Natural Clay Content for Water Retention & Erosion/Sediment Control Measures

Good day all!

I recently purchased 36 acres in Northern Arizona, Zone 7B, Juniper Pinon Woodlands. The area has free range cattle and as such the ranchers from a century ago created many water "tanks" or I would call them ponds, some in low lying areas where the water pools naturally and others man-made like swale pond/dams. There are also several other properties around mine that have small ponds in low spots as well (all appear to be natural as in no extra materials needed besides compaction of native soil). The soil is fairly redish colored, and a USDA soil survey online indicates a fair amount of clay in the soil. On my next trip I hope to take several samples and get a more accurate soil profile and idea of clay content in the soil.

My question is does anyone happen to know or can point me to a reference that indicates how much clay in the soil is needed to retain water? I am aware that conditions vary greatly and only an accurate test of my area will give me a definitive answer, but I am attempting to do some design and planning and am looking for a rough idea on this.

Also my property is on a primarily West facing slope at times fairly steep and others fairly gently. Part of the slope is covered in pioneer grass that seem to help with erosion control and the rest is bare soil. I have made 1 test swale (small), and after 1 significant rain event (I havent been there long enough to tell but it seemed to be a extra strong event above normal based on pervious erosion I observed) it completely filled with sediment. Regarding swales and ponds, is there a way to reduce this significant erosion and filling with sediment? I know adding more plants for erosion control will help but until I can start retaining water it is in short supply to establish those plants. Would adding rocks in the form of rip-rap as seen in typical erosion control reduce sediment into swales and ponds if placed directly in the channel that will flow into these features? Or perhaps focusing the erosion control plants on this area or a mixture of both?

I appreciate all knowledge and help on these matters! Thank you very much!

Daniel, isn't your red clay adobe? That's the stuff they make buildings out of, with a little help from the sun and some strengtheners. So adobe in Northern Arizona ought to be pretty easy to research.

What I would be more concerned about is how deep is the adobe on the hillside. If you hold water back and the hillside saturates, and the adobe is not very deep, it might slide. If you have any natural erosion areas where you can see the soil stratification, it might show how deep the red soil is before you get to either bedrock or other colored soils. Mine has about 8 feet of dark clay before it gets to pale chipped bedrock, so far it has not slid, and a post and barbed wire fence that is running down the length of it has not started to tip.

What I've done on my clay hillside is make trenches that split the runoff in two directions, going sideways but at a slight angle downwards, so the water is slowed, but keeps running away from straight downhill. These trenches have pretty much stayed in place and just need the occasional touching up before the rainy season.

As far as sediment in a pond, you might go on Google Earth and look at your neighbors' ponds, see if they have done anything to slow sediment. when I have put sticks and brush to try to slow it down, the water just cuts underneath them and causes erosion on my steep hillside, so that hasn't worked for me. Ponds have a maintenance factor, I guess you just have to decide if they are worth it to you.

How deep are your ponds? Do they dry up in the summer? Do you sink too much in the wet mud to be able to bulldoze out some sediment?

Excavate a test hole, obtain your soil sample, place it in a clear mason jar and shake it up to complete emulsion. Let it sit. The first layer to settle at the bottom will be sand. The next layer will be silts and loams, the third layer up will be the clay, and some slight organic matter may form an additional thin layer on the top of the clay. Your looking for a target of 30% clay content or higher to retain water. 30% is the minimum, after that you consider options of sourcing bentonite clay to add or bringing in clay which can get expensive. To address turbidity in the water as far as a swale that leads to a pond, dig a small hole/"pond" in the swale just before it reaches the pond. All of the silts and sands will settle out of the water there first before the water reaches the pond. You may fill that hole with rocks and grow reeds to further act as a biological filter. You may do the same thing if you digging a pond in a valley. place one or two small ponds/holes and fill with rock and gravel and reeds. Everything flowing down the valley will settle there first and reduce turbidity in your main pond. These small filter ponds do not need to be compacted high quality material.

The 30% clay content is specified by Geoff Lawton in his earthworks class, so I cannot give you actual reference documentation and your going to just have to take my word

Excellent! Thank you both very much for your insight and advice!

Cristo, you make an excellent point regarding hill slides with water features, something I had not taken into account. Based on my online data, it shows the clay from 2" down to 28" before hitting bedrock, I still need to dig it down to see for myself and take a sample for % of clay. This region of clay is also much lower down the hill where the slope is far less pronounced. I really like your trench idea for slowing down the water, and it is encouraging they have stayed in place! How much of an angle do you have your trenches at for reference and how steep is your slope? I am starting to see there will be some maintence with these ponds which is fine just something to consider in the entire design. Also it was interesting to note debris only caused the water to cut deeper.

Eric, thank you for the 30% figure! That was exactly the type of figure I was hoping to find so when I take my soil sample I will have a decent idea if Im close to retaining or need additional help. And I think your suggestion of a filter pond/hole is an excellent idea that by happen-stance we had already implented into the first pond. My friend designed it in to slow the water so it doesnt place a heavy load on the pond walls and cause excess erosion during heavy flow events, but with your suggestion it can also serve another purpose to collect the sand and silt and make maintence easier as I will only have 1 specific area to remove sediment from.

Daniel, I would say the trenches are not straight across the hill, but a little more downhill than that, but not as much as 45 degrees from a straight line down the hill. If a basic drawing of a feather had 45 degree lines coming off a straight line, that's what I mean by 45 degrees, and that would allow the water to move too fast. I don't want the water to race, just move at a reasonable rate, not sit and saturate.

My hillside is steep. I drew a picture of it at this thread, as I am working on an erosion spot on the edge of a flat plateau. It's a 500-foot fall over a 3,000 foot run (it's not a mile, that was an error on my drawing)

https://permies.com/t/49438/earthworks/Pond-swales-erosion#399559

And for ease of maintenance, digging a pond deeper, rather than trying to build a dam to hold back water, makes for a much more stable situation when it overflows, especially if you have gophers, voles, any digging rodents that will get into a dam. I have two family members who are constantly struggling with the dams that hold back their ponds, bad storm rains are constantly eroding around and through them. I didn't build my pond, but I thank heaven every day that the previous owner just dug deeper. When water gets out of control it's too late to go out there and try to do something in a bad storm, with pounds of clay on each boot and clinging to the shovel with every shovelful. There's plenty of things to do already on a rural piece of property without adding that to it.

I try to get my place so that when I hear heavy rain on the roof I am not up all night thinking, oh, *&%$, what's happening to the driveway? What's happening to the pond? What's happening to the hillside?