Build check dams in a swale
Timebox: half a day for a typical yard run. Output: a steep channel stepped into flat pools that water can't scour. The work is simple — the formula is the job. Spacing equals dam height divided by grade; skip the formula and you've built a speed bump in a river.
Checked 2026-09-10 · procedure re-verified against sources below
What you need
- Riprap, 4–12 in, angular — amount from the bill below
- Shovel, rake
- Line level and string — the grade number drives the spacing
- Stakes and spray paint for dam lines
Steps
- Measure the grade. Percent, along the run, in two legs if it changes slope.
- Set the spacing.
spacing = dam height ÷ grade. With 1-ft dams: 4% grade → every 25 ft; 3% → every 33 ft. Mark each line across the channel. - Key the dam. Cut a shallow trench across the channel floor and a few inches into each side slope. A dam set on the surface scours under in one season — see the object page for the failure list.
- Stack the stone. Biggest stones on the downstream face, smaller fill behind. Make the middle of the dam 2–3 inches lower than the ends: the spill point. Water goes over the middle; the ends stay anchored.
- Armor the toe. A pad of smaller stone where the falling water lands. The drop is where the energy is.
- Walk away and schedule the silt. Dams collect sediment by design; that's the maintenance bill. Empty behind each dam before the wet season, every year. If nobody in the household will do it, don't build dams — that sentence is on the check dam page and it belongs here too.
Materials bill, 40-ft run at 4% grade
| Item | Amount |
|---|---|
| Dams (1 ft high, every 25 ft) | 2 (one every 25 ft) |
| Riprap per dam, 2-ft-wide channel | ≈ 0.5 cu yd |
| Total stone | ≈ 1 cu yd |
| Time | 3–4 h including keying |
Yard-scale figures. Multiply by your run length; the formula does not change.
Where people stop
- Dams too tall. Over about 1.5 ft in a yard channel, the dam becomes a wall and fails sideways. More dams, lower — not fewer, higher.
- Spill at the edge. A dam with its low point against a side slope hands the water a head start on eroding the bank.
- Wood without armor. Timber dams rot and undercut. If you use wood, the downstream stone pad is not optional.
Where to go next
- Check dam — the spacing formula and its worked examples.
- Riprap — stone sizing by situation.
- Size a swale — the channel these dams serve.
Sources
- Minnesota Stormwater Manual, check dams — stormwater.pca.state.mn.us
- USDA NRCS — nrcs.usda.gov
- US EPA NPDES, erosion and sediment controls — epa.gov/npdes