Answer: Often, a yard is purposely graded with a slope toward a ditch or swale to direct excess rain, snowmelt and runoff water away from the foundation of the house or to direct the runoff to a desired area such as storm drains or a creek. This is usually done to prevent flooding and even a slight ditch or depression can carry a lot of water during a rainy spell. So I would strongly suggest you consult on site with a trained professional with experience in grading and drainage before you try to fill it in.
If you do decide to fill it in, it would be best to use soil that is a close match to the existing soil on site. This will help any plantings such as lawn to stay consistent between the old and new soils. It will also help avoid interface problems between the native soil and the fill. Loosen the existing soil before adding the fill so you can have a blended interface rather than two separate levels. I hope this helps.
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