When dog waste is left on the ground, rain runoff washes it (and its bacteria) into storm drains.  Since many storm sewers discharge into rivers, lakes, and streams without undergoing any treatment, bacteria from pet waste can enter our shared surface waters, impair water quality, and make it unsafe for aquatic life and human health.  In the U.S. alone, dogs produce 10 million tons of waste each year, and just one gram can contain as many as 23 million fecal coliform bacteria!  Salmonella, giardia, and other bacteria and parasites in dog waste can spread disease and impair water quality.  

On walks, always carry a baggie (preferably biodegradable!) to scoop the poop and dispose of it in the trash.  At home, clean up dog waste in your yard at least once a week and either flush it down the toilet, dispose of it in the trash, or bury it.