According to a consulted forest hydrologist, a swale is a low place in the landscape where water may or may not run sometimes. Another interesting thing about swales is that they are changeable both as a result of human activities, and those occurring in nature. A swale, into which a fair amount of water sometimes runs today, could shift or become a swale into which very little water runs by tomorrow. This change could be the result of something happening on higher ground, such as a property owner building a structure or a tree falling in the wilderness changing even slightly the direction in which runoff flows. The change could even occur as a result of another swale developing or being intentionally constructed higher up.
People construct swales, because they can be an efficient and affordable way to deal with runoff.
Water collects in these areas and rather than flowing directly into streams or rivers, slowly seeps into the soil and creeks, losing pollutants like oil and lawn fertilizer along the way through biofiltration. Whether occurring in nature or made by humans, whether they are in one spot today, or another tomorrow, any swale can provide important stormwater and habitat functions at some point in time, and it might not be a bad idea to educate the public about their uses and encourage people to construct the shallow ditch variety where they might be beneficial.