Hi there- I'm probably very close to you, as I live in N. Austin.

You didn't mention whether you'd have a pond (as opposed to a pondless w/fall), and if you are doing a pond, whether you'd have fish in it. I will assume that you will have a pond, and that it will have fish.

There will be others who can give you a better answer, but from my standpoint, limestone is actually very unstable, and at minimum you'd have constant runoff into the pond, fouling the water. This could also wreak havoc with your water quality numbers. At worst, the water could seep through the limestone, as you mentioned.

If I were in your position, I'd probably want to do as you suggested, cut into the rock, and lay a liner down, except that I wouldn't put the limestone on top of the liner; IMHO, that's asking for trouble.

Hope this helps; sounds like you have a potentially beautiful project, and I'd love to see pics of it when it gets going.

Limestone is something I stay away from largely when it comes to ponds. Many people will tell you that it will cause extensive problems with pH and other issues. I think it is largely overstated in most cases, especially if the limestone is weathered well, but in this case it could become an issue.

Pretending limestone wasn't involved I would still tell you that you will loose water. Whether it will be huge amounts every day or not, I'm not sure without seeing it. But there will be loss. Even on small concrete or retaining wall features I would put liner behind the rock/block. Does everyone, no, but you will lose water.

With that being said if you are on well water and your well can handle it, an auto fill could be the answer. You're risking making something unstable or debris issues, but it all depends on what type of feature this would be.

I'd talk to someone in your area that has experience with this type of thing. Whether that be a pond specialist, geologist, or some type of ecologist, that is up to you. Do you have a local extension office in your area? Good luck.

Thanks for the replies. this is a friendly forum (my first post here). I am rethinking the waterfall on this cliff. I may instead do a pondless fountain in the bed in front of the cliff, or just leave it alone.

What I didn't say in my original post is that I am also building a 6 ft x 6 ft raised formal pond next my patio, and my original plan was to use a weathered limestone rock as a fountain in the pond. I have a beautiful limestone rock that looks to already be weathered by water, which I can use for that purpose. Now I am wondering if the limestone will adversely affect the pH of that pond. Probably not if I were using native plants and fish, as those species live around limestone, but I plan to use exotics (goldfish, water lillies, etc.).

I guess I will need to consult with someone in the Austin Pond Society.

I think you can use limestone, but a lot of people will say otherwise. Some will tell you not to put a concrete block in the pond, I use them all the time to prop up lilies and lotus. It is largely over emphasized in the pond world.

If the stone is freshly quarried it can be more troublesome, but if it's been exposed to the elements it really shouldn't be an issue. Good Luck.

Some of us live in parts of the country where you can't really get anything but limestone. The advice I was given was to look for denser stone or weathered stone, and try to stay away from the softer stuff.

I have a question for everyone though...
In the example given by the OP, could the top of the limestone be treated with some kind of sealer that could last a couple of years before needing to be reapplied? (i.e.: concrete sealer)

The few people I have met through the APS are nice, but in my opinion the traffic on their site is not enough to generate a lot of responses when you need an answer to an issue.

For me, this forum is the best; it's only a matter of time before someone else comes onto the thread and answers your Q.

Good luck...

Whatever you decide, I'd love to see pictures of the completed project!