Check Dam

A gully is a channel cut into the ground by running water, and it deepens from the top down: water drops over a small step, scours the ground below, and the step works upstream. A check dam is a low barrier across that channel - rock, logs, wire baskets of stone, whatever is to hand - that holds the bed at one level and stops the cutting. One alone does little: they are built as a series down the gully. Sediment settles behind each until the bed fills to the top of the dam, and that filling is the point, not a problem: it rebuilds the floor of the gully. Planting into the fill once it holds is what finally stops the erosion; the dams buy time for the plants to take.

Spacing follows the gradient the finished channel should have, no steeper than about 3 per cent, so each dam's wedge of sediment reaches the foot of the next. Three rules keep one standing. The foundation must be longer than the spillway, since water falling over the crest scours the bed below and works back under. The whole length must be tamped into firm contact with the ground, because a gap concentrates flow and cuts faster than the bare gully. And it must tie into banks strong enough to hold, or water goes round the end.

What decides everything sits above the gully, not in it. If the catchment keeps shedding water at the rate that cut the channel, the dams treat a symptom and will be undermined in turn, so keeping that ground vegetated is part of the work. Coarse material settles and fine passes: particles below about 0.05 mm stay in suspension, so the series rebuilds the bed without clearing the water. Inspect after every heavy storm through the first two rainy seasons and yearly after, since a small breach left alone becomes the whole structure. One distinction matters: a check dam set in a working channel to catch silt is dug out at half its height; one in a gully is left to fill and then planted.