By Navona Gallegos

When it comes to soil care and water, the go-to saying is: slow it, sink it, spread it.

Swales are one of those classic techniques that are taught in every permaculture design course and have been used for redirecting water flow on a slope since long before the word “permaculture” was coined. My internet search, albeit brief, on the history of swales yielded nothing about their first documented use, but it’s safe to say they have been around for a long time and have certainly been in widespread use since Bill Mollison and David Holgrem introduced the term “permaculture” in the 1970’s.

There are many excellent tutorials online about how to build swales. This post will cover the fundamentals and also how I tweaked the design to fit my particular context in Northern New Mexico. I’ll also share about my experience hosting a field day in order to get swales dug on my farm. 

Introduction to Swales

The reason swales are so ubiquitous in land care is that, when applied in the appropriate situation, swales are a low tech tool that can transform a pattern of erosion in an area to a pattern of water retention and soil building.

Basically, a swale is a long trench and mound that are dug on (or nearly on) contour–that is to say, perpendicular to a slope. When water flows unimpeded straight down a slope, it picks up speed, collects debris in its path, and can carve channels that create a negative feedback loop of faster flow and more erosion. Swales mitigate that process by slowing water flow, which then gives it time to infiltrate. With increased infiltration, there’s increased plant growth, adding further to the stabilization with roots.

The trench collects water as it flows downhill and the mound redirects the water so that it spreads along the contour line instead of flowing straight downhill. Often, swales are built with a gentle 3-5% downward slope so that the swale doesn’t fill up with water and overtops; instead, the water moves slowly downhill, sometimes into a pond, sometimes off the edge into another swale. The berm creates little microclimates with more and less shade, allowing diverse planting to occur.

The image below is just one of many that can be found on the internet demonstrating how a swale can work:

As with any tool, swales are appropriate in some situations and not in others. Generally, swales may be good for 5%-20% slopes, but this really depends on the soil type, precipitation patterns, and many other factors. There’s no reason to use them on very flat land; indeed, digging trenches on flat, arid land can lower the water table and be detrimental. Similarly, there is no hard, fast rule for how many swales or how big they should be. Potential pitfalls include building a small number of big swales on steep slopes where a lot of rainfall occurs. This can cause disaster if the swales burst. In general, it’s good with any sort of rainwater harvesting earthwork to have lots of smaller structures, rather than one big one that can have a catastrophic failure. It’s also good to start slowing water as far uphill as possible so that it doesn’t pick up lots of speed and sediment by the time it gets to structures. 

Springtime Field Day

I first learned about swales in a truly life-changing permaculture design course with Starhawk and Charles Williams in 2017. Since then, I’ve used swales in lots of projects around New Mexico. But sometimes my own place is the last on my priority list. When the NM Healthy Soil Working Group offered to organize a Field Day, it was the catalyst I needed. Much to my delight, about fifteen people showed up to learn about swales and to lend a hand. The Working Group provided a delicious lunch cooked by local culinary rockstars, Nixta, who served us tacos made with all local ingredients: anasazi beans, buffalo, chile, quinoa grown on the Rio Grande, home-made blue corn tortillas, and local greens. 

The food was out-of-this-world, but for me the best part of the day was getting to meet new friends. I appreciate farmer-to-farmer education because there is always the extra benefit of inspiration and support that arises from just getting together and sharing knowledge, questions, and laughs. In this case, some of us had been neighbors for years without ever crossing paths. Now I’m exchanging tips with these new friends on how to deal with invasive plants or how to encourage beavers to return to the land. Some folks came from as far as Flagstaff and Santa Rosa, and it was a boon also to get a chance to cross-pollinate with them. And of course, as the host, I was immensely grateful for the help. Many hands make light work and, indeed, we accomplished about 80 hours worth of digging, tamping, and wheelbarrowing in just one day, all while chatting and learning together.

My farm is in Ojo Caliente, NM, an area where dry pinion and juniper desert with wide swaths of bare sandy soil abruptly abuts verdant riparian bosque with some small strips of acequia irrigated lands scattered in between. Overlooking all of this is a big mesa locally referred to as Mesa Prieta or Shadow Mountain. As that mesa erodes over time, it deposits lots of sand in the valley. The land by the river is more loam from flooding events, but where my house sits is a very sandy hill with weeds, some grass, and mostly bare dirt. The hill slopes down with a grade between 10% and 30% in various places. I’ve long thought that swales would be a good way to encourage plant cover and help keep the hillside from eroding out from under me. 

If you’ve ever made sand castles on the beach, you might have a sense of the sand on this hill. When I experimented before the Field Day with the swale design, the berms kept disintegrating like a dried sand castle. I found that they did stay when I gave them some structure by creating a backbone inside the mound with logs and/or sticks and then compacting the dirt in and on that wood in the same way one makes a Hügelkultur mound. And like a Hügelkultur bed, the wood inside the mounded part of the swale also serves to stabilize the soil by adding organic matter which feeds soil organisms that then create organic glues, giving the soil structure and increasing water infiltration while also feeding plants. To boost this process of soil building, I decided to inoculate inside the berms with a compost slurry (see the blog post: All About Inoculants).

How to build Swales

To prepare for the Field Day, a friend and I gathered woody biomass from all over the property. This included limbs from fruit trees pruned during the winter, untreated scrap lumber, and we harvested some of the abundant red willows from the bosque. I also brought a six yard load of wood chip mulch from the Santa Fe dump. We cut the smaller sticks so they had one pointed end and could serve as stakes to mark out the contour lines for the swales. They doubly served as upright posts to help hold the wood in place on the hill. All of this took one day, including the trip to and from Santa Fe.

On the day of the event, when it came time to build the swales, the first step was to find the contour lines. In order to do this, we broke into groups of three and made a really neat tool called an A-frame for each group. What I love about A-frames is that, while you can buy them with bubble levels inside, it’s easy to make them in about ten minutes out of scrap materials. All you need is three sticks, a rock, and some rope. 

1. Making an A-frame to determine contour lines

First, you make an “A” shape out of the three sticks and secure it tightly with rope; it’s important that the frame doesn’t wriggle. Then, use more rope to hang a rock from the top of the “A” so that it dangles below the crossbar of the “A.” To calibrate it so it shows you what is level, you put the legs of the “A” on a slope and mark where the rope lands on the crossbar. Then, just turn it 180 degrees so that the left leg is where the right one was and vice versa and mark where the rope lands in that position. From there, the level point is halfway between the two marks. It’s quite simple to do but one of those things that’s a lot easier to understand if you see it done. Here’s a video that shows you the process.

2. Marking contour lines

Once our A-frames were built, we went along the hillside and marked out the contour lines for four swales using our stakes. We placed the swales about twelve feet apart. 

3. Creating berms and trenches along contour

We lined the contour lines with woody biomass. After that it was time to actually dig the swales. We dug the trenches about 2’ wide and shoveled the excavated dirt over the piled up biomass to create a berm roughly 2’ wide and 2’ tall. One of the participants found that layering wood chips inside the mound helped hold the sand even better.

Since it’s important that the compost slurry not be exposed to the sun when inoculating, we added it in splashes as we went, always making sure someone was there to toss dirt on top as soon as it hit the wood. We got into a nice workflow with some people digging, some tamping and stomping the sand, a couple of us spreading the compost slurry, and some adding mulch as we went.

Finally, we topped off the swales with a layer of wood chips in keeping with the soil health principle of always having soil covered.

In this way, we were able to do two of the four swales we layed out. For erosion purposes, it would have been good to start with the swales on the top of the hill so that they could catch water flowing earlier before it built up speed going down the hill. Having no idea how long the process would take, I opted for starting on the bottom of the hill so that we weren’t having to carry wheelbarrows full of mulch from the top of the hill over already dug swales. So, the two swales we built were the lower two.

Fall Progress Report

The good news is that the swales have held up wonderfully. Indeed, I was surprised by how well intact they are five months later after one of the first true monsoon seasons we’ve had in many years. I attribute part of this to the sandy soil, which helps the rain infiltrate when it lands, but I also think that the combination of logs and branches as well as wood chips inside the swales helped them hold better. That’s a double win for the soil organisms and water retention. 

With the big rains this year, the farm is the greenest I’ve ever seen it, including on the hill where we dug the swales. A lot of the vegetation includes weedy plants such as kochia and, much to my chagrin, three foot tall, happy tumbleweeds. This comes as no surprise as these plants are showing up specifically because that area was bare, highly disturbed, and lacking in organic matter and soil biology. In this case, I’m just happy for any vegetation to start the soil building process, nevermind how prickly it is. 

But the really good news is that the hillside where we built the swales also grew a whole lot of native bunch grasses. What is more, the bunch grasses near the swale trenches are the tallest on the farm, some reaching four feet. I was surprised that the native bunch grasses by the swales were happier than those in flat areas of the farm. The grass seeds are mature and abundant this autumn. I expect we can look forward to more native grasses and less tumbleweeds in the future as over the winter months the swales will keep slowing and sinking precipitation and the wood in the swale mounds will keep decomposing and feeding the soil organisms.

Navona Gallegos is a soil ecologist and lover of life. Her passion for the wholeness of nature led her to study terrestrial ecology and specialize in soil biology. Navona’s educational background includes a BA in Environmental Science from the University of Virginia, Starhawk’s Earth Activist Training Permaculture Design Course, and Dr. Elaine Ingham’s Soil Food Web School as well as lived experience as a farmer and informal learning from a wide variety of land stewards from across the United States, Caribbean, and Central Africa.