Saddle Dam

A saddle is the natural dip in a ridge between two higher points, and the ground behind it drains out through it. A saddle dam is a wall closing the gap so the bowl behind fills instead. The dip is chosen because it is the cheapest place to build: a short wall holds a lot of water. Such a pond is usually asked to do four jobs at once - a reserve against fire, drinking water for stock, irrigation in a dry spell, and fish.

Two questions decide whether it works and neither is about the wall. The first is whether enough ground drains into the bowl, which the site does not guarantee: guidance asks for a catchment 10 to 20 times the water surface, narrowing to 5 to 20 times from grassland and widening to 20 to 40 times from woodland, where more rain soaks in first. The second is whether the soil holds water at all: about a fifth clay in the basin and rather more in the bank, compacted around a trench of clay cut down the centre line. A permanent pond averages 1.5 to 1.8 m (5 to 6 ft) deep and 3 to 3.7 m (10 to 12 ft) at the deepest, its banks falling quickly to about 0.9 m (3 ft) so rooted weed cannot take hold.

The four uses compete rather than stack. A store held full against fire cannot also be drawn down through a drought, and every litre taken for stock or crops comes out of the water the fish live in, so the claims are ranked before the outlet and basin are designed. The fire use needs two things people forget: a wide pipe run from below the surface through the bank to where a heavy vehicle can stand, and a road passable in wet weather, because without the road the pipe does nothing. Grass is sown on the bank as soon as it is finished, and stock are fenced off and watered from a trough below it. Burrowing animals are the commonest cause of a leak, discouraged by rock laid below the waterline. A slow loss is normal; a fast one means the bank is eroding from inside.