From Pennsylvania Resources Council web site:
"Rain Gardens are shallow, planted depressions that absorb runoff from impervious surfaces and allow it to infiltrate into the soil. Rain gardens are designed to have a “bowl shape” or “dip” that retains rain water as it waits to be absorbed into the soil.
Rain Gardens are planted with deep rooted, native plants. Native plants are beautiful, hardy, and once established require less maintenance than a conventional lawn. Native plants provide food and shelter for a host of native birds, butterflies and beneficial insects.
Rain Gardens reduce the initial rush of water that enters a stream during rain storms by capturing and absorbing runoff from yards, roofs, and paved surfaces. Rain gardens can absorb 30% more water than a traditional lawn. Properly designed rain gardens drain in 24 – 48 hours, can filter many common pollutants found in runoff, and help to recharge the ground water supply."*