Two-level pond with integrated waterfall: design and build

A single pond, even a successful one, remains a horizontal body of water. As soon as you add a second, higher pond that overflows into the first through a water sheet or waterfall, the garden changes scale: you hear the water, see the movement, and the sloping terrain becomes an asset rather than a constraint. A successful two-level pond rests on three things: continuous waterproofing that prevents any leaks where one level meets the other, a perfectly level overflow threshold, and a pump sized for the actual total dynamic head. Everything else - stones, plants, lighting - comes later.

Design before digging: how the hydraulics work

In a two-level pond, the water circulates in a loop. It is drawn from the lower pond, pumped through a pipe up to the upper pond, spreads across it, then overflows over the threshold. As long as the pump is running, the upper pond remains filled right up to the threshold, and a small amount of water is "in transit": in the discharge pipe, in any filter, and in the flowing water sheet.

When the pump stops (power outage, cleaning, breakdown), this water in transit flows back down into the lower pond. The upper pond, however, remains full: its level cannot fall below the threshold. The lower pond's water level therefore rises. When the pump starts again, the reverse happens: the pump must fill the pipe and raise the water sheet again before the water can flow back down, and the lower pond's level temporarily drops.

Design rule: the lower pond must be able to hold, without overflowing, all the water in transit when the pump stops, while keeping enough water above the pump when it restarts. This is why the lower pond is almost always designed significantly larger than the upper pond, and a freeboard of a few centimeters is left between the normal water level and the liner edge.

A simple calculation helps avoid mistakes. Estimate the water in transit: pipe volume (cross-section × length), filter volume if it drains, and the water sheet itself (small). Divide by the surface area of the lower pond: you get the rise in water level when the pump stops. In an 8 m² lower pond with 100 liters in transit, this is barely more than one centimeter. With a 300-liter gravity-fed filter that drains, it rises to several centimeters. It is better to know this before placing the edging stones.

The role of the upper pond: decoration, filtration, or both

The upper pond is not merely a decorative step. Several functions are possible, and the choice determines its depth and layout.

• Settling and plant pond: shallow and densely planted (irises, water mint, rushes, watercress), it serves as a constructed wetland. Water from the lower pond arrives there nutrient-rich and leaves with fewer nutrients. This is the most natural solution.
• Filter outlet pond: a gravity-fed or pressurized filter discharges its water into the upper pond, which then acts as a buffer before the drop.
• Second fish pond: possible, but it then needs to be deep enough, and you need to consider what happens in the event of a failure, since the water in the upper pond will no longer be renewed.

In a fish pond, the most robust configuration is still: pump at the lowest point of the lower pond, discharge to a filter, filter outlet into the planted upper pond, overflow downward. The upper pond then completes the filtration process, and the cascade oxygenates the water along the way.

Continuous waterproofing: where most leaks occur

A single pond rarely leaks if it has been installed correctly. A two-level pond often leaks, and almost always in the same place: where water flows from one level to the other. The reason is simple: this is where water flows continuously, splashes, and the liner edges are most difficult to raise.

One liner whenever possible

The safest solution is to cover both ponds and the waterfall area with a single EPDM sheet. No joint, no weld: water can only escape over the edges. This requires a large liner and some folds to manage at the dividing wall, but it is the most forgiving configuration. Our article on EPDM liner folds will help you handle this step properly.

Two liners: always have the upstream one over the downstream one

If two liners are necessary, the liner for the upper pond must descend over the lower pond's liner, like roof tiles. The overlap must be generous and located below the lower pond's water level, ideally bonded with a suitable joining tape. An overlap facing the wrong way, or one that is too short, will always eventually leak into the dividing wall.

Edges and capillary action

A stone placed on the liner with its base touching the soil acts like a wick: water rises by capillary action and flows behind the liner. Everywhere, and especially around the waterfall, the edge of the liner must rise above the water level, with a barrier (raised liner edge, edging) between the water and the surrounding soil. The choice of material also matters: see which liner to choose for a pond.

Wall penetrations

The pipe leading up to the upper pond, a bottom drain, or a skimmer intake sometimes has to pass through the liner. In that case, use a flanged wall penetration that clamps the liner on both sides. A simple cut and sealant will never hold up over time.

The overflow weir: natural stone, stainless steel blade or preformed waterfall

The weir is where the upper pond "speaks". Its shape determines the sound, the appearance of the waterfall and its sensitivity to adjustments.

Whatever you choose, the weir must be perfectly level. A difference of a few millimetres is enough to send all the water to one side. Place the stone or blade on a bed of mortar over protective felt and the liner, check it with a spirit level in both directions, then test it with water before permanently setting it. Beneath a weir stone, special pond sealing foam forces the water to pass over rather than underneath.

Pump and head: don't rely on the label

The large number printed on a pump's box indicates its maximum flow rate, that is, at zero head and without piping. As soon as it has to raise water by 60 cm through 10 metres of pipe, three bends and a filter, the actual flow rate drops. The concept to use is the total dynamic head: the actual elevation difference between the water surface of the lower pond and the outlet point, plus the pressure losses in the piping and equipment.

1. Measure the elevation difference between the water level of the lower pond and the outlet in the upper pond (or the filter inlet).
2. Estimate the pressure losses: they increase with the length of the pipe and the number of bends, and decrease significantly as the diameter increases. A pressure filter also adds to them.
3. Define the desired flow rate: that of the blade or waterfall (the blade manufacturer provides a range), and that of the filtration system (circulating the pond's volume in one to two hours for a fish pond).
4. Read the pump's flow-head curve and check that, at the calculated operating point, the flow rate remains within the correct range.

For details of the calculation applied to a sheet of water, see our method for choosing a pump for a sheet of water. A common mistake is buying a pump that is “just sufficient” on paper: at the slightest fouling of the filter or strainer, the sheet of water becomes a trickle. Allowing some margin and being able to adjust the flow rate (with a valve or variable-speed pump) is much more convenient.

Filtration for a two-level pond

The waterfall oxygenates the water, but it does not filter anything. Genuine filtration must therefore be integrated into the loop. Three configurations are commonly used.

• Pressurized filter: the submerged pump sends water into the filter, which can be buried or concealed, and the outlet flows into the upper pond. Simple, but the filter's head losses add to the elevation difference.
• Gravity filter above the upper pond: the pump feeds a filter placed slightly higher up, which discharges into the upper pond. This setup requires careful calculation of the water in transit when the system is stopped.
• Plant-based filtration in the upper pond: for a lightly stocked pond, a densely planted upper pond and a mechanical prefilter may be sufficient, provided the fish population is kept modest.

Also consider intake: a skimmer in the lower pond prevents leaves and pollen from accumulating, while an oversized pump strainer reduces clogging. Finally, a waterfall increases evaporation and splash losses, especially in dry, windy conditions. Reference evaporation data, such as that from the FAO, shows the influence of wind and solar radiation: in summer, regular water top-ups are normal.

Construction steps

1. Set out both ponds with a string line, taking the natural slope of the land into account.
2. Dig the lower pond first, then the upper pond, leaving an undisturbed earth wall or building a masonry retaining wall on a foundation between the two. An upper pond placed on fresh fill will eventually settle.
3. Prepare the sill: a stable horizontal area to receive the spillway or stone.
4. Lay the protective felt over all surfaces, including the walls.
5. Lay the liner, in one piece or with the upstream section overlapping the downstream section.
6. Install the wall penetrations and pipework before filling with water.
7. Partially fill with water so the liner settles into place, then install the sill and check it with a level.
8. Test the pump for several hours before trimming excess liner and finishing the edges.
9. Leak test: stop the pump overnight and note the level of each pond separately. A drop in the upper pond alone indicates a leak there; a drop that occurs only when the pump is running indicates the waterfall area.

Common mistakes

• Lower pond too small: it overflows whenever the pump stops, or the pump draws in air when restarting.
• Uneven sill: the spillway narrows to a trickle on one side, and water splashes outside the liner.
• Pond liner overlap facing the wrong way: a slow leak, often invisible, in the intermediate wall.
• Raised pond on fill: settlement after one or two winters, causing the sill to tilt.
• Pump selected based on maximum flow rate: at the actual elevation gain, the waterfall lacks water.
• Cascade left running all winter in a koi pond: the waterfall mixes the layers of water and cools the bottom, where the fish stay. In cold weather, many koi keepers reduce or turn off the waterfall and maintain gentler circulation.

Generic practical examples

Common case: sloping ground and family garden

The ground slopes down 70 cm along the length of the garden. A lower pond of around ten square metres is created for goldfish and water lilies, along with a shallow upper pond of two to three square metres planted with irises and water mint. A 60 cm stainless steel spillway provides the transition. A submersible pump supplies a pressure filter hidden behind the rockery, which discharges into the upper pond. The whole setup remains easy to maintain.

Common case: koi pond with a raised filter pond

The deep lower pond houses the koi. The pump sends the water to a multichamber filter, then to a planted upper pond that completes the purification process before overflowing over a wide sill stone. The pump is selected with a good margin on the curve, and a valve makes it possible to reduce the waterfall in winter.

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For more on the waterfall itself: How to build a pond waterfall?.

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Frequently asked questions

— What size should the upper pond be relative to the lower pond?

Significantly smaller, often between one-fifth and one-third of the lower pond's surface area. The key is that the lower pond can accommodate the water in transit flowing back down when the pump stops.

— Should a single liner be used for both levels?

This is the safest solution. With two liners, the upper pond's liner must overlap the lower pond's liner extensively, below the water level, like roof tiles.

— How do you choose the pump for a two-level pond?

On its flow-head curve, at the point corresponding to the actual elevation difference plus the pressure losses in the pipes and filter, while allowing a margin for fouling.

— Why does the water flow on only one side of the sill?

Because the sill is not perfectly horizontal, or because settling has occurred. It must be leveled again; a few millimeters are enough to unbalance the sheet of water.

In summary

• The lower pond, which is larger, accommodates the water flowing back down when the pump stops.
• Continuous waterproofing, or overlapping the upstream liner over the downstream liner, prevents leaks as water passes from one level to the other.
• The sill, whether made of stone or stainless steel sheet, must be perfectly level and sealed.
• Choose the pump based on the actual head height, with a margin.
• The waterfall oxygenates the water, but does not filter it: plan for a proper filter or a heavily planted pond.

Last updated: October 3, 2026.