Hi
My conditions are so different i hesitated to reply but I built an above ground 5x10 foot pool to collect rainwater . Evolved into a water garden/ fish pool. have never used pumps or filtration but do use a pump for irrigation
Your idea is so different from mine as well as conditions
can't really help you . Good luck!!! gary

The age old question! You will get answers that support both filter and non-filter. In the end, it comes out to keeping your habitat balanced. I use a filter because, after all, it's man made & my fish produce a lot of nastiness & eat all my filtering plants.

Like gary, I have a catch pond from a french drain. It essentially has no circulation or much care. It does not stink. I did put a few goldfish in it to control mosquitoes but you could probably use mosquito dunks instead. The issue I wonder about is if your pond would dry out if there are long dry spells.

I have an above-ground swimming pool that fills from the gutters on my garage roof. It has a pump and filter, but I never use it. Lots of goldfish, plenty of plants. I occasionally siphon water from the bottom of the pond to use in my garden.

My concern with your design is that you're putting the pond at the BOTTOM of the slope; that would mean the water is running over and eroding a lot of ground, then dumping dirt in your pond.

If the water is coming from a particular source, like downspouts, try to catch it directly, without letting it run over ground.

If it's just rain falling on the ground, you may be able to create a swale (I think that's what it's called), a wide, shallow ditch that collects water from a large area; it could then direct the water into your pond, maybe via a waterfall over your slope.

I've run ponds with and without pumps. Pumps don't really have anything to do with whether a pond "stinks" or not. I've never actually seen any pond that I would say stank. If you get in there in stir the muck some sure, they all stink, pump or not.

Like steiconi said runoff fed means soil entering the pond.

In addition, a pond in a woodland will collect a lot of leaves. That can create a lot of muck, basically becomes a compost pile. Still wouldn't actually stink at least imo. The entire floor of a woodland is a compost pile.

You'd also have to be very careful about the edge of the liner where the water enters the pond. You have to have extra liner buried deep with stone on top or the water is like to wash under the liner and push the whole liner into the pond.

Floating liner is a serious issue with ponds constructed in low areas. Once the low spot fills with water above the liner top there is no weight holding down the liner. Water is just as happy to go under the liner and the liner will float, with a bubble of air in the middle that's above water. After the rain the water sinks into the ground, the liner falls back into place but with very little water inside.

Thank you so much, people; these comments are all extremely helpful to me; since I have no experience, it's important to understand what kind of issues one has to face and consider.
I could bring water in to top it off to prevent drying out; I often have to do this anyway for irrigation.I guess I'm also thinking that, with plants like waterlilies shading the surface, this would help slow down evaporation anyway.
I do have to study the question of how to handle the soil/gravel issue. I think I would have to devise a filtering system,or some way of canalizing the water...steiconi, this is an excellent point and I'm going to have to study the whole "swale" thing. I don't have much room for one at the top of my slope, true, because that's the road/path,but I may be able to think up something...
I was planning on building a stone and cement pond; I think my conditions are just too rough and wild for using pond liners,so that wouldn't be an issue. The litter thing could turn into a benefit : all that lucious muck for my garden would be welcome! In any case I think that it would require maintenance (but I think all garden ponds do)...any other comments or ideas??? all are welcome, since I'm a total newcomers to this stuff! regards, bart

Lillies, like all plants, use a lot of water. They use it when they "breath" and also in the conversion of sun into food. I'm not 100% sure, it does vary by plant, but I think you may lose more water thru the leaves of the lilies than you would from evaporation if the lilies weren't there.

If this pond was to "save" water and you were having to top it off with a hose I'm sure the net result would be wasting water.

A stone and cement pond is extremely expensive to do correctly. Many people don't think they have to build these with steel reinforcement (rebar) or have to use as much concrete as properly built structure. Yet for some unexplained reason they think their structure will actually hold water. So they go to all the trouble and expense, the pond doesn't hold water, and then post in pond forums asking whether putting some goop over the cracked concrete will solve their problems. Well, they ask the question, but almost always they've already decided that they're going to do that and are just looking for someone who has the same experience level to tell them "Great idea" so they can proceed knowing it will work. It doesn't.

You can read lots of these posts in any pond forum. Just search "crack concrete". Learning from other people's mistake is a great money and time saver.

On the other hand laying a liner first and then putting whatever concrete stones over the the liner will hold water. Because no rebar is needed (or desired) and the concrete/mortar/stone can be very thin you end up saving money. The protective layer on top of the liner can help protect the liner from animals.

While EPDM liner is very good there are cheaper alternatives like used billboards depending on how long you want the structure to last.

Personally I doubt the "saved" water would ever get remotely close to be being worth while from a cash perspective and certainly not from a saving the planet perspective.

Since any project to be used for irrigation would be a big project and because we're talking about the flow of surface water you should consider whether you have the right or not to do this project. I have absolutely no idea where you're planning this project or your plans but you should check with your local Fish&Game, EPA, city and state to make sure your project isn't severely damaging stuff you don't own, like a wetland that's on your property or downhill from your property.

Many people think "their" property means they own everything and can do whatever they like however they like. Seems especially true for people new to these types of projects.

Your neighbors and the rest of us, who most likely own at least some water rights on your property have a different perspective. We have created Fish&Game, the EPA and other agencies to protect our rights and stop people from stealing our resources. Wish we didn't didn't need these agencies, but sure seems like we do. It could save you a lot of hassle later to check first.

Your neighbors also have rights that can be affected when you change drainage.

Some of these agencies can be very helpful too. Fish&Game will probably send someone out to look over the site with you and offer good suggestions, maybe even have money to help you with your project.

Be aware that just because Fish&Game says it's OK doesn't mean it's OK with EPA, or your city, or your state. You are the only person responsible to make sure you have the right to do whatever it is you want to do. Water rights and ownership is very complex in the US because it is such a valuable resource. More valuable than gold or oil because water is forever and needed for life. So people (all of us) take protecting their rights pretty serious.

Wow, waterbug,didn't mean to offend you!
I live in Italy,so there's no Fish and Game, etc. I have no neighbours to my garden; it's a large mainly woodland property surrounded by other woodlands.
I certainly was planning on using rebar and metal to reinforce! Ideally I'd make my pond out of stone, but there are not a whole lot of "hard" rocks on my property; most of them are very friable,so I'll be stuck using at least some cement. I could see lining the resultant basin with plastic or maybe liner, but I'm not sure what is available over here,or the cost.
The problem with my land is/ was that the former owner made a big mess ,bulldozing a very wide path (more like a narrow road, though un-paved) down the hillside,dumping building debris all over the place,so this was what made erosion an issue in the first place.
I don't want to collect the water to save money, I just hate to see all that precious rain water going to waste. What's more, in the present situation, it runs unchecked down into the garden, eroding as it goes,so I have to do something in any case! The idea is to try to make something that will be useful but also aesthetically pleasing, since the area is right in the middle of my garden. It could collect rain water in the rainy season,then during the drought of summer, once it started evaporating too much, I could top it off with water brought in. It would look better than the large plastic water tanks that I presently use! though I won't get rid of them, either; just conceal them behind a hedge or something...
I think I better try to put the info that I'm not in the USA on my user-name logo...

You have confused information with offense.

Like I said: I have absolutely no idea where you're planning this project or your plans but you should check with your local...
I sure don't know Italian law. If you feel you don't have to check with your local government then build away. Lots of people here in the US also don't think they have to either. There's a dude named Andy Johnson here in the US who is in the news lately looking at huge fines. Like $75,000 per day, not that he will actually ever have to pay that, but he will still have to pay a lot just to repair the damage he did. Same type of project as yours. Me personally I don't really care. I enjoy reading about these yahoos. We have a lot of them in the US and it's very entertaining. The news shows love them, great ratings.

You said you wanted to save the water for other use. But if you're actually looking for erosion control there are many cheaper and better methods. Saving water and getting erosion control is more of dam type deal. Not too easy. You don't have any laws against building dams either? Sweet. That's one of the issues old Mr Johnson has saying what he built was a pond when it was actually a dam. I doubt he even knows the difference.

A pond you top off wouldn't really be of much use for erosion control because when the rains came you would be greatly limiting the amount of water the pond could hold. Rain would still overflow and erode. And that's the big issue with dams. Even if you dig a hole it can still be dam. If the soil bank on the down slope side erodes when the pond overflows the whole thing, concrete, liner or whatever, can suddenly flow downhill. It's the main reason we really don't let any yahoo build dams anymore here in the US. Used to kill a lot of people and everyone got a little tired of it.

I really don't understand how you would be saving precious rain water if you have to top off during dry spells. But I don't know the details of how that would work.

Here in Phoenix we have lots of rain water catch basins. The city requires these for many buildings to reduce flooding. They're nothing more than basically a dirt pond, no liner, and then the "pond" is landscaped. Many plants can handle being submerged for a day to two once in a great while.

That allows some water to sink into the ground inside the basin and also collect. So it holds the maximum amount of water for the least digging. In theory the water might even end up in our aquifer which is where we get some of our water. That's true in many locations around the world...rainwater isn't "wasted", it sinks into the ground which purifies it and we can pump it out and use it. So normally the only reason to catch water is to save money or do a term we have here called greenwashing. That's using lots of resources in order to appear to be saving resources. It's a big business here.

If you want to collect water I suggest looking into a deep liner pond filled with rock. When there is no water it looks like part of the landscape. When rain comes you can store a lot of water between the rocks. Or if money is no problem there are several manufacturers of underground catch basins that can be covered with rocks for the same effect only it holds more water. You can also make such systems DIY with plastic drums.

I don't really see how a pond would achieve your goals off the top of my head. Seems like kind of the hard way. But those are the only ideas I could come up with.

OK, anybody else have any thoughts?

Hmmmm... I think you might want to research sodium bentonite. It's a type of natural clay that expands when it's wet. It's used in (among other things) golf course water features.

If you're aiming for something "natural" you could do a stone bed along the path of most erosion and trench it so that you were directing the water along that path. If you end that in a hole filled with sodium bentonite you will have something that will at least hold water. It might be a start to see if the whole idea is feasible.

Thank you,people! Steiconi, double thanks for mentioning swales : though a simple and fundamental concept (in a sense, the rows in a veggie garden are sort of tiny swales, in a way), I never heard the term before, didn't know what they were,and the more i research them, the more enthusiastic i am feeling!!! This concept could resolve not only my erosion problem, but be of major help to my entire garden's soil...the pond will have to wait; I'm going to start planning my first swale...