Watch this 3 min video on why stormwater is regulated:

To summarize: When rain falls on hard surfaces like rooftops, driveways, parking lots and roadways it runs over those areas and in urbanized areas often off into a storm drain. This is great for preventing flooding, but has a hidden impact on water quality. When rain runs off/over hard surfaces it picks up various pollutants along the way (excess fertilizer, oil, brake dust, sand, salt, etc.) and carries them with it into our storm drains which empty directly into your local stream, pond, river or the ocean. This results in green algal blooms, fish kills, closed swimming and fishing areas, and other impairments to our water ways.

This diffuse or "nonpoint source" pollution is the next big challenge in cleaning our water bodies. Under the federal Clean Water Act, states establish a program (the MS4 General Permit) that regulates how municipalities (and  institutions)  manage their stormwater to help reduce the number of pollutants being carried into our water bodies.

The video below goes into more depth how community decision about land use and development can lead to polluted stormwater runoff. The MS4 General Permit is intended to address this source of pollution.