Sue, been there, done it!
The first time I attempted this many years ago, I cut the preformed and added a paint tray as a spillway. The paint tray, I disguised with pebbles attached with silicone seal.
It looked okay, but unfortunately the water worked it's way back up the underside of the spillway, due to capillary action and I ended up with a water loss.

So I figured the answer was, to install the preformed pond, so the liner was completely under it and up all around it, to the same height as the preformed pond's rim.
This is the way you would do it, when building a rock waterfall, you want to encapsulate the whole thing with liner, up it's sides. Then any water that does manage to creep or leak away, goes back into the pond.
"Horton"

So were you able to successfully cut the pre-form just the way you wanted to or did it start to crack and give you an opening that you didn't bargain for? It scares the heck out of me thinking of cutting the preform and possibly ruining it but I really can't think of any other way to incorporate it without severely tipping it, which would look silly and be hard to hide. A paint tray, huh? That was pretty clever!
Sue

Hey there. With the encouragement of Horton and some others here I did the same thing with a 100 gallon round pre form. I tried heating the side with a heat gun to shape the opening and while it worked, I didn't stay satisfied for long and I bought a metal spillway and that has kept me content for now. There is a forum for bogs on garden web that might be helpful. I found it so 3 or 4 years ago but a bog may be more complicated than you think. A vegetative filter might be a better choice.
The spill ways are available online but mine was made by a local metal fabricator. Sometimes you just have to make the first cut while keeping your fingers crossed. ??? Or something like that.
Actually, it is not so much different than making a Skippy.Sandy

I ordered my spillways from Gene at KoiVillage. Glenda

Sue, I did not have any problems cutting the pre-formed shell, I used a jig/sabre saw with a fine toothed blade.
Just take your time doing it and it cuts like butter.

When I fitted the paint tray spillway, I bolted it onto the pre-formed pond with 1/4" nuts and bolts, then slobbered silicone seal all around the edges.

I have never tried it myself, but one of the old members, who used to post to this forum, used Gorilla Glue, when he was sealing/joining plastics. He said it worked great as a glue and sealer.
"Horton"

Wow, thanks everyone - that's the encouragement I needed - a preform veggie filter/waterfall it is! In the past I have become depressed/anxious/apprehensive when thinking about this major pond re-build I need to do as soon as the weather breaks, but now I am actually getting excited and looking forward to it! Of course I have to get the main pond and liner done QUICKLY so that I can get the fish out of their tiny holding cell but after that I can take my time and enjoy being creative. I didn't realize you could purchase "spillways" . . . I'm so used to making things by myself I never even looked but I'm on my way now to the suggested websites to either do some shopping or steal some ideas. Thanks again!
Sue

I got some of the Gorilla Glue and threw it away after the first use. It was like GreatStuff except thinned down so it ran all over the place as it was expanding. It was a mess that could not be cleaned up. If you use it, be sure to practice before you work on the actual project. I ruined a $45 pot. Sandy

One of my "waterfalls" is a 300 gallon stock tank. I used a reciprocating saw to cut the opening for the weir. I screwed in a stainless steel angle iron for the support and had a piece of flat rock cut to fit. I secured it all with black great stuff foam. Here is a pic.
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So what you're saying Sandy is, "Don't monkey about with Gorilla Glue"!!!??? LOL
"Horton"

lsst, thanks for this info from me, too. I want to make a veggie filter from an oval stock tank and I was wondering how to make the spillway. Glad Sue asked. Vanessa

I have two rubbermaid stock tanks as part of my three pond system. The third part is a regular liner pond. One of the stock tanks flows into the other one through a two inch PVC pipe that has a bulkhead fitting in the side of the tank. The top of the hole is a little over an inch below the rim, so I get a constant water level about two inches below the rim. It isn't exactly a waterfall, but it works well and doesn't leak. My waterfall is from the upper pond(liner pond) into the stock tank.

This might not be what you want, but it is another option if you aren't too attached to the idea of a natural looking waterfall.

Magdaloonie,
Stock tanks make great veggie filters.
The angle iron gives a lot of support and a stone yard cut the piece of stone for me for about a dollar a cut. Use stainless steel for the angle iron and nut and bolts.

I made my weir about 8 inches from the top and the stone depth took away about 1.5 inches. It ended up being about 6.5 inches deep.

OK, back to this again. The closer it gets to spring the more panicky I get because I don't have an ideal picture in my head of what I'm going to build in the next few weeks. I think I've decided that I'm going to have the waterfall in the middle of the pond this time (rather than at one end) but I wonder if that is going to cause problems. Obviously the water cascading and falling into the center will go both to the left and right and I'll put the skimmer at one end but what about the water at the other end - will it become stagnant and accumulate a lot of debris since there won't be anything pulling or pushing the water from that corner? I know ideally you should put the waterfall at one end and the skimmer/pump at the other, but I'd like to change the design this time.

Also, you've all convinced me that I do want to use my pre-form holding tank as an "upper pond" and I'll cut it to make it the source of my waterfall, but that means I'm still going to have to use a biofalls type of device to get the clean water INTO that upper pond. My yard is flat so I'm afraid making a three-tier pond is going to look too phoney and HOW do you successfully hide that ugly biofall bucket? I hate to enclose it or surround it totally in rocks because there are times you really like to get in there and clean it out or tip it or reposition it, etc. Anyone understand my concerns? I know it's hard to answer not being able to see my landscape but I don't know if I can nicely have "three tiers" on a flat surface without it looking too artificial, but I need three tiers if I'm going to use the preform as my "upper pond and waterfall", right?

I'm going to drive myself mad plotting and planning... I think I need to take a week off and just get out there and start DOING.
Sue

Thanks for all the great ideas...Did have a question in regards to the silicone and Great Stuff, does anyone have problems with it effecting the fish...Wouldn't want to have a beautiful waterfall and then kill the fish :(