Hello Larry, I'll probably be crucified for suggesting it, but with a 45' long piece I'd go with EPDM (rubber) roofing from one of your local roofing wholesalers... just look up building supplies in the phone book, that should get you started. Some on here will say it isn't "approved" for pond use but before there were "approved-pond liners" this is something folks would use. I have it as my liner. Can't remember if it is Carlisle or Firestone's brand. No problems to report though, it's been trouble free. I believe the roll I purchased was a full-size sheet, 25' x 50'. Cost less than $300 US. They delivered because most building supply houses are used to doing so; delivered right to the hole I had excavated using their special forklift/spider-lift. If you're getting something as narrow as 10' (x 45'), that sounds like a deep stream, maybe? Good luck.

Having had the misfortune of having a fair number of packaged concrete mix bags stacked that were rained on rather heavily several times, I can probably add a couple of cautions. ( I was injured and could not get into the yard to take care of the problem.) Stacked bags do not absorb moisture evenly resulting in very poor structure. Some of it results in good sized chunks and some in crumbles that don't stick together. The paper bags deteriorate and rip apart allowing the separate cured contents to slip and slide or just crumble away to cause loss of any original structure. Once a bag has deteriorated, it is impossible to pick up or move the resulting concrete without it falling apart. Mortar doesn't help since air and water pockets are formed that cause uneven expansion and contraction. This destroys any cohesion in the cement. Even if there is EPDM on top of the stack. the bags will absorb moisture from the ground. Once again this will cause uneven curing and a collapsing structure. The top will rapidly (depends on weathering conditions, number of bags stacked, etc) become uneven causing any coping stone placed on top to become uneven. The idea of a stone wall built on the outside of the stack may be feasible (much like ancient rubble filled walls) but any debris will eventually work its way down between the ground and the EPDM without an inner wall as well. Coping stones would need to be placed on the walls for proper support, rather than the bags. By their nature stucco and plaster coatings are thin and very fragile. Moist conditions make them more fragile and pretty much temporary. Casting or pouring cement is a much better use of the material.

I hate to post a photo because the pond doesn't have its liner yet. It is looking pretty rough right now because we have been inundated with rain for the past week or so and have not been able to do squat with the pond. When we stacked the cement bags, we did not remove the bags because until it rained on the bags the cement inside was still loose. So, now with all this rain the bags are disintegrating and they look awful. At least we now can see the cement underneath and, yes, I think it will look like gray fieldstone once the wrappings are off. But right now? Sort of hideous, in my opinion. But I PROMISE to post photos when -- or IF -- we can ever get back out there and do some more work, hideous or not. You will at least get an idea of what it is going to look like when finished. Gary -- I think my stones will have rounded edges. We plan to go shopping for them next week as our pool 'advisor' has told us of a great place to find them. I sure hope it gets hot again because the EPDM is going to be pretty stiff as we try to position it into the bottom of the pond. So, naturally, I'm hoping for a short heat wave!

Having had a natural clay pond for over three years now, I must beg to differ with some of the above responses. It is very natural. Yes, the water level goes up and down but the critters move right along with it and it never ever dries completely up partly because I can top it off the same way each of you can top off yours when they lose water by evaporation or other means. I have no liner cheapo or otherwise, pinned in or otherwise (it would only collapse eventually anyway because of the shifting clay). The pond is clear and I can see all the way to the bottom on any given day. There are snails, frogs, fish along with other abundant wildlife that visit the pond. I don't think anyone should be discouraged from digging in clay and I think calling them victims is very unnecessary. Liners haven't been around forever but ponds sure have. If any "victims" have any questions about natural clay ponds, feel free to email me privately. It can be done and done successfully and beautifully.