For waterfalls and streams that are lined with rubber or PVC

The process of elimination must continue. In which part of the stream is the leak occurring?

1. Run the stream and waterfalls. Check the inlet hose is not leaking backwards

2. Wait until all the sections of the stream and waterfall are full of water and it is ready to flow. If they were well constructed then water should be held to a certain extent all the way down the stream. If there was a liner of underlay put on top of the liner to protect it from the rockwork sitting on it, then the water will siphon down to the bottom and into the pool through the underlay. Let us just hope that it isn't siphoning anywhere else.

3. Leave the stream or waterfall pools to stand and suspect those areas that lose their water entirely.

4. If there is no perceptible or unexplained loss over a few hours then try running the stream and watching for stray rivulets as the water builds up. Some setups take a while for the full force of the water to build up. If there is nothing obvious by way of leaks or splashes then this might need further investigation during a very dry spell.

One place that needs a very careful examination is where the stream liner comes down to the pool level. Is there any chance that water could being forced back up between the liners? It is also here and at other waterfalls that water can travel considerable distances sideways and even upwards behind stonework.

Notice how the liner is supported at the sides well above the level of the water flow by being sandwiched between the rocks. The pond water and plants will obscure this from view.

5. During a dry spell, with minor excavations check for signs of dampness and seepage. If you do find some you can probably trace it back to a place where the water is pushed out between stone and cement and liner. Water forcing down behind stone work on a water fall can sometimes find nowhere else to go apart from up. 'Weep holes' to reduce the pressure should have been inserted at the base of the waterfall.

6. Still no dampness? There may be a hole in the liner underneath the face of a waterfall caused by a knock during construction. These are generally quite small holes, otherwise someone would have noticed the damage during construction and so would not cause undue leakage beyond that expected with evaporation. If there is no visible leak in the liner either when its flowing or not, then suspect animal interference. Lets face it, you have created a perfect environment for many hole burrowing animals and if anything your waterfall intrudes on their potential home. They must think it a small recompense to borrow a little of the rubber you have draped over their hillside to bulk up their nesting material.

CURE :-So now the investigation gets to the serious - 'take it apart and have look !' or 'knock it down and start again !' in stages.

Resort to the "Perfect Pond Recipe Book" or Designing and Creating Water Gardens to put it back together. If there are short falls in the techniques of construction, now is the opportunity to get it right. If there are holes to be discovered, it is the only way you will get to them. Therefore start at the most suspicious areas and work up and then down. Alternatively if there are no clues, start on the easy bits.

BUT try the cure for seepages and pressure first just in case.

SEEPAGES:- In normal circumstances you would think that if you have water flowing down a channel confined by the sides of that channel, it would manage to contain that water. But when you have two layers, one of which the water flows over and also flows under, then you get some new rules. In order to stop water getting pushed up the sides by water coming up from behind with force, you must create holes in the upper level i.e. the stone work, to relieve the pressure. Let it flow.

When you create a stream or waterfall on top of a waterproof membrane, the temptation is to glue everything down with cement and backfill with cement behind all the facing stones. This works excellently to begin with as the cement excludes any water from travelling under the stonework. But as time goes by, the cement relinquishes its grip on the liner and allows a thin smear of water to travel between it and the stone. This is when the problems start.

A stream and waterfall I built in Somerset using very little cement except at the outfalls.

I have found that the most successful waterfalls and streams I have built in the past have used as little cement as possible. If you confine yourself to just cementing in the sill stone, the flanking stones and the top of the facing or mirror stone, you can backfill behind the rest with round smooth pea gravel. This, with the very slow moving water underneath the stone, fills in the gaps and displaces the faster moving water that is then forced to run over the top..

If you build your streams and waterfalls with stones just placed in self- supporting positions, there will never be too much upheaval even if major repairs are required in the future.

Brick faces to 'mirror-style' water falls can have small 'weep holes' provided by copper and plastic tubes placed in the pointing of the brickwork at the bottom of the waterfall. These drain away the water at the face of the liner.

If and when you discover a hole in the liner, then the repair is the easy part (refer to cures in leaking liners). The jigsaw of replacing the stonework is a challenge, but once done, it is a satisfying achievement. A Polaroid or digital photograph can be 'memory jogger' to the actual replacement of the larger stones in their original positions. For those of you fortunate enough to possess a camcorder, you could film yourself dismantling it and run the tape backwards to see how it goes back together!

Undiscoverable leaks in streams can be the bane of your life, and all other pool problems can pale into insignificance in comparison. But intermittent water loss is sometimes more easy to explain.

I once met a longhaired golden Retriever that would drink copious quantities of water from her master's stream when it was running. When it was very hot she would lie in the bottom of the steam letting the flow wash over her as she blocked it up. She was totally unperturbed by the cascade of water she would create, tumbling over the rockery and into and over the rest of the garden. I was called in three times to cure the pond leak before I discovered that.

Large quantities of leaves in the fall can have the same effect, so in the Autumn consider netting the stream as well as the pool, or turn it off.

N.B. ENSURED WATER LEVEL:-

To ensure that if your filter were to clog up or your fountain or waterfall were suddenly to start distributing the liquid contents of the pool over the rest of the garden leaving your plants high and dry and your fish floundering in half an inch of water, then read on:-

The pump that is providing any water supply to any of these items should sit in a highsided bucket at its normal level in the pond e.g. the thickness of a brick off the bottom. In this way, if a potential disaster were to begin to occur then when the water level began to sink below the level of the side of the bucket then the pump would run dry. This may burn out the pump, although many good pumps would only temporarily seize or trip the RCD installed for your outside electrics. (If you haven't got one, get one.)

If the pump does burn out then this is the opportunity to get one with a float switch attached. This will reliably turn off the pump long before the water level gets dangerously low.

Another alternative is to install a mechanical or electronic water level detection device that will turn off the water when the level of the pool drops below a certain point.

Avoid linking something like this to an automatic top up device. These can only be depended upon in clean undisturbed environments e.g. in isolated sumps that are linked to the pool. I have seen the disastrous consequences of having pools linked with electronic and mechanical devices, so beware. There is always a point at some time in the future when such a device will fail and usually in a way that sticks open rather than seizing shut, and sods law dictates that that is at the worst possible time as far as you are concerned.

If you want to overcome any water level problems with any 'top-up' devices then purely mechanical are the most reliable, but make them accessible and easy to maintain. They must have a non-return valve installed before the outlet that prevents any reverse back flow of contaminated water back up the pipe-work - easily obtainable from all builders merchants and easy to plumb in. Also remember, 'topping up' helps to make your water green.