Storm drains are separate and distinct from sanitary sewer systems. The separation of storm sewers from sanitary sewers helps prevent sewage treatment plants becoming overwhelmed by infiltration/inflow during a rainstorm, which could discharge untreated sewage into the environment.
Many storm drainage systems drain untreated storm water into rivers or streams. In the US, many local governments conduct public awareness campaigns about this, lest people dump waste into the storm drain system. In Cleveland, Ohio, for example, all new catch basins installed have inscriptions on them not to dump any waste, and usually include a fish imprint as well. Trout Unlimited Canada recommends that a yellow fish symbol be painted next to existing storm drains.