A swale is like a ditch, but shallower and wider. You usually see these beside roads. When there is a lot of water, it fills up and slowing down the flow of water on its way to the creek. When the weather is dry, the swale is empty and grass and other plants grow.
By holding the water temporarily in a swale, it allows some water to soak into the ground, some to evaporate and mostly it slows the water headed for the neighbourhood creek or stream.
This helps protect the habitat at the creek or stream so little critters, like toads and newts don’t get swept away in the swift current that occurs in typical developments.
- See more at: http://www.surrey.ca/city-services/3646.aspx#sthash.uHTwzrnr.dpuf