Well there are many different ways to build a waterfall so this is an open-ended question.
I would suggest using a biological waterfall filter box, which makes the starting point of the waterfall a breeze and gives you a filter that is easy to hide in your landscape.
We typically take the soil from the excavation of the pond and create a berm (or mound) to build the waterfall. It is important to compact the soil and then excavate shelves for your waterfall. Slate is commonly used for nice looking waterfall spillways, but large flat boulders or stones can be used effectively too.
Here is a photo of a waterfall that has been sculpted out of the excavated soil. This is approximately a 1500 - 2000 gallon pond.
The important thing to note here: You need to outline your expectations of the waterfall before you start to build!
How wide do you want the waterfall spillways?
How tall will the waterfall be?
Do you want a raging waterfall or something subtle?
Do you want a wishbone (split spillway) waterfall?
Are you going to use slate as the spillway or flat boulders?
Answering these questions before hand can save you plenty of cash, time and headache.
The answer to these questions will determine what size plumbing you will need to use, the size of the biological waterfall filter you will want to purchase, as well as be the deciding factor of what pump you will need to fulfill your expectations, in turn determining what size skimmer you will need to have to house your pump(s).
Remember the size pump you need to meet your expectations may require more electricity than you are willing to spend, so this may change everything.
It's good to get these details outlined before you spend any money!
Here are a couple of important factors you will help calculate what is needed to meet your expectations.
We suggest 1000 to 1500 g.p.h. (gallons per hour) over a slate spillway to give you a fantastic looking waterfall. You can use less gph for a subtle waterfall and may require more if you want raging waters! So with this knowledge you can get a ball park of what size pump you will need.
3 foot spill way needs a 3000 to 4500 gph pump.
4 foot spill way needs a 4000 to 5500 gph pump.
A wish bone waterfall waterfall with a 2 foot spill way and a 3 foot spill way needs a 5000 gph pump to a 7500 gph pump.
1.5 inch plumbing can handle approx. 2700 gph
2 inch plumbing can handle approx. 5000 gph
And then you will need to know what the max flow of the waterfall filter and skimmer.
There are a variety of pond kits available on the market today that have already made these calculations for you so breath easy!
However if you are planning something custom we can gladly help you make the right decision before you get started!
Feel free to post pics of your potential waterfall site location, expectations, and questions here and we will do the best to respond quickly.