An easy to way to track the flow of water on your property

A hole dug in seemingly dry ground filling with water.

It is pouring here in Western Washington, always a good time to talk about water. This week we used the mini-excavator on our tractor to dig holes for five large river birch trees (Betula nigra). Within minutes, they began to fill with subsurface flows of water and a high water table. This section of property is a bit deceiving. Grass grows well. The sod is nice and solid. We can drive a tractor on this section year round without worrying about it ripping up the grass or sinking. However, if I had followed my inclination to put in a line of drainage-loving conifers, I likely would have watched them struggle.

When I bought this property, I knew there was some seasonal flooding in area but I did not have a full understanding of how the water flowed underneath the surface.  From a bird's-eye view, there was clearly a bit of higher ground but there aren't any huge elevation changes. My neighbor's field has a very slight slope my direction but looks relatively flat. The flow of water was not quite as obvious as the water-filled hole would make it seem. Thankfully, the OSU PDC required I learn how to map the flow of water on my site.

Ignoring any more-sophisticated rules related to geology or groundwater hydrology, there is a very simple rule of thumb: water flows downhill at right angles to contour.

For most of us muggles, this is as far as we ever need to get. If you go to your county property records, you should be able to pull up a basic contour map. Grab a screenshot of your property and any relevant neighboring properties, print it out, and grab a pen. Find a contour line with a high number. Find the neighboring contour line with a lower number. Draw an arrow perpendicular to the high contour line pointing towards the low contour line. That is the direction water is going to flow. If you plot arrows around every contour of your property, the flow of water becomes immediately clear.

Below, I've plotted a small section of my property (below) and portions of the neighboring parels above. This makes it more obvious that, not only is water going to flow from the high ground on my property, both of my neighbors are on enough of a slope that water will flow from their properties, too. This results in a small amount of standing water that then flows across the surface of my property during the winter months. It also results in a high water table and subsurface flows much of the year. This is not where one would plant a cactus.

A topographic map showing the direction of water flowing at right angles to contour lines. Blue arrows show the general flow of water. (Black lines are property information redacted.)

After mapping out the flow of water on my site as well as the flow of water onto my site from neighboring properties, I gulped and re-evaluated the entire design plan. Even though the ground was seemingly firm and dry, we did not want to risk flooding during records rains or dealing with soggy, muddy ground around a building, particularly with livestock. We ended up relocating the site of future structures, which are now built and we are happy to have them on as high ground as possible.

This also helped me figure out what not to plant in this area. No fruit trees. No conifers that want good drainage. Nothing that is going to experience wet feet, immediately get a fungus, and collapse in a heap. (Sometime, I will write about the time I put a swale in front of a fruit tree and caused its instant demise.) Instead, I've been planting trees that will love the water. Red alder, willows, dogwood, and now some river birch. They will punch large root systems through the soil, creating air pockets that allow for better water infiltration. During the shoulder seasons, they will suck excess water out of the ground. They will also add much needed height ecological diversity in marginal sections of pasture that would really prefer to be wetland.

Now, if I lived in more of a dryland climate, I'd consider planting trees closer to these water flows but not where their roots could suffocate in flash floods. If I were managing livestock on a larger scale, I could use this to mark out summer or winter pastures, climate depending. Context is everything. On that note, put on your rubber boots, get out there in a big storm, and follow the water in real time. Having a sense of where it "should" flow helped me read a new landscape. Now, having worked with the landscape for a few years, it all feels obvious but it didn't at the start. If I was considering purchase a new property, one of the first things I would do is print out a topo map, track the flows, and then see what the landscape already has to say about it.