There are two basic kinds of garden waterfalls: attached to a pond, and pondless. How do you choose which to build?
If you already have an established water garden, or are excited about a big home-improvement project, garden waterfalls with ponds may be the best option for you. These types of garden waterfalls have a water intake that pumps water out of your preexisting pond and raises it to the top of the waterfall, where an engineered course of rocks arranged to look natural forms a track for the water to flow back down. This kind of waterfall can be attached to your pond filter and circulator in order to unify the system and help keep the fountain pump clean.
Garden waterfalls can benefit your pond by creating controlled water circulation that is gentle enough to leave your water plants happily undisturbed, while adding oxygen to the water and discouraging insect infestation. The disadvantage of adding a waterfall is that you may not always desire the visual effect of rippling water on your pond. If this is the case you can make an enclosed grotto or bay around the outlet of the waterfall, using rocks, gravel, or plant life. This will allow the main part of your pond to remain a still-surfaced reflecting pool.
A pondless waterfall is more appropriate and easier to install if you do not have the space for a full-fledged water garden. This type of waterfall is built similarly to a pond-attached waterfall. The main difference is that the rock formation is set above a hidden, buried reservoir whose surface is usually concealed with a gravel bed. The water appears to vanish underground, but in reality it is simply dripping past the gravel back into the reservoir. These waterfalls are easy to maintain and refill, since the waterfall assembly can be simply lifted off the reservoir for cleaning.
Note that a pondless waterfall doesn’t have to be small; it can be styled as a stream, traveling across most of your garden! The main difference between the types of waterfalls is the reservoir at the bottom. A pondless reservoir can be preferable if your household contains small children or young pets, since they can drown easily in very shallow depths of water. Concealing the reservoir underground is safer for them, since it removes the risk of drowning in a pond.