
How to calculate the size of your pump
A submersible pump placed in the pond will power a small waterfall. The pump recycles water via flexible, reinforced tubing to the top, where it falls over the rocks to the bottom again. The size of the pump needed depends on the width of the watercourse, the height of the waterfall and the desired rate of flow. As a rule of thumb, a one-inch- (2.5-centimetre-) wide stream of water about 1/2 inch (one centimetre) deep flows over the edge of a waterfall at the rate of 100 gallons per hour.
To calculate the number of gallons per hour your waterfall requires, multiply its width by 100. For example, a 12-inch- (30-centimetre-) wide face needs a pump that moves 1,200 gph. And for a three-foot- (90-centimetre-) high waterfall (the distance the water will fall into your pond) you'll need a pump powerful enough to lift 1,200 gph to that height. For a less forceful flow, a smaller pump can be used. If in doubt, take your waterfall's measurements to a pond specialist for advice.