Swales: what they do, how to build one, and when not to
A swale is a shallow ditch that slows rainwater. Dug level across a slope, it holds water so it soaks in. Built with a gentle fall, it carries water away slowly. Both work only in the right place, and both need a planned overflow. Start by watching your slope in the next storm.
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Before you dig, walk the slope in a real storm and watch where water runs.[6] Then check it against sink it or shed it: a swale is the right tool only where soaking water in is safe.
Steps and parts from a published swale building guide.[4] Not to scale.
One word, two very different ditches
Ask a permaculture designer and a civil engineer what a swale is, and you will get two answers.
- The level swale. A ditch dug across the slope, on contour, with the soil piled downhill as a berm. Water stops, spreads along it and soaks in.[3][4] Its job is infiltration.
- The drainage swale. A shallow planted channel with a gentle fall, usually 1 to 2%, that carries runoff somewhere else while slowing and cleaning it.[2] Sonoma's local stormwater guide says at least 2%.[1] Its job is conveyance.
Same word. Opposite intent. One holds water on your land. The other moves it off, gently. Know which one you are building, and which one a contractor means.
Kinds of swales
| Type | What it does |
|---|---|
| Level swale and berm | Holds runoff along the contour so it soaks in.[3][4] |
| Off-contour swale or keyline | Set slightly off level to move water from wet valleys out toward drier ridges.[7] |
| Grassed channel | A planted ditch that carries and slows runoff. The least pollutant removal of the drainage types.[2] |
| Dry swale or bioswale | A channel over an engineered soil bed that filters runoff, then drains.[2] |
| Wet swale | Reaches groundwater and keeps a shallow permanent pool with wetland plants.[2] |
| Rock-lined swale | Uses rock instead of grass. Needs an overflow in saturated clay or a high water table.[1] |
What a swale does well
- Slows runoff so sediment, metals and bacteria settle out before the water moves on.[2]
- Spreads water across a slope instead of letting it gather into a gully.[3][6]
- Puts water into the soil, where trees and shrubs on the berm can reach it after the rain stops.[3][4]
- Recharges groundwater. Occidental's WATER Institute frames this as keeping water in the landscape instead of the old "pave and pipe" approach.[5]
Why timing matters in Sonoma County
Our rain comes in a short, wet season. From October 2024 to September 2025, 98% of Santa Rosa's rain fell from November to March.[14]
Our calculation from NOAA California-Nevada River Forecast Center data, station STSC1: 38.46 of 39.30 inches.[14]
That cuts both ways. A swale can bank winter water in the soil for a long, dry summer. But it fills when the ground is already soaked, which is exactly when clay soils and slopes are least able to take more.
How much a swale really holds
Our calculation. Method: 50 ft x 2 ft x 1 ft = 100 cubic feet x 7.48 gallons. Runoff = 2,000 sq ft x inches x 0.623 gallons. Assumes all of it reaches the swale, as from a roof or packed ground.
A 50 foot swale with a foot of water holds about 750 gallons. Send it the runoff from 2,000 square feet and it is full after about six tenths of an inch of rain. Our biggest storms drop far more.
So the spillway is not an afterthought. It is half the design. Plan where the overflow goes and use it as a resource, ideally into the next swale or a planted area downhill.[6][4]
When not to dig a swale
- On unstable or steep slopes. Adding water to a slope, from downspouts, irrigation or poor drainage, is a known landslide trigger.[12][1]
- In heavy clay or a high winter water table. The water has nowhere to go. Soak-in features there need an overflow or are a poor fit.[1][13]
- Near the house or septic system. Rain garden guides say at least 10 feet from the house and 25 to 40 feet from a septic system.[1][13] We apply the same caution to swales.
- Where a drainage swale is steep. Fast water in a channel erodes it.[2]
- Where you need a permit. In unincorporated Sonoma County, a drainage permit is required before any drainage improvement, unless exempt.[15] See when you need a county permit to change drainage.
The swale trend, and how to be smart about it
Swales became a signature of permaculture design. They also drew a backlash. The British permaculture writer Patrick Whitefield titled a 2014 post "One of Permaculture's Holy Cows: the Death of the Swale."[16]
Some of the tools have thin evidence. A California farm water group notes that no research has measured how much water keyline design saves, though it adds that the benefits are considered substantial.[7]
Our view: a swale is a tool, not a badge. The test is the site, not the trend.
- Watch the land through a wet season first.[6]
- Start small and high on the slope, where water is slow and easy to manage.[6]
- Ask what the swale is for. If the answer is "everyone has one," pick a simpler tool.
Alternatives worth knowing
| Tool | Best for |
|---|---|
| Fiber rolls (straw wattles) | Bare slopes. Trenched in on contour and staked, they slow sheet flow and catch sediment. Not for concentrated flow.[9] |
| Berms and basins, half-moon catchments | Single trees and small areas. A low mound with a shallow dip uphill.[3][10] |
| One-rock dams | Small gullies. A single layer of rock lets floods slide over instead of pushing against it.[10] |
| Gabions | Channels that need hard armor. Wire baskets of rock that flex with the ground, but the wire can wear out.[11] Larger work may need an engineer and a permit. |
| Rain gardens | Roof water near the house, on soil that drains.[13][1] |
| Living groundcover | Almost everywhere. Soil kept covered with plants absorbs and holds more water.[6] |
Bill Zeedyk and Jan-Willem Jansens, writing about rock structures in gullies, put it simply: "it's the new vegetation that stops erosion, not the structure itself."[10] Our view: the same holds for most of this table.
Hugelkultur and swales
Layers as described by Oklahoma State University Extension.[8] Not to scale.
A hugelkultur is a raised bed built over logs, branches and organic matter that soak up water like a sponge.[8]
- It works best laid along the contour, across the flow of water.[8] Our view: that makes it a natural partner for a swale berm.
- Avoid black walnut, cedar, fresh cuttings that resprout, and treated wood.[8]
- Expect it to sink as the wood breaks down, and to tie up nitrogen at first.[8]
Building a small level swale yourself
- Mark the contour. Use a calibrated A-frame level and pivot it along the slope.[4]
- Save the topsoil. Strip it and set it aside.[4]
- Dig and build the berm. Dig just below the line and pile the soil downhill. Give the ditch a flat, level bottom and gentle sides.[4]
- Cap the ends and check level. High spots make water pool unevenly.[4]
- Cut the spillway. At the same level, armored with stone or grass, leading to a safe place.[4]
- Plant and mulch. Put the topsoil back on the berm, then plant and mulch everything, including the ditch.[4]
Before any of it: call 811 two working days before you dig, and check whether you need a permit.[1][15]
Local help in Sonoma County
- Permaculture Artisans, Sebastopol. An ecological landscape design-build firm that lists rain gardens and swales among its work.[17]
- Occidental Arts and Ecology Center, WATER Institute, Occidental. Watershed education and the home of "slow it, spread it, sink it."[5]
- Daily Acts, Petaluma. Ran a free online rain garden and swale webinar in October 2024.[18]
- Sonoma Resource Conservation District. Publisher of the free local stormwater guide.[1]
These are listings, not paid placements or recommendations. We have not hired them.
What it means for you
Our view: a well-placed swale is one of the best ways to keep winter rain working on your land. A badly placed one floods a foundation or loosens a hill. The difference is observation, not effort.
Next: decide which water to keep and which to send on in sink it or shed it, then see slow it, spread it, sink it for other small steps.
Sources
- 1. Sonoma Resource Conservation District and RCD of Santa Cruz County, Slow It. Spread It. Sink It. Store It! (2nd ed., June 2015)
- 2. US EPA, Stormwater Best Management Practice: Grassed Swales (EPA-832-F-21-031P, Dec 2021)
- 3. University of Arizona Extension, Passive Water Harvesting (AZ1564, 2012)
- 4. The Permagarden Approach, How to Construct a Swale (USAID-supported SCALE guide)
- 5. Occidental Arts and Ecology Center, WATER Institute
- 6. Brad Lancaster, The Eight Rainwater-Harvesting Principles
- 7. California Agricultural Water Stewardship Initiative, Keyline Design
- 8. Oklahoma State University Extension, Creating a Hugelkultur for Gardening with Stormwater Management Benefits (HLA-6478, Feb 2026)
- 9. CASQA Construction BMP Handbook, SE-5 Fiber Rolls (Dec 2019)
- 10. Bill Zeedyk and Jan-Willem Jansens, An Introduction to Erosion Control (2nd ed., 2006), Quivira Coalition
- 11. ASDSO Dam Safety Toolbox, Gabions (citing FEMA P-1015)
- 12. Utah Geological Survey, Guide to Recognizing and Reducing Landslide Damage
- 13. University of Nebraska-Lincoln Extension, Rain Garden Design for Homeowners (G1758, 2007)
- 14. NOAA CNRFC, Monthly Precipitation Summary, Water Year 2025
- 15. Permit Sonoma, Storm Water (Drainage) Permits
- 16. Patrick Whitefield, One of Permaculture's Holy Cows: the Death of the Swale (2014)
- 17. Permaculture Artisans, Sebastopol
- 18. City of Petaluma, Daily Acts: Retain the Rain, Rain Gardens and Swales webinar (Oct 15, 2024)
Checked 2026-10-12. Rules and programs change. If something here is out of date, the source above wins. Spot an error? See our corrections policy.
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