Water is the most common liquid on the planet. It falls from the clouds in the sky and forms lakes, oceans, and rivers. It can provide and store energy. Water is essential to plants and animals, but many people and regions of the world do not have sufficient access to clean water.
When water quickly runs over the land it doesn't have sufficient time to be absorbed into the soil. Instead it can cause erosion as it races over and carries off any accumulated topsoil, organic matter, and silt. Fast moving water can also create undesirable crevasses and gullies. Swales help to slow water to allow it to be absorbed into the soil to recharge groundwater.