Discharging runoff directly to a stream encourage stream bank erosion or channel deepening.   Discharging directly to a stream also increases the pollutant load in the streams including sediment, pesticides, fertilizers, and oil and grease that are found in typical urban runoff.
	This slide shows a vegetated (plants besides grass) grass swale that slows down runoff and filters some of the pollutant load before it reaches the stream.  (Note: Grass itself does not slow down runoff much.)
	The next slides show examples of BMPS that slow down and/or address storm water.