Water from rain and melting snow either seeps into the ground or “runs off” to lower areas, making its way into streams, lakes and other water bodies (rivers). On its way, runoff water can pick up and carry many substances that pollute water. Some - like pesticides, fertilizers, oil and soap – are harmful in any quantity. Others - like sediment from construction, bare soil, or agricultural land, or pet waste, grass clippings and leaves - can harm creeks, rivers and lakes in sufficient quantities.
In addition to rain and snowmelt, various human activities like watering, car washing, and malfunctioning septic tanks can also put water onto the land surface. Here, it can also create runoff that carries pollutants to creeks, rivers and lakes. Polluted runoff generally happens anywhere people use or alter the land. For example, in developed areas, none of the water that falls on hard surfaces like roofs, driveways, parking lots or roads can seep into the ground. These impervious surfaces create large amounts of runoff that pick up pollutants.
The runoff flows from gutters and storm drains to streams. Runoff not only pollutes' but erodes stream banks. The mix of pollution and eroded dirt muddies the water and causes problems downstream.