All floods and most polluted waterways have one thing in common: stormwater runoff. Stormwater runoff is the fastest growing type of water pollution in Pennsylvania. When rain falls or snow melts faster than it can soak into the ground, the runoff ends up in local waterways. When more runoff reaches those waterways than they can hold, they flood. Stormwater runoff isn’t just muddy, it’s dirty too. Water running off streets and parking lots carries chemical fertilizer, oil, animal waste, and trash into the waterways.