How to plan a garden stream that suits your space

Start with the lie of the land

Before you dig anything, work out how your garden actually falls. Water only travels downhill, and a stream that has to be pushed up a slope will look wrong and cost you a fortune in pump running costs. Stand at the highest point you'd like the water to start and sight along the garden. You can get a decent reading with a long timber batten, a spirit level and a tape measure: rest the batten on the ground, level it, and measure the gap at the far end. Do this in a few places and note the drop over each run.

A fall of roughly 1:60 to 1:30 — about 15mm to 30mm per metre — gives you a lovely, audible trickle. Flatter than that and the water sits in puddles; much steeper and you're better off designing a series of small waterfalls, each with a 150mm to 300mm drop into a shallow pool, rather than a single tumbling channel. If your garden is genuinely level, you can still have a stream by building up a low mound at the source and using a reservoir below, but be honest with yourself about the extra work.

Test your soil and think about lining

Dig a test hole about 400mm deep and fill it with water. If it drains away within a few hours, you're on sand or gravel and you will need a liner, full stop. If it holds water for days, you're on clay — which sounds helpful, but heavy clay shifts and cracks in dry summers, so a flexible liner is still the sensible choice.

For most garden streams, a 1mm EPDM or 0.75mm–1mm butyl liner laid over a geotextile underlay will give you 25 to 40 years of service. Remember to allow extra liner for folds, overlaps and the banks — add at least 300mm to every dimension, more on sloping ground. If you're building a formal rill, a sealed concrete or brick channel with a render and pond paint finish looks crisp, but it will crack if the ground moves.

Watch the water table in winter. In a wet clay garden, groundwater can push a liner upwards like a balloon. A shallow drainage trench alongside the stream, or a slightly raised design, avoids a miserable spring repair job.

Follow the sun and the shade

Aim for a stream that gets four to six hours of sun a day. Full sun all day in a shallow channel encourages blanketweed and green water; deep shade means fallen leaves, poor plant growth and a stream that always looks a bit gloomy. A route that catches morning sun and afternoon shade is close to ideal.

Sunlight also decides what you can plant. Flowering marginals such as marsh marigold, water forget-me-not and flowering rush need decent light to perform. In shade, stick to ferns, hostas and sedges at the water's edge. Never site a pond directly beneath a deciduous tree — you'll spend every October fishing out leaves, and the rotting vegetation will feed algae all winter.

Plan the route, shape and depth

Lay a hosepipe or rope on the ground and move it around until the curve looks natural. Streams in nature meander, widen at bends and narrow between them. Avoid a straight line from the patio to the fence unless you're deliberately going formal.

• Width: 300mm to 600mm is plenty for a garden stream and much easier to line neatly than a wide channel.
• Depth: 50mm to 100mm of water over the liner, with a slightly deeper 150mm plunge pool at each fall.
• Pond depth: 450mm minimum for wildlife, 600mm or more in one zone so frogs and newts can overwinter below the frost line.
• Edges: build in a pebble beach or gently ramped margin so hedgehogs, frogs and birds can get in and out. Vertical sides are a death trap.

Keep the stream at least 300mm away from house walls and fence posts, and leave a 600mm maintenance strip down one side. Constant damp against timber or brickwork causes real damage over time.

Kit, power and practicalities

You'll need a pump, a reservoir or pond at the bottom to draw from, and a hidden return pipe or hose back up to the source. Size the pump by head height as well as flow: a pump rated at 4,000 litres per hour at zero head may only deliver 1,500 litres per hour at 1.5m of lift. Check the flow curve, not the headline figure. Aim to move the total volume of your system through the pump every one to two hours.

Think about power early. An outdoor RCD-protected socket, or a low-voltage setup, is far cheaper to install before you pave or plant. If you're in a conservation area, a listed building, or you're raising ground levels near a protected tree, check with your local authority before you start — a quiet word now saves a costly argument later.

Finally, think about children and grandchildren. Even 100mm of water is a hazard for a toddler. A rigid metal grille set just below the surface, or a shallow rill with no standing pool, keeps the feature safe without spoiling it.

Wildlife, planting and a maintenance rhythm

Plant in baskets of aquatic compost topped with gravel so fish and wildlife can't root them out. Include two or three oxygenators such as hornwort for water clarity, and avoid invasive species — floating pennywort, parrot's feather and water fern are illegal to plant in the wild and will take over a small pond fast.

Once running, the upkeep is modest if you're consistent. Net or skim fallen leaves weekly through autumn, rinse the pump intake monthly, and top up with rainwater from a butt rather than tap water where you can — tap water adds nutrients that feed algae. Leave the pump running through winter to stop ice sealing the system, and check the water level after any dry spell. Do that, and a well-planned stream will reward you for decades.