Adding a waterfall or a stream to a pond

A waterfall or a short stream is a pump lifting water out of the pond and letting it run back down. That sounds simple, and the hardware often is. The disappointments are nearly always the same three: the flow at the top is much weaker than the box suggested, the water runs out over the side of the stream, or the pond level drops for no obvious reason.

Decide what the feature is for

A waterfall is mostly for the sound and the look. Moving water can also help oxygen in a fish pond in hot weather. A wildlife pond does not need one. If you add one there, keep it gentle and make sure the pump intake is guarded so tadpoles and larvae are not drawn in.

Decide the height and length before you buy anything. A low cascade a hand's height above the water and a stream running several metres down a slope are very different jobs for a pump.

Head height

Head is the vertical distance the pump lifts the water: from the pond surface up to the point where the water comes out at the top of the fall or stream. It is not the length of the hose, and it is not the depth the pump sits at. The length of the hose and the bends in it matter as well, because friction in the pipe also costs flow.

The pump size page works through this using a pump maker's own instructions, which add an allowance for hose friction to the measured height. The planner takes a waterfall height and shows the range it uses.

Maximum flow is not outlet flow

The big number on a pump box is the maximum flow. That is the flow with no lift at all. As soon as the pump has to push water up to a waterfall, the flow drops, and at the pump's maximum head it falls to nothing.

What you need is the flow at your head, which comes from the maker's performance curve or table. Look for:

• The flow at the height of your outlet plus the friction allowance, not at zero.
• The maximum head, which should be comfortably above your height, not just above it.
• The hose size the maker recommends. A narrower hose than the pump outlet costs flow.

If a listing gives only a maximum flow and no curve, you cannot size it against a waterfall from that page. Ask the maker for the curve, or choose a pump that publishes one. The pump size page shows this with real maker figures. A pump bought for its maximum flow and then asked to lift water a metre or more often gives a trickle where a sheet was expected.

How much flow looks right over a lip depends on how wide the lip is and what effect you want. This page does not give a litres-per-hour figure per centimetre of width. Weir and spillway makers often publish their own guidance. Use theirs for the product you buy.

One pump or two

If the pond already has a filter pump, the waterfall can sometimes run off the return from the filter, as long as the filter type allows it. The filter size page explains which filters can push water on uphill and which drain back by gravity. A gravity-return box filter cannot feed a waterfall above it.

A separate pump just for the feature is simpler to control and to switch off. It is also another load on the supply.

Liner for a waterfall or stream

A stream needs its own liner, and it needs more than people expect. The sheet has to cover the bed, climb both banks, and continue past the edge, just like the pond liner. Measure the stream the same way as the liner size page measures a pond: length plus twice the depth plus an overlap on each side, for each direction.

• Overlap the stream liner over the pond liner at the bottom, so water running down always lands on the upper sheet and flows onto the lower one. Never lay it so water can run under a join.
• Make the banks of the stream higher than the water will ever run. Stones in the bed raise the water level and push it sideways.
• Build a small header pool or a weir box at the top so the water spreads before it falls, rather than squirting out of a hose end.

Water that escapes over the side of a stream comes from the pond. If the level drops while the waterfall runs and stops dropping when it is off, the feature is the first place to look. The leaking pond guide has the rest of that check.

Size the pond as well

A long stream holds water while the pump runs. When the pump stops, that water drains back. A small pond can drop noticeably while a large stream is running, and overflow when it stops. Check the pond is big enough to feed the feature. See how big should a garden pond be.

Electricity goes to an electrician

A waterfall pump is a mains appliance sitting in water. Any new outdoor supply, socket or circuit for it is a job for a qualified electrician, and in general terms a new circuit outside is the kind of work the Part P rules cover. We do not give wiring instructions. Use an RCD, test it, keep the cable off the mowing line, and read pond pump electrics before you plan the cable route. Plan the route with the electrician while the stream is still being dug, not after the stones are in.

Children

A waterfall is attractive to small children. Splashing water draws them to the edge, and the stones around a stream are uneven and slippery. If young children use the garden, the child safety decisions apply to the stream and the header pool as well as the pond.

Why is my waterfall weaker than the pump box says?

The box figure is maximum flow at zero head. At the top of a waterfall the pump is lifting water, and the hose adds friction, so the real flow is lower. Check the maker's curve at your height. Also clean the strainer.

Does a stream need underlay?

Yes, under the liner, in the same way as the pond. Stones in the stream bed sit on the liner, so the protection underneath matters.

Can I run the waterfall all the time?

That depends on the pump, the fish and the running cost. Enter the pump's wattage from the maker in the planner to see a rough electricity cost. A wildlife pond does not need it running.