If you don't want to have the risk of gullies forming from rain runoff then yes you should have structures to slow that water down and move it in a manner that won't allow erosion to get a foot hold.
The other part of this is that by slowing the water down it has the opportunity to soak into the soil where plants can make use of it all down the slope (gardens of vegetables, berry bushes, fruittrees, etc.) instead of just around the lake front.
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stephanie howe wrote:my site has a 10-15 degree slope down to a lake. since runoff water will just end up in the lake anyway, is there any reason for me to construct swales on my site? Thank you
Bryant answered that question
do i need swales?
The title question needs more observation. When you have a heavy rain event does it always soak into the ground before ruining along the surface?
For example I have such a slope which is almost pure sand, even when the soil is bare I can not get the water to run on the surface, It soaks down to the clay layer that makes op the rest of my field. There it come out in a natural swale that forms a big Z with some shallow ponds that hold the water until it soaks into the clay.
It would be wasted effort to put swales in the sand. I am carefully restoring the natural swale, filling in the ditches that were an attempt to drain the standing water so they could plow. The natural swale has a slope of less than 5 degrees across 2 acres.
I'm not sure you asked the right question. There is a difference between "need" and "any reason for". If your slope has had the same ecology for years, and it has not eroded, no need for swales. If you maintain a good plant cover on it and it doesn't erode, no "need". If you like the look of swales, then there is a reason. If you plow it, swales probably might be needed. If you are trying to capture water on the slope, then there can be a reason and a need. For any of "us" to answer your question properly, we'd probably have reason for and need of more information. --All the while keeping in mind that major earth moving is very upsetting to the soil, the biology of the soil and the bacteria in the soil. (Not to mention the Fairies often don't really like it either.)
Here in my part of Ohio, the plant growth is so luxurious, the soil so rich, the climate so regular, and the land so nicely rolling, that if our farming practices are correct we don't need swales. Just sometimes contour planting. In other parts of our great and wonderful country it gets very dry, then very wet, the land can be steep enough, the soil poor enough, etc., so that sometimes earth moving is needed.
One of the basic principles of permaculture is observing, relating to, being mindful of, acting appropriately towards, ..all the circumstances of a situation. To just base a decision on just one aspect of a circumstance may be to miss all kinds of reasons and needs to do something else. To just act for one reason or need out of context to all the other needs and reasons is, for the most part, industrial farming. To consider everything is more permie.
~~~P.S. All this is sometimes my problem with some permaculture advocates. Some of them give advice on what they have been taught, or what they have done, according to what works in a particular ecology. But there are rather large differences in land, weather, soil, moisture, so that the practices of one place simply do not apply to all places. We had a fellow here one time that had learned perma. for out west. So he built lots of water catchments all over the garden. We have so much prefect rain here that we don't need to catch water. So we ended up with lots of little mosquito "ponds". It took me two years to straighten out his "perma mess" after he left. ~~So my advice to folks is to consider everything, choose what works best for your situation, be happy.
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As we age and the climate change(man-made or million yr cycle), a swale might make a big difference in just 10yrs.
The stream prefers sub surface water flow over surface run off so help the river, esp if you might get animals/manure in the future.
The first thing I would say is that there is more than one water management technique to help water soak into the soil. To determine which techniques to use from the menu of techniques, several factors need to be determined. Here are just a few questions I'd ask myself to start (more questions should be asked):
1 - What is my goal? What am I trying to do with the property, especially the land between the top of the hill and the lake?
2 - What are my desires for the lake if it is on my property? What are the water rights laws in my state/county?
3 - What is the soil make up of the land between the top of the hill and the lake?
4 - What is currently growing on the land and what are the characteristics of those plants and how do they impact my goals for the land?
I have found that for pasture on slope the plants of a wide variety of forage can do a very nice job of slowing the water down enough to allow for percolation if I let it grow and die off seasonally and not mow or hay it. Now, this may not be an option for you, but this also adds significant organic matter to the soil, a key factor in percolation. The slope on my property was drawing from a large area (5 acres) and at the bottom of the hill the pasture was spongy even on dry days with many water-loving plants that are not the best beefcattle forage (curly dock). After having let the pasture in this section grow and seasonally die off for two years, we are no longer experiencing any wet conditions at the bottom of the hill and I believe I can now place rotationally mob-grazed livestock on this pasture without fear of erosion or run off problems because the level of organic matter in the soil has risen to such a degree that it can percolate large rain events (we have about 40-44 inches of precip each year).
Now, that’s just one technique. But in two years I have changed the pasture’s organic matter ratio in the soil and its hydrologic shed. Today, I would only cut into the land and alter structure, such as with a swale, only when absolutely necessary.
Result vary on myriad factors that are contextually relevant to you on your property. Be wary of blanket solutions to specific challenges.