What it is
A small barrier across a drainage line. Rock, log, or brush. Not a reservoir wall.
The job is simple. Slow the water. Sediment drops out behind the dam. A flat, moist terrace builds up. Plants move in.
Halve the speed of water and you cut its erosive power by 75 percent. That single physics fact is the whole reason check dams work. Farmers in Tigray, Rajasthan, the Deccan, and the American Southwest have been stacking stones across gullies for centuries on that principle.
Why it works
A gully is an accelerating channel. Each rain pulse cuts deeper. Each cut steepens the gradient. The system runs away from you.
Drop a 40 cm rock barrier across the throat and the gradient breaks. Water pools, slows, drops its load. A second dam downstream catches what the first missed. By the third or fourth structure, the channel reads as a staircase instead of a chute.
The accumulated sediment is the real prize. That wedge of trapped silt is a planting bed delivered by the storm itself.
Build it
Shape. A shallow crescent, ends higher than the center. Overflow goes over the middle, not around the sides. Water that finds the edge cuts a new gully within one season.
Height. 30 to 60 cm for small gullies. Rarely above one meter even in big channels. You want water flowing over the top during storms, not piling behind it.
Spacing. The top of each downstream dam should sit level with the base of the one above it. Steep gully: dams close together. Gentle slope: further apart.
Materials. Loose rock is the standard. It is permeable, so some flow seeps through and the structure does not fail catastrophically. Logs work in forest: lay them across, pin with stakes driven into the bank. They rot in 5 to 10 years, by which time the sediment and roots hold on their own. Brush bundles work on small gullies and decompose as plants take over.
Tend it
The sediment wedge is a planting opportunity. Wait one or two wet seasons for material to accumulate, then go in with pioneer species that can take wet feet.
In temperate zones, willow and alder cuttings struck directly into the silt will root through the first growing season. In the tropics, vetiver and Leucaena do similar work. Use whatever native colonises disturbed wet ground in your area.
Vetiver grass earns special mention. Roots reach 3 to 4 meters. Planted as a line immediately upstream of a rock dam, it becomes a living filter that catches silt, reinforces the structure, and outlasts the rock. Sterile, non-spreading, tolerant of both flood and drought.
Within five years a well vegetated terrace can read as natural ground. The gully is gone.
When it goes wrong
The dam blows out at the edges. You built it flat or convex instead of crescent. Water found the bank and cut around.
The spacing is too generous. Water regains speed between dams and scours the bed. Add intermediate structures.
You built one big dam instead of ten small ones. A single 1.5 m wall in a steep gully will fail in the first serious storm. Stack the small ones.
At scale
Tigray, Ethiopia. Two decades of check dams plus terracing plus grazing exclusion. Thousands of kilometers of gullies stabilised. Dry springs flowing again. Water tables up.
Rajasthan, India. Tarun Bharat Sangh built thousands of crescent earthen johads. Rivers that had been dry for decades resumed perennial flow. Cost per structure: minimal. Labour: local. Benefits: compounding.
Same rule at every scale. Slow it. Spread it. Sink it.