In Brooklyn, we have what is called a "combined sewer system" that drains both waste water (from our toilets, showers, etc.) and stormwater from the storm drains on the streets. On a dry sunny day, the sewer flow is slow and steady to a local water treatment plant. Waste water is safely treated and returned back into NY Harbor (the Gowanus Canal is part of NY Harbor). But when it rains, the water treatment plant gets overwhelmed and cannot accommodate the increased flow of water, causing the combined sewage to overflow into local waterways.
In addition, impervious surfaces cover much of our city. As a result, more stormwater flows directly into drains (called "runoff"), ultimately ending up in our local waterways, carrying with with it everything on the street. Watch the video to see a model of this "combined sewer system" in action.
Watch the two videos below to learn about how to construct a scientific explanation. Keep in mind that developing a scientific explanation is the ultimate goal of the scientist. A scientific explanation is the first of two paragraphs in a conclusion, which is the last step in the scientific method.