Check dam
A check dam is a small barrier — rock, or wood with a stake — laid across a swale or channel. It steps one long slope into a flight of nearly flat pools, so water drops 8 inches at a dam instead of scouring 8 feet of grade. It is the cheapest way to put a swale on ground that is steeper than it likes.
The spacing formula
Dams are spaced so the toe of one dam sits at the same elevation as the crest of the next one downhill. That geometry gives:
spacing = dam height ÷ channel grade
A 1-ft-high dam on a 4% grade: 1 ÷ 0.04 = 25 ft between dams. On 2% grade the same dam goes 50 ft. If your grade number came from an eyeball, the spacing is fiction — measure it.
Worked example
| Run | Grade | Dam height | Spacing | Dams per 100 ft |
|---|---|---|---|---|
| Side yard | 5% | 1 ft | 20 ft | 5 |
| Back slope | 3% | 0.75 ft | 25 ft | 4 |
| Long shallow run | 1.5% | 0.5 ft | 33 ft | 3 |
Steeper ground wants more dams and lower ones; a 2-ft dam on a 10% slope is a wall that will fail sideways. Keep dams under about 1.5 ft in yard channels.
What a dam does and does not do
- It slows water; it does not store much. The pool behind a 1-ft dam is shallow and small. Dams make a steep swale survivable; they do not turn it into a pond.
- It collects sediment — on purpose. That is the maintenance bill: the pool fills, the dam gets silt behind it, and someone must empty it. Budget for it or don't build dams.
- It needs an armored spill point. Water goes over the middle; the middle washes first. Rock dams are their own armor. Wood dams need a stone pad on the downstream side.
Materials, honestly compared
Rock dams (6–12 in stone) survive freeze-thaw and mowing mistakes. Wood dams (pressure-treated timber staked through) are faster to build and look intentional, but rot at ground contact and walk downhill under frost. If you can carry the stone, carry the stone — sized like riprap.
Where to go next
- Build check dams in a swale — half a day, rock and stakes.
- Grade — the number in the formula.
- Riprap — the armor rules.
Sources
- Minnesota Stormwater Manual, check dam guidance — stormwater.pca.state.mn.us
- USDA NRCS — nrcs.usda.gov
- US EPA NPDES, construction erosion controls — epa.gov/npdes