Stormwater is rainwater and melted snow that flows over streets, parking lots, rooftops, and other hard surfaces instead of soaking into the ground. When rain falls on a forest or field, most of it seeps into the soil. But when it falls on pavement or buildings, it has nowhere to go except to rush downhill, picking up speed as it travels.

This rushing water becomes a problem because it collects trash, oil, chemicals, and dirt as it flows, carrying all that pollution into storm drains, streams, and eventually lakes or oceans. Cities spend millions managing stormwater through drainage systems, underground pipes, and special ponds designed to slow the water down and filter out pollutants before they reach natural waterways.

You can see stormwater in action during a heavy rainstorm: watch how water streams down your driveway or street, forming little rivers in the gutters. Engineers design stormwater systems to prevent flooding and protect water quality. Some cities now use green solutions like rain gardens and permeable pavement that let water soak into the ground naturally, the way it would in a forest. When you see a storm drain with a sign saying “Dump No Waste: Drains to Bay,” that's because stormwater can flow directly into natural water bodies without being cleaned at a treatment plant first.