Easy — if you’re outside, just look down. You’re standing in a watershed. A watershed is the area of land where water falls and drains into a common outlet. Watersheds can be small in footprint or large enough to encompass an entire landscape – like the water that drains into the rivers, bayous, and creeks which flow into Galveston Bay to the Gulf of Mexico.
As the water flows, it often picks up pollutants, which may have harmful effects on the ecology of the watershed and, ultimately, on the reservoir, bay, or ocean.
Not all water flows directly to the sea, however. When rain falls on dry ground, it can soak into the ground. This groundwater remains in the soil where it will eventually seep into the nearest stream. In other areas, where the soil contains a lot of hard clay, very little water may seep into the ground. Instead, it quickly runs off to lower ground.
During periods of heavy rain, water may run off of impervious surfaces, such as parking lots, roads, buildings, and other structures, because it has nowhere else to go. These surfaces act as “fast lanes” that transport the water directly into storm drains. The excess water volume can quickly overwhelm streams and rivers, causing them to overflow and possibly result in floods.