A water rill is a formal, narrow stream, with crisp, clean edges. It may be sunk into the ground or may form a raised pool, and can be used to introduce a contemporary feel to a garden. It can also be used to evoke the feeling of the Moorish style, as seen in the gardens of the Alhambra, Spain.

The sharp lines of a rill can be created by using a concrete base lined with a butyl liner. The canal may form an isolated element of still water, or it can be set along a slight gradient so that it flows in to feed a larger pool or water feature.

To build a sunken water rill, use the following method:

Digging the Channel

Mark out the area of your rill; a finished width of between 30-60 cm (12-24 in) and depth of 30-45 cm (12-18 in) should be sufficient. Make sure you take into account the width of the concrete blocks for the sides and the depth of the concrete base. Use a plank of wood and a spirit level to check that the top of the rill is level on either side.

Creating a Gradient (Optional)

If your rill is to feed another water feature, create a gradient along its length using levelling pegs. A gradient of approximately 1 in 70 to 80 (2.5 cm/2 m or 1 in/6 ft) should work well.

Take a number of levelling pegs so that you can space them in pairs every 2 m (6 ft) down the length of the rill. Make a mark on the first pair of pegs 2.5 cm (1 in) down from the top and drive them into the base of the rill up to the mark. Mark the second pair 5 cm (2 in) down from the top, and each subsequent pair a further 2.5 cm (1 in) down each time. Drive in the pairs of pegs every 2 m (6 ft) so that the tops are level with the first pair; use a spirit level to check. When you have finished positioning all the pegs, rake and remove the soil so that it is level with the mark on each peg. This will create a gentle gradient for the water to flow along. Remove the pegs when you have finished.

Forming the Base

Tamp the soil down along the base of the channel and pour in a 10-15 cm (4-6 in) layer of concrete. When the concrete has hardened slightly, roughen the surface in a strip the width of the concrete blocks around the perimeter of the base. This will provide a key when you add the concrete blocks for the sides.

Cover the top of the rill with a protective plastic sheet if rain is forecast and leave the concrete to set.

Building the Sides

Once the concrete has set, line the sides of the channel using concrete blocks laid in a stretcher bond. Build the sides up so that the top of the blocks will be just above the level of the water when complete.

When the mortar has set, line the channel with a polyester matting or other cushioning material. Alternatively, spread a 2.5 cm (1 cm) layer of sand and rake it level.

Fit the liner into the channel, smoothing out creases and folds and secure it along the edges temporarily with large stones. Any overlaps in the liner should be joined with a seam of at least 15 cm (6 in). If your canal will flow into another feature, make sure that you arrange the overlap so that water flows away from the join, not into it.

You may want to consider adding a layer of overlapping slates or smooth, dark coloured paddlestones (flat pebbles) along the base of the rill to create a decorative effect.

Edging the Rill

Fill the rill with water and then trim the liner, leaving about 15 - 20 cm (6-8 in) to lie underneath the edging stones of the rill. Install the edging of your rill, setting the stones or slabs to overhang the top by 2.5-5 cm, (1-2 in). This will create an interesting line of shadow and will hide the top of the liner.