do you have a leak in the pump hose? if its buried then you might not see the water spot. Your waterfall might be splashing water out. Stream might have a small leak... Just some suggestions

One of the smaller ponds (the last of 3) did have a small leak so I removed the hose and now have it pouring into the first small pond. The water seems to be flowing out of the hose end very rapidly. Would it still do this with a leak? Would the water really drain that fast from a small hole in the hose? I want to avoid digging up the hose if possible.

No splashing that I am seeing. Plus we are talking about draining a few inches in the pond in a couple of hours not over night. It would have to be some major splashing to drain that fast. I will try having the hose pour straight into the main pond next. That way we can eliminate the splash idea completely.

Thanks for the ideas!

Do you have any leakage when the pump is off? If not, then the leak can be isolated to the stream rather than the pond. Any low spots in the liner of the stream? If there are, lift the liner a little and backfill as needed. Any excessive debris? I have had string algae form in the stream that raised the water level enough to raise the water level of the stream over and out. Any overhanging plants to wick the water out? Check the hose connections. Sometimes the hose clamps rust out and may need replacing depending on what kind of clamp was initially used. Any split in the hose?
Let us know what you find. It takes only a small slit to create havoc.

No, it does not drain when the pump is off. At least it does not go down that fast. I dunno about excessive debris, does not seem like the water is going anywhere except the thin air. I will look closer this afternoon and take some pictures. It honestly seems like the pump is sending the water somewhere else, just crazy that it is draining so fast.

These types of leaks can very very difficult to find. Especially in a smaller pond because it doesn't take much water to go down a couple of inches.

The first thing to consider is that in a multiple pool pond when you first turn on the pump it takes a while for the upper pools to overflow and for water to get back to the lowest pool where the pump is located. So the lowest pool drops when the pump is first turned on. This is called draw down. Larger the pump the greater the draw down. Also the smaller the lowest pool by surface area compared to the upper pools the greater the draw down.

The next thing to consider is that all evaporation, from all the pools, appears only in the lowest pool. This loss can appear to be very dramatic.

However, draw down and evaporation does not explain loosing a few inches in a couple of hours. Some people do sometimes use words that are a little more expressive than actual events indicate. You don't want to talk yourself into searching for a non-existent leak. They're the hardest to find. So double check this wasn't caused by draw down or evaporation. I assume it wasn't.

Please use as precise terms as you can. Size of each pool (surface area) and loss in actual number of inches rather than "a few" and in what time period rather than "couple of hours". That can save you a lot of wild goose chases.

The next source of mystery leaks is if it rained before the the leak was detected. There's a weird thing where liners float, dumping out water and then settle back. I assume this is not true in your case. But worth asking.

That leaves an actual leak.

With the pool sizes and inches of loss the number of gals lost can be computed. That gives you an idea of what you're looking for. A few hundred gals can disappear into soil with little sign. When you get to thousands of gals the problem can appear on the surface, but even then it depends on soil.

By far the most common place for leaks is waterfalls and streams. Next likely is the edge of a pond where the liner has sunk into the soil because a foot step or a rock has pushed it down over time.

How to find the leak depends greatly on how the pond was built. Most of these types of ponds aren't made very well which leads to these types of adventures. Normally just a hole in the dirt, maybe underlayment, liner and then some rocks at least around the edges. With these I start by looking at the edge of the liner completely around the pond. That means moving some rocks. I make one pass fast to see if I get lucky and the problem is obvious. Next pass I move more rocks and repeat until I have the entire edge of the liner exposed. By edge I mean the cut edge. Don't assume a fold is above the water line, you have to see the cut edge. If you can't follow the cut edge around the entire pond you are only wasting your time unless you get lucky.

The spillways from an upper pool to a lower pool are a primary suspect so those liner edges are my first concern. But it depends on the pond and what easiest to eliminate first.

In larger ponds we bypass pools to narrow the down the problem. Bypassing the top pool and leak goes away you know the problem is in the top pool or its spillway. In smaller ponds I normally skip this. Depends on the pond construction, number of rocks to move.

Holes in a liner are much less likely, because of your description, especially if this is a EPDM. It would be helpful to known the liner type. For these you fill the pools to see which one drops and the water line becomes the line where the hole is located. The seal around the skimmer and any bulk heads isn't likely either because of your description.

If I've removed all the rocks at the edges and still find no muddy soil I start the pond back up. Now I can see the water level and liner and can double check check low spots.

If the leak has now disappeared, even though I couldn't find the leak, I know the problem was with the rocks or spillways. I rebuild sides and spillways properly (concrete block around the edges under the liner and a gap between liners on the spillways to prevent capillary action. I replace the rocks around the edge and test.

In many cases it becomes clear pretty fast that the pond was not built very well and there's really not much point wasting time looking for a leak. To me, if someone has paid me to fix a leak I'm not going to move a few rocks to stop a leak that will just be back in a month or two. It's normally easy enough to fix the pond because that's the actual problem and so the owner has a much less chance of future problems. But it depends on the owner. Many people want cheap, cheap, cheap but expect perfect results forever. Those people will never be happy.

The first place to check is around the waterfall.

So, problem area found. It has to be in one of the smaller pools/water fall. I pulled the pump return out and had it going straight into the large pond overnight and did not have any water loss.

Thanks everyone for the tips!

Yup, that's the first place we always check when we see a
large drop in the water level.

Glad you frond it, that's a big relief alrighty!