Dan, It certainly wasn't my intent to criticize and most likely the picture is not giving me a true analysis of the situation, but I also live in Oklahoma and I know the kind of rain events we have. I think you do have a real problem there and I admire your recognition of the situation and your attempt to rectify it. I don't think it would be hard to fix so why let all of that silt build up in you pond if you don't need to. I think the problem has an easy solution and will bring with it another fix. Since you have a rock surface under that good soil, the water has no where to go except down the hill, so it will go through the soil, hit the rock, and flow toward your pond taking soil with it. You can see from you digging that the sediment building up in the area is creating good soil, so let's think about keeping it where it is useful to you and not in the bottom of your pond which is a real problem.
This is what I would do as an experiment and it should only cost a few dollars and a couple of hours of your time. I would search for a place where I could drive a short t-post on each side of the gully that is washing out. Since there is rock there, that might be the most difficult part. I would drive the t-post and attach a piece of welded wire fencing across the erosion path, getting it as low to the ground as possible, but your post need to be opposite each other and allow you to erect a fence which will be perpendicular to the flow of the water. If you have rock that you can put behind the fencing (on the uphill side), that would be ideal, but you could also use logs if that is all you have. They won't last forever, but they will last a few years and give you a chance to see if that fixes your problem, and time to collect some rock for a long term solution. You will not be creating a dam, just a gabion to slow down the water flow and catch that soil. The soil becomes another bonus, as you noted when you dug there. If you can keep it there then you will have a great place to grow something desirable as it builds up over time.
Now let's talk about swales. Creating a swale on contour, simply means that you have found the level of the land and this is where you are going to put your swale. So think like this, if I plugged the end of a garden hose, filled it full of water, and plugged the other end and laid it around the curve of my hill so that when I removed the end caps, the water stayed in the hose, or just slowly trickled out, then I have found the nearest thing to level on my land. If it gushes out one end, then I have a ditch, not a swale. Of course, this is not the way you determine it, just a way to think about it. You want to hold water in this swale along the entire length of the swale. A swale isn't really shaped like a "U". The bottom of the swale is perfectly flat and in Oklahoma will likely be clay, because you have removed the top soil, and it is part of the mound on the downhill side of the swale. The backside of the swale is not a "U" either, because it needs to be angled gently to let the surface water flow downward into the swale. If not a gradual drop, you will just erode the side of the swale and create a ditch in the process. Forget the mound for a minute and just think about the excavated part. You have a flat bottom ditch, probably with a clay bottom, with a gentle slope on the high side that is going to allow the surface water to gentle flow downward just as it always did. The difference is that you are catching it and holding the full length of the swale. Pretend again that the mound isn't there. The idea is to let the water that is caught in the swale, slowly infiltrate into the downside of the hill through the wall of the swale. So the water is now soaking in, below surface level, not just flowing over the surface. It will move more slowly going through the soil than it did running over the surface, so you have held and slowed the water flow. Since gravity is in charge here, it is always going to go down. Now, back to the mound..you have created a mound withe the dirt you removed from the hole you dug, and piled it on the low side of the swale, you have added back the top soil, and left it in a soft mound on the downhill side. Since it has water flowing UNDER it, not through it except for rainfall, it will wick up water and become a perfect place to plant valuable trees that will be well watered.
You now have a little gabion that is going to catch the soil, but allow the water to flow through at a gentler pace. I hope I have explained this so that it is clear, up if not, just ask and I will try again. I think you can protect the pond, and save the soil that is coming onto you property anyway. A bonus.
